Provided by: tcl8.5-doc_8.5.19-4_all 
NAME
subst - Perform backslash, command, and variable substitutions
SYNOPSIS
subst ?-nobackslashes? ?-nocommands? ?-novariables? string ________________________________________________________________________________________________________________
DESCRIPTION
This command performs variable substitutions, command substitutions, and backslash substitutions on its
string argument and returns the fully-substituted result. The substitutions are performed in exactly the
same way as for Tcl commands. As a result, the string argument is actually substituted twice, once by
the Tcl parser in the usual fashion for Tcl commands, and again by the subst command.
If any of the -nobackslashes, -nocommands, or -novariables are specified, then the corresponding
substitutions are not performed. For example, if -nocommands is specified, command substitution is not
performed: open and close brackets are treated as ordinary characters with no special interpretation.
Note that the substitution of one kind can include substitution of other kinds. For example, even when
the -novariables option is specified, command substitution is performed without restriction. This means
that any variable substitution necessary to complete the command substitution will still take place.
Likewise, any command substitution necessary to complete a variable substitution will take place, even
when -nocommands is specified. See the EXAMPLES below.
If an error occurs during substitution, then subst will return that error. If a break exception occurs
during command or variable substitution, the result of the whole substitution will be the string (as
substituted) up to the start of the substitution that raised the exception. If a continue exception
occurs during the evaluation of a command or variable substitution, an empty string will be substituted
for that entire command or variable substitution (as long as it is well-formed Tcl.) If a return
exception occurs, or any other return code is returned during command or variable substitution, then the
returned value is substituted for that substitution. See the EXAMPLES below. In this way, all
exceptional return codes are “caught” by subst. The subst command itself will either return an error, or
will complete successfully.
EXAMPLES
When it performs its substitutions, subst does not give any special treatment to double quotes or curly
braces (except within command substitutions) so the script
set a 44
subst {xyz {$a}}
returns “xyz {44}”, not “xyz {$a}” and the script
set a "p\} q \{r"
subst {xyz {$a}}
returns “xyz {p} q {r}”, not “xyz {p\} q \{r}”.
When command substitution is performed, it includes any variable substitution necessary to evaluate the
script.
set a 44
subst -novariables {$a [format $a]}
returns “$a 44”, not “$a $a”. Similarly, when variable substitution is performed, it includes any
command substitution necessary to retrieve the value of the variable.
proc b {} {return c}
array set a {c c [b] tricky}
subst -nocommands {[b] $a([b])}
returns “[b] c”, not “[b] tricky”.
The continue and break exceptions allow command substitutions to prevent substitution of the rest of the
command substitution and the rest of string respectively, giving script authors more options when
processing text using subst. For example, the script
subst {abc,[break],def}
returns “abc,”, not “abc,,def” and the script
subst {abc,[continue;expr {1+2}],def}
returns “abc,,def”, not “abc,3,def”.
Other exceptional return codes substitute the returned value
subst {abc,[return foo;expr {1+2}],def}
returns “abc,foo,def”, not “abc,3,def” and
subst {abc,[return -code 10 foo;expr {1+2}],def}
also returns “abc,foo,def”, not “abc,3,def”.
SEE ALSO
Tcl(3tcl), eval(3tcl), break(3tcl), continue(3tcl)
KEYWORDS
backslash substitution, command substitution, variable substitution