jammy (7) elvish-language.7.gz
Introduction
This document describes the Elvish programming language. It is both a specification and an advanced
tutorial. The parts of this document marked with either notes or called out as examples are non-
normative, and only serve to help you understand the more formal descriptions.
Examples in this document might use constructs that have not yet been introduced, so some familiarity
with the language is assumed. If you are new to Elvish, start with the learning materials.
Source code encoding
Elvish source code must be Unicode text encoded in UTF-8.
In this document, character is a synonym of Unicode codepoint (https://en.wikipedia.org/wiki/Code_point)
or its UTF-8 encoding.
Lexical elements
Whitespace
In this document, an inline whitespace is any of the following:
• A space (U+0020);
• A tab (U+0009);
• A comment: starting with # and ending before (but not including) the next carriage return, newline or
end of file;
• A line continuation: a ^ followed by a newline ("\n"), or a carriage return and newline ("\r\n").
A whitespace is any of the following:
• An inline whitespace;
• A carriage return (U+000D);
• A newline (U+000A).
Metacharacters
The following metacharacters serve to introduce or delimit syntax constructs:
Metacharacter Use
──────────────────────────────────────────────────────
$ Referencing variables
* and ? Forming wildcard
| Separating forms in a pipeline
& Marking background pipelines;
introducing key-value pairs
; Separating pipelines
< and > Introducing IO redirections
( and ) Enclosing output captures
[ and ] Enclosing list literals, map literals
or function signature
{ and } Enclosing lambda literals or brace
expressions
The following characters are parsed as metacharacters under certain conditions:
• ~ is a metacharacter if it appears at the beginning of a compound expression, in which case it is
subject to tilde expansion;
• = is a metacharacter when used for terminating map keys or option keys, or denoting legacy assignment
form or temporary assignments.
Single-quoted string
A single-quoted string consists of zero or more characters enclosed in single quotes ('). All enclosed
characters represent themselves, except the single quote.
Two consecutive single quotes are handled as a special case: they represent one single quote, instead of
terminating a single-quoted string and starting another.
Examples: '*\' evaluates to *\, and 'it''s' evaluates to it's.
Double-quoted string
A double-quoted string consists of zero or more characters enclosed in double quotes ("). All enclosed
characters represent themselves, except backslashes (\), which introduces escape sequences. Double
quotes are not allowed inside double-quoted strings, except after backslashes.
The following escape sequences are supported:
• \cX, where X is a character with codepoint between 0x40 and 0x5F, represents the codepoint that is 0x40
lower than X. For example, \cI is the tab character: 0x49 (I) - 0x40 = 0x09 (tab). There is one
special case: A question-mark is converted to del; i.e., \c? or \^? is equivalent to \x7F.
• \^X is the same as \cX.
• \[0..7][0..7][0..7] is a byte written as an octal value. There must be three octal digits following
the backslash. For example, \000 is the nul character, and \101 is the same as A, but \0 is an invalid
escape sequence (too few digits).
• \x.. is a Unicode code point represented by two hexadecimal digits.
• \u.... is a Unicode code point represented by four hexadecimal digits.
• \U...... is a Unicode code point represented by eight hexadecimal digits.
• The following single character escape sequences:
• \a is the “bell” character, equivalent to \007 or \x07.
• \b is the “backspace” character, equivalent to \010 or \x08.
• \f is the “form feed” character, equivalent to \014 or \x0c.
• \n is the “new line” character, equivalent to \012 or \x0a.
• \r is the “carriage return” character, equivalent to \015 or \x0d.
• \t is the “tab” character, equivalent to \011 or \x09.
• \v is the “vertical tab” character, equivalent to \013 or \x0b.
• \\ is the “backslash” character, equivalent to \134 or \x5c.
• \" is the “double-quote” character, equivalent to \042 or \x22.
An unsupported escape sequence results in a parse error.
Note: Unlike most other shells, double-quoted strings in Elvish do not support interpolation. For
instance, "$name" simply evaluates to a string containing $name. To get a similar effect, simply
concatenate strings: instead of "my name is $name", write "my name is "$name. Under the hood this is a
compounding operation.
Bareword
A string can be written without quoting – a bareword, if it only includes the characters from the
following set:
• ASCII letters (a-z and A-Z) and numbers (0-9);
• The symbols !%+,-./:@\_;
• Non-ASCII codepoints that are printable, as defined by unicode.IsPrint
(https://godoc.org/unicode#IsPrint) in Go’s standard library.
Examples: a.txt, long-bareword, elf@elv.sh, /usr/local/bin, 你好世界.
Moreover, ~ and = are allowed to appear without quoting when they are not parsed as metacharacters.
Note: since the backslash (\) is a valid bareword character in Elvish, it cannot be used to escape
metacharacter. Use quotes instead: for example, to echo a star, write echo "*" or echo '*', not echo \*.
The last command just writes out \*.
Value types
String
A string is a (possibly empty) sequence of bytes.
Single-quoted string literals, double-quoted string literals and barewords all evaluate to string values.
Unless otherwise noted, different syntaxes of string literals are equivalent in the code. For instance,
xyz, 'xyz' and "xyz" are different syntaxes for the same string with content xyz.
Strings that contain UTF-8 encoded text can be indexed with a byte index where a codepoint starts, which
results in the codepoint that starts there. The index can be given as either a typed number, or a string
that parses to a number. Examples:
• In the string elv, every codepoint is encoded with only one byte, so 0, 1, 2 are all valid indices:
~> put elv[0]
▶ e
~> put elv[1]
▶ l
~> put elv[2]
▶ v
• In the string 世界, each codepoint is encoded with three bytes. The first codepoint occupies byte 0
through 2, and the second occupies byte 3 through 5. Hence valid indices are 0 and 3:
~> put 世界[0]
▶ 世
~> put 世界[3]
▶ 界
Such strings may also be indexed with a slice (see documentation of list for slice syntax). The range
determined by the slice is also interpreted as byte indices, and the range must begin and end at
codepoint boundaries.
The behavior of indexing a string that does not contain valid UTF-8-encoded Unicode text is unspecified.
Note: String indexing will likely change.
Number
Elvish supports several types of numbers. There is no literal syntax, but they can be constructed by
passing their string representation to the num builtin command:
• Integers are written in decimal (e.g. 10), hexadecimal (e.g. 0xA), octal (e.g. 0o12) or binary (e.g.
0b1010).
NOTE: Integers with leading zeros are now parsed as octal (e.g. 010 is the same as 0o10, or 8), but
this is subject to change (#1372 (https://b.elv.sh/1371)).
• Rationals are written as two exact integers joined by /, e.g. 1/2 or 0x10/100 (16/100).
• Floating-point numbers are written with a decimal point (e.g. 10.0) or using scientific notation (e.g.
1e1 or 1.0e1). There are also three additional special floating-point values: +Inf, -Inf and NaN.
Digits may be separated by underscores, which are ignored; this permits separating the digits into groups
to improve readability. For example, 1000000 and 1_000_000 are equivalent, so are 1.234_56e3 and
1.23456e3, or 1_2_3 and 123.
The string representation is case-insensitive.
Strings and numbers
Strings and numbers are distinct types; for example, 2 and (num 2) are distinct values.
However, by convention, all language constructs that expect numbers (e.g. list indices) also accept
strings that can be converted to numbers. This means that most of the time, you can just use the string
representation of numbers, instead of explicitly constructing number values. Builtin numeric commands
follow the same convention.
When the word number appears unqualified in other sections of this document, it means either an
explicitly number-typed value (typed number), or its string representation.
When a typed number is converted to a string (e.g. with to-string), the result is guaranteed to convert
back to the original number. In other words, eq $x (num (to-string $x)) always outputs $true if $x is a
typed number.
Exactness
Integers and rationals are exact numbers; their precision is only limited by the available memory, and
many (but not all) operations on them are guaranteed to produce mathematically correct results.
Floating-point numbers are IEEE 754 (https://en.wikipedia.org/wiki/IEEE_754) double-precision. Since
operations on floating-point numbers in general are not guaranteed to be precise, they are always
considered inexact.
This distinction is important for some builtin commands; see exactness-preserving commands.
List
A list is a value containing a sequence of values. Values in a list are called its elements. Each
element has an index, starting from zero.
List literals are surrounded by square brackets [ ], with elements separated by whitespace. Examples:
~> put [lorem ipsum]
▶ [lorem ipsum]
~> put [lorem
ipsum
foo
bar]
▶ [lorem ipsum foo bar]
Note: In Elvish, commas have no special meanings and are valid bareword characters, so don’t use them to
separate elements:
~> li = [a, b]
~> put $li
▶ [a, b]
~> put $li[0]
▶ a,
A list can be indexed with the index of an element to obtain the element, which can take one of two
forms:
• A non-negative integer, an offset counting from the beginning of the list. For example, $li[0] is the
first element of $li.
• A negative integer, an offset counting from the back of the list. For instance, $li[-1] is the last
element $li.
In both cases, the index can be given either as a typed number or a number-like string.
A list can also be indexed with a slice to obtain a sublist, which can take one of two forms:
• A slice $a..$b, where both $a and $b are integers. The result is sublist of $li[$a] up to, but not
including, $li[$b]. For instance, $li[4..7] equals [$li[4] $li[5] $li[6]], while $li[1..-1] contains
all elements from $li except the first and last one.
Both integers may be omitted; $a defaults to 0 while $b defaults to the length of the list. For
instance, $li[..2] is equivalent to $li[0..2], $li[2..] is equivalent to $li[2..(count $li)], and
$li[..] makes a copy of $li. The last form is rarely useful, as lists are immutable.
Note that the slice needs to be a single string, so there cannot be any spaces within the slice. For
instance, $li[2..10] cannot be written as $li[2.. 10]; the latter contains two indices and is
equivalent to $li[2..] $li[10] (see Indexing).
• A slice $a..=$b, which is similar to $a..$b, but includes $li[$b].
Examples:
~> li = [lorem ipsum foo bar]
~> put $li[0]
▶ lorem
~> put $li[-1]
▶ bar
~> put $li[0..2]
▶ [lorem ipsum]
Map
A map is a value containing unordered key-value pairs.
Map literals are surrounded by square brackets; a key/value pair is written &key=value (reminiscent to
HTTP query parameters), and pairs are separated by whitespaces. Whitespaces are allowed after =, but not
before =. Examples:
~> put [&foo=bar &lorem=ipsum]
▶ [&foo=bar &lorem=ipsum]
~> put [&a= 10
&b= 23
&sum= (+ 10 23)]
▶ [&a=10 &b=23 &sum=33]
The literal of an empty map is [&].
Specifying a key without = or a value following it is equivalent to specifying $true as the value.
Specifying a key with = but no value following it is equivalent to specifying the empty string as the
value. Example:
~> echo [&a &b=]
[&a=$true &b='']
A map can be indexed by any of its keys. Unlike strings and lists, there is no support for slices, and
.. and ..= have no special meanings. Examples:
~> map = [&a=lorem &b=ipsum &a..b=haha]
~> echo $map[a]
lorem
~> echo $map[a..b]
haha
You can test if a key is present using has-key and enumerate the keys using the keys builtins.
Note: Since & is a metacharacter, key-value pairs do not have to follow whitespaces; [&a=lorem&b=ipsum]
is equivalent to [&a=lorem &b=ipsum], just less readable. This might change in future.
Pseudo-map
A pseudo-map is not a single concrete data type. It refers to concrete types that behave like maps with
some restrictions.
A pseudo-map has a fixed set of keys whose values can be accessed by indexing like you would for a
regular map. Similarly, you can use commands like keys and has-key on such objects.
Unlike a normal map, it is currently not possible to create a modified version of an existing pseudo-map:
it is not possible to create a pseudo-map with new keys, without existing keys, or with a different value
for a given key.
The pseudo-map mechanism is often used for introspection. For example, exceptions, user-defined
functions, and $buildinfo are pseudo-maps.
Nil
The value $nil serves as the initial value of variables that are declared but not assigned.
Boolean
There are two boolean values, $true and $false.
When converting non-boolean values to the boolean type, $nil and exceptions convert to $false; such
values and $false itself are booleanly false. All the other non-boolean values convert to $true; such
values and $true itself are booleanly true.
Exception
An exception carries information about errors during the execution of code.
There is no literal syntax for exceptions. See the discussion of exception and flow commands for more
information about this data type.
An exception is a pseudo-map with a reason field, which is in turn a pseudo-map. The reason pseudo-map
has has a type field identifying how the exception was raised, and further fields depending on the type:
• If the type field is fail, the exception was raised by the fail command.
In this case, the content field contains the argument to fail.
• If the type field is flow, the exception was raised by one of the flow commands.
In this case, the name field contains the name of the flow command.
• If the type field is pipeline, the exception was a result of multiple commands in the same pipeline
raising exceptions.
In this case, the exceptions field contains the exceptions from the individual commands.
• If the type field starts with external-cmd/, the exception was caused by one of several conditions of
an external command. In this case, the following fields are available:
• The cmd-name field contains the name of the command.
• The pid field contains the PID of the command.
• If the type field is external-cmd/exited, the external command exited with a non-zero status code. In
this case, the exit-status field contains the exit status.
• If the type field is external-cmd/signaled, the external command was killed by a signal. In this case,
the following extra fields are available:
• The signal-name field contains the name of the signal.
• The signal-number field contains the numerical value of the signal, as a string.
• The core-dumped field is a boolean reflecting whether a core dump was generated.
• If the type field is external-cmd/stopped, the external command was stopped. In this case, the
following extra fields are available:
• The signal-name field contains the name of the signal.
• The signal-number field contains the numerical value of the signal, as a string.
• The trap-cause field contains the number indicating the trap cause.
Examples:
~> put ?(fail foo)[reason]
▶ [&content=foo &type=fail]
~> put ?(return)[reason]
▶ [&name=return &type=flow]
~> put ?(false)[reason]
▶ [&cmd-name=false &exit-status=1 &pid=953421 &type=external-cmd/exited]
File
There is no literal syntax for the file type. This type is returned by commands such as file:open and
path:temp-file. It can be used as the target of a redirection rather than a filename.
A file object is a pseudo-map with fields fd (an int) and name (a string). If the file is closed the fd
will be -1.
Function
A function encapsulates a piece of code that can be executed in an ordinary command, and takes its
arguments and options. Functions are first-class values; they can be kept in variables, used as
arguments, output on the value channel and embedded in other data structures. Elvish comes with a set of
builtin functions, and Elvish code can also create user-defined functions.
Note: Unlike most programming languages, functions in Elvish do not have return values. Instead, they
can output values, which can be captured later.
A function literal, or alternatively a lambda, evaluates to a user-defined function. The literal syntax
consists of an optional signature list, followed by a code chunk that defines the body of the function.
Here is an example without a signature:
~> f = { echo "Inside a lambda" }
~> put $f
▶ <closure 0x18a1a340>
One or more whitespace characters after { is required: Elvish relies on the presence of whitespace to
disambiguate function literals and braced lists.
Note: It is good style to put some whitespace before the closing } for symmetry, but this is not required
by the syntax.
Functions defined without a signature list do not accept any arguments or options. To do so, write a
signature list. Here is an example:
~> f = [a b]{ put $b $a }
~> $f lorem ipsum
▶ ipsum
▶ lorem
There must be no space between ] and {; otherwise Elvish will parse the signature as a list, followed by
a lambda without signature:
~> put [a]{ nop }
▶ <closure 0xc420153d80>
~> put [a] { nop }
▶ [a]
▶ <closure 0xc42004a480>
Like in the left hand of assignments, if you prefix one of the arguments with @, it becomes a rest
argument, and its value is a list containing all the remaining arguments:
~> f = [a @rest]{ put $a $rest }
~> $f lorem
▶ lorem
▶ []
~> $f lorem ipsum dolar sit
▶ lorem
▶ [ipsum dolar sit]
~> f = [a @rest b]{ put $a $rest $b }
~> $f lorem ipsum dolar sit
▶ lorem
▶ [ipsum dolar]
▶ sit
You can also declare options in the signature. The syntax is &name=default (like a map pair), where
default is the default value for the option; the value of the option will be kept in a variable called
name:
~> f = [&opt=default]{ echo "Value of $opt is "$opt }
~> $f
Value of $opt is default
~> $f &opt=foobar
Value of $opt is foobar
Options must have default values: Options should be optional.
If you call a function with too few arguments, too many arguments or unknown options, an exception is
thrown:
~> [a]{ echo $a } foo bar
Exception: need 1 arguments, got 2
[tty], line 1: [a]{ echo $a } foo bar
~> [a b]{ echo $a $b } foo
Exception: need 2 arguments, got 1
[tty], line 1: [a b]{ echo $a $b } foo
~> [a b @rest]{ echo $a $b $rest } foo
Exception: need 2 or more arguments, got 1
[tty], line 1: [a b @rest]{ echo $a $b $rest } foo
~> [&k=v]{ echo $k } &k2=v2
Exception: unknown option k2
[tty], line 1: [&k=v]{ echo $k } &k2=v2
A user-defined function is a pseudo-map. If $f is a user-defined function, it has the following fields:
• $f[arg-names] is a list containing the names of the arguments.
• $f[rest-arg] is the index of the rest argument. If there is no rest argument, it is -1.
• $f[opt-names] is a list containing the names of the options.
• $f[opt-defaults] is a list containing the default values of the options, in the same order as $f[opt-
names].
• $f[def] is a string containing the definition of the function, including the signature and the body.
• $f[body] is a string containing the body of the function, without the enclosing brackets.
• $f[src] is a map-like data structure containing information about the source code that the function is
defined in. It contains the same value that the src function would output if called from the function.
Variable
A variable is a named storage location for holding a value. The following characters can be used in
variable names (a subset of bareword characters) without quoting:
A variable exist after it is declared (either explicitly using var or implicitly using the legacy
assignment form), and its value may be mutated by further assignments. It can be used as an expression
or part of an expression.
Note: In most other shells, variables can map directly to environmental variables: $PATH is the same as
the PATH environment variable. This is not the case in Elvish. Instead, environment variables are put
in a dedicated E: namespace; the environment variable PATH is known as $E:PATH. The $PATH variable, on
the other hand, does not exist initially, and if you have defined it, only lives in a certain lexical
scope within the Elvish interpreter.
You will notice that variables sometimes have a leading dollar $, and sometimes not. The tradition is
that they do when they are used for their values, and do not otherwise (e.g. in assignment). This is
consistent with most other shells.
Variable suffix
There are two characters that have special meanings and extra type constraints when used as the suffix of
a variable name:
• If a variable name ends with ~, it can only take callable values, which are functions and external
commands. Such variables are consulted when resolving ordinary commands. The default value is the
builtin nop command.
• If a variable name ends with :, it can only take namespaces as values. They are used for accessing
namespaced variables.
Scoping rule
Elvish has lexical scoping. A file or an interactive prompt starts with a top-level scope, and a
function literal introduce new lexical scopes.
When you use a variable, Elvish looks for it in the current lexical scope, then its parent lexical scope
and so forth, until the outermost scope:
~> x = 12
~> { echo $x } # $x is in the global scope
12
~> { y = bar; { echo $y } } # $y is in the outer scope
bar
If a variable is not in any of the lexical scopes, Elvish tries to resolve it in the builtin: namespace,
and if that also fails, fails with an error:
~> echo $pid # builtin
36613
~> echo $nonexistent
Compilation error: variable $nonexistent not found
[interactive], line 1:
echo $nonexistent
Note that Elvish resolves all variables in a code chunk before starting to execute any of it; that is why
the error message above says compilation error. This can be more clearly observed in the following
example:
~> echo pre-error; echo $nonexistent
Compilation error: variable $nonexistent not found
[tty], line 1: echo pre-error; echo $nonexistent
When you assign a variable, Elvish does a similar searching. If the variable cannot be found, instead of
causing an error, it will be created in the current scope:
~> x = 12
~> { x = 13 } # assigns to x in the global scope
~> echo $x
13
~> { z = foo } # creates z in the inner scope
~> echo $z
Compilation error: variable $z not found
[tty], line 1: echo $z
One implication of this behavior is that Elvish will not shadow your variable in outer scopes.
There is a local: namespace that always refers to the current scope, and by using it it is possible to
force Elvish to shadow variables:
~> x = 12
~> { local:x = 13; echo $x } # force shadowing
13
~> echo $x
12
After force shadowing, you can still access the variable in the outer scope using the up: namespace,
which always skips the innermost scope:
~> x = 12
~> { local:x = 14; echo $x $up:x }
14 12
The local: and up: namespaces can also be used on unshadowed variables, although they are not useful in
those cases:
~> foo = a
~> { echo $up:foo } # $up:foo is the same as $foo
a
~> { bar = b; echo $local:bar } # $local:bar is the same as $bar
b
It is not possible to refer to a specific outer scope.
You cannot create new variables in the builtin: namespace, although existing variables in it can be
assigned new values.
Closure semantics
When a function literal refers to a variable in an outer scope, the function will keep that variable
alive, even if that variable is the local variable of an outer function that that function has returned.
This is called closure semantics (https://en.wikipedia.org/wiki/Closure_(computer_programming)), because
the function literal “closes” over the environment it is defined in.
In the following example, the make-adder function outputs two functions, both referring to a local
variable $n. Closure semantics means that:
1. Both functions can continue to refer to the $n variable after make-adder has returned.
2. Multiple calls to the make-adder function generates distinct instances of the $n variables.
~> fn make-adder {
n = 0
put { put $n } { n = (+ $n 1) }
}
~> getter adder = (make-adder)
~> $getter # $getter outputs $n
▶ 0
~> $adder # $adder increments $n
~> $getter # $getter and $setter refer to the same $n
▶ 1
~> getter2 adder2 = (make-adder)
~> $getter2 # $getter2 and $getter refer to different $n
▶ 0
~> $getter
▶ 1
Variables that get “captured” in closures are called upvalues; this is why the pseudo-namespace for
variables in outer scopes is called up:. When capturing upvalues, Elvish only captures the variables
that are used. In the following example, $m is not an upvalue of $g because it is not used:
~> fn f { m = 2; n = 3; put { put $n } }
~> g = (f)
This effect is not currently observable, but will become so when namespaces become introspectable
(https://github.com/elves/elvish/issues/492).
Expressions
Elvish has a few types of expressions. Some of those are new compared to most other languages, but some
are very similar.
Unlike most other languages, expressions in Elvish may evaluate to any number of values. The concept of
multiple values is distinct from a list of multiple elements.
Literal
Literals of strings, lists, maps and functions all evaluate to one value of their corresponding types.
They are described in their respective sections.
Variable use
A variable use expression is formed by a $ followed by the name of the variable. Examples:
~> foo = bar
~> x y = 3 4
~> put $foo
▶ bar
~> put $x
▶ 3
If the variable name only contains the following characters (a subset of bareword characters), the name
can appear unquoted after $ and the variable use expression extends to the longest sequence of such
characters:
• ASCII letters (a-z and A-Z) and numbers (0-9);
• The symbols -_:~. The colon : is special; it is normally used for separating namespaces or denoting
namespace variables;
• Non-ASCII codepoints that are printable, as defined by unicode.IsPrint
(https://godoc.org/unicode#IsPrint) in Go’s standard library.
Alternatively, $ may be followed immediately by a single-quoted string
(https://elv.sh/ref/language.html#single-quoted-string) or a double-quoted string
(https://elv.sh/ref/language.html#double-quoted-string), in which cases the value of the string specifies
the name of the variable. Examples:
~> "\n" = foo
~> put $"\n"
▶ foo
~> '!!!' = bar
~> put $'!!!'
▶ bar
Unlike other shells and other dynamic languages, local namespaces in Elvish are statically checked. This
means that referencing a nonexistent variable results in a compilation error, which is triggered before
any code is actually evaluated:
~> echo $x
Compilation error: variable $x not found
[tty], line 1: echo $x
~> f = { echo $x }
compilation error: variable $x not found
[tty 1], line 1: f = { echo $x }
If a variable contains a list value, you can add @ before the variable name; this evaluates to all the
elements within the list. This is called exploding the variable:
~> li = [lorem ipsum foo bar]
~> put $li
▶ [lorem ipsum foo bar]
~> put $@li
▶ lorem
▶ ipsum
▶ foo
▶ bar
Note: Since variable uses have higher precedence than indexing, this does not work for exploding a list
that is an element of another list. For doing that, and exploding the result of other expressions (such
as an output capture), use the builtin all command.)
Output capture
An output capture expression is formed by putting parentheses () around a code chunk. It redirects the
output of the chunk into an internal pipe, and evaluates to all the values that have been output.
~> + 1 10 100
▶ 111
~> x = (+ 1 10 100)
~> put $x
▶ 111
~> put lorem ipsum
▶ lorem
▶ ipsum
~> x y = (put lorem ipsum)
~> put $x
▶ lorem
~> put $y
▶ ipsum
If the chunk outputs bytes, Elvish strips the last newline (if any), and split them by newlines, and
consider each line to be one string value:
~> put (echo "a\nb")
▶ a
▶ b
Trailing carriage returns are also stripped from each line, which effectively makes \r\n also valid line
separators:
~> put (echo "a\r\nb")
▶ a
▶ b
Note 1. Only the last newline is ever removed, so empty lines are preserved; (echo "a\n") evaluates to
two values, "a" and "".
Note 2. One consequence of this mechanism is that you can not distinguish outputs that lack a trailing
newline from outputs that have one; (echo what) evaluates to the same value as (print what). If such a
distinction is needed, use slurp to preserve the original bytes output.
If the chunk outputs both values and bytes, the values of output capture will contain both value outputs
and lines. However, the ordering between value output and byte output might not agree with the order in
which they happened:
~> put (put a; echo b) # value order need not be the same as output order
▶ b
▶ a
Note: If you want to capture the stdout and stderr byte streams independent of each other, see the
example in the run-parallel documentation.
Exception capture
An exception capture expression is formed by putting ?() around a code chunk. It runs the chunk and
evaluates to the exception it throws.
~> fail bad
Exception: bad
Traceback:
[interactive], line 1:
fail bad
~> put ?(fail bad)
▶ ?(fail bad)
If there was no error, it evaluates to the special value $ok:
~> nop
~> put ?(nop)
▶ $ok
Exceptions are booleanly false and $ok is booleanly true. This is useful in if (introduced later):
if ?(test -d ./a) {
# ./a is a directory
}
Note: Exception captures do not affect the output of the code chunk. You can combine output capture and
exception capture:
output = (error = ?(commands-that-may-fail))
Braced list
A braced list consists of multiple expressions separated by whitespaces and surrounded by braces ({}).
There must be no space after the opening brace. A braced list evaluates to whatever the expressions
inside it evaluate to. Its most typical use is grouping multiple values in a compound expression.
Example:
~> put {a b}-{1 2}
▶ a-1
▶ a-2
▶ b-1
▶ b-2
It can also be used to affect the order of evaluation. Examples:
~> put *
▶ foo
▶ bar
~> put *o
▶ foo
~> put {*}o
▶ fooo
▶ baro
Note: When used to affect the order of evaluation, braced lists are very similar to parentheses in C-like
languages.
Note: A braced list is an expression. It is a syntactical construct and not a separate data structure.
Elvish currently also supports using commas to separate items in a braced list. This will likely be
removed in future, but it also means that literal commas must be quoted right now.
Indexing
An indexing expression is formed by appending one or more indices inside a pair of brackets ([]) after
another expression (the indexee). Examples:
~> li = [foo bar]
~> put $li[0]
▶ foo
~> li = [[foo bar] quux]
~> put $li[0][0]
▶ foo
~> put [[foo bar]][0][0]
▶ foo
If the expression being indexed evaluates to multiple values, the indexing operation is applied on each
value. Example:
~> put (put [foo bar] [lorem ipsum])[0]
▶ foo
▶ lorem
~> put {[foo bar] [lorem ipsum]}[0]
▶ foo
▶ lorem
If there are multiple index expressions, or the index expression evaluates to multiple values, the
indexee is indexed once for each of the index value. Examples:
~> put elv[0 2 0..2]
▶ e
▶ v
▶ el
~> put [lorem ipsum foo bar][0 2 0..2]
▶ lorem
▶ foo
▶ [lorem ipsum]
~> put [&a=lorem &b=ipsum &a..b=haha][a a..b]
▶ lorem
▶ haha
If both the indexee and index evaluate to multiple values, the results generated from the first indexee
appear first. Example:
~> put {[foo bar] [lorem ipsum]}[0 1]
▶ foo
▶ bar
▶ lorem
▶ ipsum
Compounding
A compound expression is formed by writing several expressions together with no space in between. A
compound expression evaluates to a string concatenation of all the constituent expressions. Examples:
~> put 'a'b"c" # compounding three string literals
▶ abc
~> v = value
~> put '$v is '$v # compounding one string literal with one string variable
▶ '$v is value'
When one or more of the constituent expressions evaluate to multiple values, the result is all possible
combinations:
~> li = [foo bar]
~> put {a b}-$li[0 1]
▶ a-foo
▶ a-bar
▶ b-foo
▶ b-bar
The order of the combinations is determined by first taking the first value in the leftmost expression
that generates multiple values, and then taking the second value, and so on.
Tilde expansion
An unquoted tilde at the beginning of a compound expression triggers tilde expansion. The remainder of
this expression must be a string. The part from the beginning of the string up to the first / (or the
end of the word if the string does not contain /), is taken as a user name; and they together evaluate to
the home directory of that user. If the user name is empty, the current user is assumed.
In the following example, the home directory of the current user is /home/xiaq, while that of the root
user is /root:
~> put ~
▶ /home/xiaq
~> put ~root
▶ /root
~> put ~/xxx
▶ /home/xiaq/xxx
~> put ~root/xxx
▶ /root/xxx
Note that tildes are not special when they appear elsewhere in a word:
~> put a~root
▶ a~root
If you need them to be, use a braced list:
~> put a{~root}
▶ a/root
Wildcard expansion
Wildcard patterns are expressions that contain wildcards. Wildcard patterns evaluate to all filenames
they match.
In examples in this section, we will assume that the current directory has the following structure:
.x.conf
a.cc
ax.conf
foo.cc
d/
|__ .x.conf
|__ ax.conf
|__ y.cc
.d2/
|__ .x.conf
|__ ax.conf
Elvish supports the following wildcards:
• ? matches one arbitrary character except /. For example, ?.cc matches a.cc;
• * matches any number of arbitrary characters except /. For example, *.cc matches a.cc and foo.cc;
• ** matches any number of arbitrary characters including /. For example, **.cc matches a.cc, foo.cc and
b/y.cc.
The following behaviors are default, although they can be altered by modifiers:
• When the entire wildcard pattern has no match, an error is thrown.
• None of the wildcards matches . at the beginning of filenames. For example:
• ?x.conf does not match .x.conf;
• d/*.conf does not match d/.x.conf;
• **.conf does not match d/.x.conf.
Wildcards can be modified using the same syntax as indexing. For instance, in *[match-hidden] the *
wildcard is modified with the match-hidden modifier. Multiple matchers can be chained like
*[set:abc][range:0-9]. In which case they are OR’ed together.
There are two kinds of modifiers:
Global modifiers apply to the whole pattern and can be placed after any wildcard:
• nomatch-ok tells Elvish not to throw an error when there is no match for the pattern. For instance, in
the example directory put bad* will be an error, but put bad*[nomatch-ok] does exactly nothing.
• but:xxx (where xxx is any filename) excludes the filename from the final result.
• type:xxx (where xxx is a recognized file type from the list below). Only one type modifier is allowed.
For example, to find the directories at any level below the current working directory: **[type:dir].
• dir will match if the path is a directory.
• regular will match if the path is a regular file.
Although global modifiers affect the entire wildcard pattern, you can add it after any wildcard, and the
effect is the same. For example, put */*[nomatch-ok].cpp and put *[nomatch-ok]/*.cpp do the same thing.
On the other hand, you must add it after a wildcard, instead of after the entire pattern: put
*/*.cpp[nomatch-ok] unfortunately does not do the correct thing. (This will probably be fixed.)
Local modifiers only apply to the wildcard it immediately follows:
• match-hidden tells the wildcard to match . at the beginning of filenames, e.g. *[match-hidden].conf
matches .x.conf and ax.conf.
Being a local modifier, it only applies to the wildcard it immediately follows. For instance, *[match-
hidden]/*.conf matches d/ax.conf and .d2/ax.conf, but not d/.x.conf or .d2/.x.conf.
• Character matchers restrict the characters to match:
• Character sets, like set:aeoiu;
• Character ranges like range:a-z (including z) or range:a~z (excluding z);
• Character classes: control, digit, graphic, letter, lower, mark, number, print, punct, space, symbol,
title, and upper. See the Is* functions here (https://godoc.org/unicode) for their definitions.
Note the following caveats:
• Local matchers chained together in separate modifiers are OR’ed. For instance, ?[set:aeoiu][digit]
matches all files with the chars aeoiu or containing a digit.
• Local matchers combined in the same modifier, such as ?[set:aeoiu digit], behave in a hard to explain
manner. Do not use this form as the behavior is likely to change in the future.
• Dots at the beginning of filenames always require an explicit match-hidden, even if the matcher
includes .. For example, ?[set:.a]x.conf does not match .x.conf; you have to ?[set:.a match-
hidden]x.conf.
• Likewise, you always need to use ** to match slashes, even if the matcher includes /. For example
*[set:abc/] is the same as *[set:abc].
Order of evaluation
An expression can use a combination of indexing, tilde expansion, wildcard and compounding. The order of
evaluation is as follows:
1. Literals, variable uses, output captures and exception captures and braced lists have the highest
precedence and are evaluated first.
2. Indexing has the next highest precedence and is then evaluated first.
3. Expression compounding then happens. Tildes and wildcards are kept unevaluated.
4. If the expression starts with a tilde, tilde expansion happens. If the tilde is followed by a
wildcard, an exception is raised.
5. If the expression contains any wildcard, wildcard expansion happens.
Here an example: in ~/$li[0 1]/* (where $li is a list [foo bar]), the expression is evaluated as follows:
1. The variable use $li evaluates to the list [foo bar].
2. The indexing expression $li[0] evaluates to two strings foo and bar.
3. Compounding the expression, the result is ~/foo/* and ~/bar/*.
4. Tilde expansion happens; assuming that the user’s home directory is /home/elf, the values are now
/home/elf/foo/* and /home/elf/bar/*.
5. Wildcard expansion happens, evaluating the expression to all the filenames within /home/elf/foo and
/home/elf/bar. If any directory is empty or nonexistent, an exception is thrown.
To force a particular order of evaluation, group expressions using a braced list.
Command forms
A command form is either an ordinary command, a special command or an legacy assignment form. All of
three different types have access to IO ports, which can be modified via redirections.
When Elvish parses a command form, it applies the following process to decide its type:
• If the command form contains an unquoted equal sign surrounded by inline whitespaces, it is an ordinary
assignment.
• If the first expression in the command form contains a single string literal, and the string value
matches one of the special commands, it is a special command.
• Otherwise, it is an ordinary command.
Ordinary command
An ordinary command form consists of a command head, and any number of arguments and options.
The first expression in an ordinary command is the command head. If the head is a single string literal,
it is subject to static resolution:
• If a variable with name head~ (where head is the value of the head) exists, the head will evaluate as
if it is $head~; i.e., a function invocation.
• If the head contains at least one slash, it is treated as an external command with the value as its
path relative to the current directory.
• Otherwise, the head is considered “unknown”, and the behavior is controlled by the unknown-command
pragma:
• If the unknown-command pragma is set to external (the default), the head is treated as the name of an
external command, to be searched in the $E:PATH during runtime.
• If the unknown-command pragma is set to disallow, such command heads trigger a compilation error.
If the head is not a single string literal, it is evaluated as a normal expression. The expression must
evaluate to one value, and the value must be one of the following:
• A callable value: a function or external command.
• A string containing at least one slash, in which case it is treated like an external command with the
string value as its path.
Examples of commands using static resolution:
~> put x # resolves to builtin function $put~
▶ x
~> f~ = { put 'this is f' }
~> f # resolves to user-defined function $f~
▶ 'this is f'
~> whoami # resolves to external command whoami
elf
Examples of commands using a dynamic callable head:
~> $put~ x
▶ x
~> (external whoami)
elf
~> { put 'this is a lambda' }
▶ 'this is a lambda'
Note: The last command resembles a code block in C-like languages in syntax, but is quite different under
the hood: it works by defining a function on the fly and calling it immediately.
Examples of commands using a dynamic string head:
~> x = /bin/whoami
~> $x
elf
~> x = whoami
~> $x # dynamic strings can only used when containing slash
Exception: bad value: command must be callable or string containing slash, but is string
[tty 10], line 1: $x
The definition of barewords is relaxed when parsing the head, and includes <, >, and *. These are all
names of numeric builtins:
~> < 3 5 # less-than
▶ $true
~> > 3 5 # greater-than
▶ $false
~> * 3 5 # multiplication
▶ 15
Arguments and options can be supplied to commands. Arguments are arbitrary words, while options have
exactly the same syntax as key-value pairs in map literals. They are separated by inline whitespaces and
may be intermixed:
~> echo &sep=, a b c # &seq=, is an option; a b c are arguments
a,b,c
~> echo a b &sep=, c # same, with the option mixed within arguments
a,b,c
Note: Since options have the same syntax as key-value pairs in maps, &key is equivalent to &key=$true:
~> fn f [&opt=$false]{ put $opt }
~> f &opt
▶ $true
Note: Since & is a metacharacter, it can be used to start an option immediately after the command name;
echo&sep=, a b is equivalent to echo &sep=, a b, just less readable. This might change in future.
Special command
A special command form has the same syntax with an ordinary command, but how it is executed depends on
the command head. See special commands.
Legacy assignment form
If any argument in a command form is an unquoted equal sign (=), the command form is treated as an
assignment form: the arguments to the left of =, including the head, are treated as lvalues, and the
arguments to the right of = are treated as values to assign to those lvalues.
If any lvalue refers to a variable that doesn’t yet exist, it is created first.
This is a legacy syntax that will be deprecated in future. Use the var special command to declare
variables, and the set special command set the values of variables.
Temporary assignment
You can prepend any command form with temporary assignments, which gives variables temporarily values
during the execution of that command.
In the following example, $x and $y are temporarily assigned 100 and 200:
~> x y = 1 2
~> x=100 y=200 + $x $y
▶ 300
~> echo $x $y
1 2
In contrary to normal assignments, there should be no whitespaces around the equal sign =. To have
multiple variables in the left-hand side, use braces:
~> x y = 1 2
~> fn f { put 100 200 }
~> {x,y}=(f) + $x $y
▶ 300
If you use a previously undefined variable in a temporary assignment, its value will become the empty
string after the command finishes. This behavior will likely change; don’t rely on it.
Since ordinary assignments are also command forms, they can also be prepended with temporary assignments:
~> x=1
~> x=100 y = (+ 133 $x)
~> put $x $y
▶ 1
▶ 233
Temporary assignments must all appear at the beginning of the command form. As soon as something that is
not a temporary assignments is parsed, Elvish no longer parses temporary assignments. For instance, in
x=1 echo x=1, the second x=1 is not a temporary assignment, but a bareword.
Note: Elvish’s behavior differs from bash (or zsh) in one important place. In bash, temporary
assignments to variables do not affect their direct appearance in the command:
bash-4.4$ x=1
bash-4.4$ x=100 echo $x
1
Note: Elvish currently supports using the syntax of temporary assignments for ordinary assignments, when
they are not followed by a command form; for example, a=x behaves like an ordinary assignment a = x.
This will likely go away; don’t rely on it.
IO ports
A command have access to a number of IO ports. Each IO port is identified by a number starting from 0,
and combines a traditional file object, which conveys bytes, and a value channel, which conveys values.
Elvish starts with 3 IO ports at the top level with special significance for commands:
• Port 0, known as standard input or stdin, and is used as the default input port by builtin commands.
• Port 1, known as standard output or stdout, and is used as the default output port by builtin commands.
• Port 2, known as standard error or stderr, is currently not special for builtin commands, but usually
has special significance for external commands.
Value channels are typically created by a pipeline, and used to pass values between commands in the same
pipeline. At the top level, they are initialized with special values:
• The value channel for port 0 never produces any values when read.
• The value channels for port 1 and 2 are special channels that forward the values written to them to
their file counterparts. Each value is put on a separate line, with a prefix controlled by $value-out-
indicator. The default prefix is ▶ followed by a space.
When running an external command, the file object from each port is used to create its file descriptor
table. Value channels only work inside the Elvish process, and are not accessible to external commands.
IO ports can be modified with redirections or by pipelines.
Redirection
A redirection modifies the IO ports a command operate with. There are several variants.
A file redirection opens a file and associates it with an IO port. The syntax consists of an optional
destination IO port (like 2), a redirection operator (like >) and a filename (like error.log):
• The destination IO port determines which IO port to modify. It can be given either as the number of
the IO port, or one of stdin, stdout and stderr, which are equivalent to 0, 1 and 2 respectively.
The destination IO port can be omitted, in which case it is inferred from the redirection operator.
When the destination IO port is given, it must precede the redirection operator directly, without
whitespaces in between; if there are whitespaces, otherwise Elvish will parse it as an argument
instead.
• The redirection operator determines the mode to open the file, and the destination IO port if it is not
explicitly specified.
• The filename names the file to open.
Possible redirection operators and their default FDs are:
• < for reading. The default IO port is 0 (stdin).
• > for writing. The default IO port is 1 (stdout).
• >> for appending. The default IO port is 1 (stdout).
• <> for reading and writing. The default IO port is 1 (stdout).
Examples:
~> echo haha > log
~> cat log
haha
~> cat < log
haha
~> ls --bad-arg 2> error
Exception: ls exited with 2
Traceback:
[interactive], line 1:
ls --bad-arg 2> error
~> cat error
/bin/ls: unrecognized option '--bad-arg'
Try '/bin/ls --help' for more information.
IO ports modified by file redirections do not support value channels. To be more exact:
• A file redirection using < sets the value channel to one that never produces any values.
• A file redirection using >, >> or <> sets the value channel to one that throws an exception when
written to.
Examples:
~> put foo > file # will truncate file if it exists
Exception: port has no value output
[tty 2], line 1: put foo > file
~> echo content > file
~> only-values < file
~> # previous command produced nothing
Redirections can also be used for closing or duplicating IO ports. Instead of writing a filename, use
&fd (where fd is a number, or any of stdin, stdout and stderr) for duplicating, or &- for closing. In
this case, the redirection operator only determines the default destination FD (and is totally irrevelant
if a destination IO port is specified). Examples:
~> date >&-
date: stdout: Bad file descriptor
Exception: date exited with 1
[tty 3], line 1: date >&-
~> put foo >&-
Exception: port has no value output
[tty 37], line 1: put foo >&-
If you have multiple related redirections, they are applied in the order they appear. For instance:
~> fn f { echo out; echo err >&2 } # echoes "out" on stdout, "err" on stderr
~> f >log 2>&1 # use file "log" for stdout, then use (changed) stdout for stderr
~> cat log
out
err
Redirections may appear anywhere in the command, except at the beginning, (this may be restricted in
future). It’s usually good style to write redirections at the end of command forms.
Special commands
Special commands obey the same syntax rules as normal commands, but have evaluation rules that are custom
to each command. Consider the following example:
~> or ?(echo x) ?(echo y) ?(echo z)
x
▶ $ok
In the example, the or command first evaluates its first argument, which has the value $ok (a truish
value) and the side effect of outputting x. Due to the custom evaluation rule of or, the rest of the
arguments are not evaluated.
If or were a normal command, the code above is still syntactically correct. However, Elvish would then
evaluate all its arguments, with the side effect of outputting x, y and z, before calling or.
Declaring variables: var {#var}
The var special command declares local variables. It takes any number of unqualified variable names
(without the leading $). The variables will start out having value $nil. Examples:
~> var a
~> put $a
▶ $nil
~> var foo bar
~> put $foo $bar
▶ $nil
▶ $nil
To set alternative initial values, add an unquoted = and the initial values. Examples:
~> var a b = foo bar
~> put $a $b
▶ foo
▶ bar
Similar to set, at most one of variables may be prefixed with @ to function as a rest variable.
When declaring a variable that already exists, the existing variable is shadowed. The shadowed variable
may still be accessed indirectly if it is referenced by a function. Example:
~> var x = old
~> fn f { put $x }
~> var x = new
~> put $x
▶ new
~> f
▶ old
Setting the value of variables or elements: set {#set}
The set special command sets the value of variables or elements.
It takes any number of lvalues (which refer to either variables or elements), followed by an equal sign
(=) and any number of expressions. The equal sign must appear unquoted, as a single argument.
An lvalue is one of the following:
• A variable name (without $).
• A variable name prefixed with @, for packing a variable number of values into a list and assigning to
the variable.
This variant is called a rest variable. There could be at most one rest variable.
Note: Schematically this is the reverse operation of exploding a variable when using it, which is why
they share the @ sign.
• A variable name followed by one or more indices in brackets ([]), for assigning to an element.
The number of values the expressions evaluate to and lvalues must be compatible. To be more exact:
• If there is no rest variable, the number of values and lvalues must match exactly.
• If there is a rest variable, the number of values should be at least the number of lvalues minus one.
All the variables to set must already exist; use the var special command to declare new variables.
Examples:
~> var x y z
~> set x = foo
~> put $x
▶ foo
~> x y = lorem ipsum
~> put $x $y
▶ lorem
▶ ipsum
~> set x @y z = a b
~> put $x $y $z
▶ a
▶ []
▶ b
~> set x @y z = a b c d
~> put $x $y $z
▶ a
▶ [b c]
▶ d
~> set y[0] = foo
~> put $y
▶ [foo c]
If the variable name contains any character that may not appear unquoted in variable use expressions, it
must be quoted even if it is otherwise a valid bareword:
~> var 'a/b'
~> set a/b = foo
compilation error: lvalue must be valid literal variable names
[tty 23], line 1: a/b = foo
~> set 'a/b' = foo
~> put $'a/b'
▶ foo
Lists and maps in Elvish are immutable. As a result, when assigning to the element of a variable that
contains a list or map, Elvish does not mutate the underlying list or map. Instead, Elvish creates a new
list or map with the mutation applied, and assigns it to the variable. Example:
~> var li = [foo bar]
~> var li2 = $li
~> set li[0] = lorem
~> put $li $li2
▶ [lorem bar]
▶ [foo bar]
Deleting variables or elements: del {#del}
The del special command can be used to delete variables or map elements. Operands should be specified
without a leading dollar sign, like the left-hand side of assignments.
Example of deleting variable:
~> x = 2
~> echo $x
2
~> del x
~> echo $x
Compilation error: variable $x not found
[tty], line 1: echo $x
If the variable name contains any character that cannot appear unquoted after $, it must be quoted, even
if it is otherwise a valid bareword:
~> 'a/b' = foo
~> del 'a/b'
Deleting a variable does not affect closures that have already captured it; it only removes the name.
Example:
~> x = value
~> fn f { put $x }
~> del x
~> f
▶ value
Example of deleting map element:
~> m = [&k=v &k2=v2]
~> del m[k2]
~> put $m
▶ [&k=v]
~> l = [[&k=v &k2=v2]]
~> del l[0][k2]
~> put $l
▶ [[&k=v]]
Logics: and, or, coalesce {#and-or-coalesce}
The and special command outputs the first booleanly false value the arguments evaluate to, or $true when
given no value. Examples:
~> and $true $false
▶ $false
~> and a b c
▶ c
~> and a $false
▶ $false
The or special command outputs the first booleanly true value the arguments evaluate to, or $false when
given no value. Examples:
~> or $true $false
▶ $true
~> or a b c
▶ a
~> or $false a b
▶ a
The coalesce special command outputs the first non-nil value the arguments evaluate to, or $nil when
given no value. Examples:
~> coalesce $nil a b
▶ a
~> coalesce $nil $nil
▶ $nil
~> coalesce $nil $nil a
▶ a
~> coalesce a b
▶ a
All three commands use short-circuit evaluation, and stop evaluating arguments as soon as it sees a value
satisfying the termination condition. For example, none of the following throws an exception:
~> and $false (fail foo)
▶ $false
~> or $true (fail foo)
▶ $true
~> coalesce a (fail foo)
▶ a
Condition: if {#if}
TODO: Document the syntax notation, and add more examples.
Syntax:
if <condition> {
<body>
} elif <condition> {
<body>
} else {
<else-body>
}
The if special command goes through the conditions one by one: as soon as one evaluates to a booleanly
true value, its corresponding body is executed. If none of conditions are booleanly true and an else
body is supplied, it is executed.
The condition part is an expression, not a command like in other shells. Example:
fn tell-language [fname]{
if (has-suffix $fname .go) {
echo $fname" is a Go file!"
} elif (has-suffix $fname .c) {
echo $fname" is a C file!"
} else {
echo $fname" is a mysterious file!"
}
}
The condition part must be syntactically a single expression, but it can evaluate to multiple values, in
which case they are and’ed:
if (put $true $false) {
echo "will not be executed"
}
If the expression evaluates to 0 values, it is considered true, consistent with how and works.
Tip: a combination of if and ?() gives you a semantics close to other shells:
if ?(test -d .git) {
# do something
}
However, for Elvish’s builtin predicates that output values instead of throw exceptions, the output
capture construct () should be used.
Conditional loop: while {#while}
Syntax:
while <condition> {
<body>
} else {
<else-body>
}
Execute the body as long as the condition evaluates to a booleanly true value.
The else body, if present, is executed if the body has never been executed (i.e. the condition evaluates
to a booleanly false value in the very beginning).
Iterative loop: for {#for}
Syntax:
for <var> <container> {
<body>
} else {
<body>
}
Iterate the container (e.g. a list). In each iteration, assign the variable to an element of the
container and execute the body.
The else body, if present, is executed if the body has never been executed (i.e. the iteration value has
no elements).
Exception control: try {#try}
(If you just want to capture the exception, you can use the more concise exception capture construct ?()
instead.)
Syntax:
try {
<try-block>
} except except-varname {
<except-block>
} else {
<else-block>
} finally {
<finally-block>
}
Only try and try-block are required. This control structure behaves as follows:
1. The try-block is always executed first.
2. If except is present and an exception occurs in try-block, it is caught and stored in except-varname,
and except-block is then executed. Example:
~> try { fail bad } except e { put $e }
▶ ?(fail bad)
Note that if except is not present, exceptions thrown from try are not caught: for instance, try {
fail bad } throws bad; it is equivalent to a plain fail bad.
Note that the word after except names a variable, not a matching condition. Exception matching is
not supported yet. For instance, you may want to only match exceptions that were created with fail
bad with except bad, but in fact this creates a variable $bad that contains whatever exception was
thrown.
3. If no exception occurs and else is present, else-block is executed. Example:
~> try { nop } else { echo well }
well
4. If finally-block is present, it is executed. Examples:
~> try { fail bad } finally { echo final }
final
Exception: bad
Traceback:
[tty], line 1:
try { fail bad } finally { echo final }
~> try { echo good } finally { echo final }
good
final
5. If the exception was not caught (i.e. except is not present), it is rethrown.
Exceptions thrown in blocks other than try-block are not caught. If an exception was thrown and either
except-block or finally-block throws another exception, the original exception is lost. Examples:
~> try { fail bad } except e { fail worse }
Exception: worse
Traceback:
[tty], line 1:
try { fail bad } except e { fail worse }
~> try { fail bad } except e { fail worse } finally { fail worst }
Exception: worst
Traceback:
[tty], line 1:
try { fail bad } except e { fail worse } finally { fail worst }
Function definition: fn {#fn}
Syntax:
fn <name> <lambda>
Define a function with a given name. The function behaves in the same way to the lambda used to define
it, except that it “captures” return. In other words, return will fall through lambdas not defined with
fn, and continues until it exits a function defined with fn:
~> fn f {
{ echo a; return }
echo b # will not execute
}
~> f
a
~> {
f
echo c # executed, because f "captures" the return
}
a
c
TODO: Find a better way to describe this. Hopefully the example is illustrative enough, though.
The lambda may refer to the function being defined. This makes it easy to define recursive functions:
~> fn f [n]{ if (== $n 0) { put 1 } else { * $n (f (- $n 1)) } }
~> f 3
▶ (float64 6)
Under the hood, fn defines a variable with the given name plus ~ (see variable suffix). Example:
~> fn f { echo hello from f }
~> var v = $f~
~> $v
hello from f
Language pragmas: pragma {#pragma}
The pragma special command can be used to set pragmas that affect the behavior of the Elvish language.
The syntax looks like:
pragma <name> = <value>
The name must appear literally. The value must also appear literally, unless otherwise specified.
Pragmas apply from the point it appears, to the end of the lexical scope it appears in, including
subscopes.
The following pragmas are available:
• The unknown-command pragma affects the resolution of command heads, and can take one of two values,
external (the default) and disallow. See ordinary command for details.
Note: pragma unknown-command = disallow enables a style where uses of external commands must be
explicitly via the e: namespace. You can also explicitly declare a set of external commands to use
directly, like the following:
pragma unknown-command = disallow
var ls = $e:ls~
var cat = $e:cat~
# ls and cat can be used directly;
# other external commands must be prefixed with e:
Pipeline
A pipeline is formed by joining one or more commands together with the pipe sign (|).
For each pair of adjacent commands a | b, the standard output of the left-hand command a (IO port 1) is
connected to the standard input (IO port 0) of the right-hand command b. Both the file and the value
channel are connected, even if one of them is not used.
Elvish may have internal buffering for both the file and the value channel, so a may be able to write
bytes or values even if b is not reading them. The exact buffer size is not specified.
Command redirections are applied before the connection happens. For instance, the following writes foo
to a.txt instead of the output:
~> echo foo > a.txt | cat
~> cat a.txt
foo
A pipeline runs all of its command in parallel, and terminates when all of the commands have terminated.
Pipeline exception
If one or more command in a pipeline throws an exception, the other commands will continue to execute as
normal. After all commands finish execution, an exception is thrown, the value of which depends on the
number of commands that have thrown an exception:
• If only one command has thrown an exception, that exception is rethrown.
• If more than one commands have thrown exceptions, a “composite exception”, containing information all
exceptions involved, is thrown.
If a command threw an exception because it tried to write output when the next command has terminated,
that exception is suppressed when it is propagated to the pipeline.
For example, the put command throws an exception when trying to write to a closed pipe, so the following
loop will terminate with an exception:
~> while $true { put foo } > &-
Exception: port has no value output
[tty 9], line 1: while $true { put foo } > &-
However, if it appears in a pipeline before nop, the entire pipeline will not throw an exception:
~> while $true { put foo } | nop
~> # no exception thrown from previous line
Internally, the put foo command still threw an exception, but since that exception was trying to write to
output when nop already terminated, that exception was suppressed by the pipeline.
This can be more clearly observed with the following code:
~> var r = $false
~> { while $true { put foo }; set r = $true } | nop
~> put $r
▶ $false
The same mechanism works for builtin commands that write to the byte output:
~> var r = $false
~> { while $true { echo foo }; set r = $true } | nop
~> put $r
▶ $false
On UNIX, if an external command was terminated by SIGPIPE, and Elvish detected that it terminated after
the next command in the pipeline, such exceptions will also be suppressed by the pipeline. For example,
the following pipeline does not throw an exception, despite the yes command being killed by SIGPIPE:
~> yes | head -n1
y
Background pipeline
Adding an ampersand & to the end of a pipeline will cause it to be executed in the background. In this
case, the rest of the code chunk will continue to execute without waiting for the pipeline to finish.
Exceptions thrown from the background pipeline do not affect the code chunk that contains it.
When a background pipeline finishes, a message is printed to the terminal if the shell is interactive.
Code Chunk
A code chunk is formed by joining zero or more pipelines together, separating them with either newlines
or semicolons.
Pipelines in a code chunk are executed in sequence. If any pipeline throws an exception, the execution
of the whole code chunk stops, propagating that exception.
Exception and Flow Commands
Exceptions have similar semantics to those in Python or Java. They can be thrown with the fail command
and caught with either exception capture ?() or the try special command.
If an external command exits with a non-zero status, Elvish treats that as an exception.
Flow commands – break, continue and return – are ordinary builtin commands that raise special “flow
control” exceptions. The for, while, and peach commands capture break and continue, while fn modifies
its closure to capture return.
One interesting implication is that since flow commands are just ordinary commands you can build
functions on top of them. For instance, this function breaks randomly:
fn random-break {
if eq (randint 2) 0 {
break
}
}
The function random-break can then be used in for-loops and while-loops.
Note that the return flow control exception is only captured by functions defined with fn. It falls
through ordinary lambdas:
fn f {
{
# returns f, falling through the innermost lambda
return
}
}
Namespaces and Modules
Like other modern programming languages, but unlike traditional shells, Elvish has a namespace mechanism
for preventing name collisions.
Syntax
Prepend namespace: to command names and variable names to specify the namespace. The following code
e:echo $E:PATH
uses the echo command from the e: namespace and the PATH variable from the E: namespace. The colon is
considered part of the namespace name.
Namespaces may be nested; for example, calling edit:location:start first finds the edit: namespace, and
then the location: namespace inside it, and then call the start function within the nested namespace.
Special namespaces
The following namespaces have special meanings to the language:
• local: and up: refer to lexical scopes, and have been documented above.
• e: refers to externals. For instance, e:ls refers to the external command ls.
Most of the time you can rely on static resolution rules of ordinary commands and do not need to use
this explicitly, unless a function defined by you (or an Elvish builtin) shadows an external command.
• E: refers to environment variables. For instance, $E:USER is the environment variable USER.
This is always needed, because unlike command resolution, variable resolution does not fall back onto
environment variables.
• builtin: refers to builtin functions and variables.
You don’t need to use this explicitly unless you have defined names that shadows builtin counterparts.
Modules
Apart from the special namespaces, the most common usage of namespaces is to reference modules, reusable
pieces of code that are either shipped with Elvish itself or defined by the user.
Importing modules with use
Modules are imported using the use special command. It requires a module spec and allows a namespace
alias:
use $spec $alias?
The module spec and the alias must both be a simple string literal. Compound strings such as 'a'/b are
not allowed.
The module spec specifies which module to import. The alias, if given, specifies the namespace to import
the module under. By default, the namespace is derived from the module spec by taking the part after the
last slash.
Module specs fall into three categories that are resolved in the following order:
1. Relative: These are relative to the file containing the use command.
2. User defined: These match a user defined module in a module search directory.
3. Pre-defined: These match the name of a pre-defined module, such as math or str.
If a module spec doesn’t match any of the above a “no such module” exception is raised.
Examples:
use str # imports the "str" module as "str:"
use a/b/c # imports the "a/b/c" module as "c:"
use a/b/c foo # imports the "a/b/c" module as "foo:"
Pre-defined modules
Elvish’s standard library provides the following pre-defined modules that can be imported by the use
command:
• edit is only available in interactive mode. As a special case it does not need importing via use, but
this may change in the future.
• epm
• math
• path
• platform
• re
• readline-binding
• store
• str
• unix is only available on UNIX-like platforms (see $platform:is-unix)
User-defined modules
You can define your own modules in Elvish by putting them under one of the module search directories and
giving them a .elv extension (but see relative imports for an alternative). For instance, to define a
module named a, you can put the following in ~/.config/elvish/lib/a.elv (on Windows, replace ~/.config
with ~\AppData\Roaming):
~> cat ~/.config/elvish/lib/a.elv
echo "mod a loading"
fn f {
echo "f from mod a"
}
This module can now be imported by use a:
~> use a
mod a loading
~> a:f
f from mod a
Similarly, a module defined in ~/.config/elvish/lib/x/y/z.elv can be imported by use x/y/z:
~> cat .config/elvish/lib/x/y/z.elv
fn f {
echo "f from x/y/z"
}
~> use x/y/z
~> z:f
f from x/y/z
In general, a module defined in namespace will be the same as the file name (without the .elv extension).
There is experimental support for importing modules written in Go. See the project repository
(https://github.com/elves/elvish) for details.
Circular dependencies
Circular dependencies are allowed but has an important restriction. If a module a contains use b and
module b contains use a, the top-level statements in module b will only be able to access variables that
are defined before the use b in module a; other variables will be $nil.
On the other hand, functions in module b will have access to bindings in module a after it is fully
evaluated.
Examples:
~> cat a.elv
var before = before
use ./b
var after = after
~> cat b.elv
use ./a
put $a:before $a:after
fn f { put $a:before $a:after }
~> use ./a
▶ before
▶ $nil
~> use ./b
~> b:f
▶ before
▶ after
Note that this behavior can be different depending on whether the REPL imports a or b first. In the
previous example, if the REPL imports b first, it will have access to all the variables in a:
~> use ./b
▶ before
▶ after
Note: Elvish caches imported modules. If you are trying this locally, run a fresh Elvish instance with
exec first.
When you do need to have circular dependencies, it is best to avoid using variables from the modules in
top-level statements, and only use them in functions.
Relative imports
The module spec may begin with ./ or ../ to introduce a relative import. When use is invoked from a file
this will import the file relative to the location of the file. When use is invoked from an interactive
prompt, this will import the file relative to the current working directory.
Scoping of imports
Namespace imports are lexically scoped. For instance, if you use a module within an inner scope, it is
not available outside that scope:
{
use some-mod
some-mod:some-func
}
some-mod:some-func # not valid
The imported modules themselves are also evaluated in a separate scope. That means that functions and
variables defined in the module does not pollute the default namespace, and vice versa. For instance, if
you define ls as a wrapper function in rc.elv:
fn ls [@a]{
e:ls --color=auto $@a
}
That definition is not visible in module files: ls will still refer to the external command ls, unless
you shadow it in the very same module.
Re-importing
Modules are cached after one import. Subsequent imports do not re-execute the module; they only serve
the bring it into the current scope. Moreover, the cache is keyed by the path of the module, not the
name under which it is imported. For instance, if you have the following in
~/.config/elvish/lib/a/b.elv:
echo importing
The following code only prints one importing:
{ use a/b }
use a/b # only brings mod into the lexical scope
As does the following:
use a/b
use a/b alias