Provided by: vnlog_1.40-1_all 
NAME
vnl-filter - filters vnlogs to select particular rows, fields
SYNOPSIS
$ cat run.vnl
# time x y z temperature
3 1 2.3 4.8 30
4 1.1 2.2 4.7 31
6 1 2.0 4.0 35
7 1 1.6 3.1 42
$ <run.vnl vnl-filter -p x,y,z | vnl-align
# x y z
1 2.3 4.8
1.1 2.2 4.7
1 2.0 4.0
1 1.6 3.1
$ <run.vnl vnl-filter -p i=NR,time,'dist=sqrt(x*x + y*y + z*z)' | vnl-align
# i time dist
1 3 5.41572
2 4 5.30471
3 6 4.58258
4 7 3.62905
$ <run.vnl vnl-filter 'temperature >= 35' | vnl-align
# time x y z temperature
6 1 2.0 4.0 35
7 1 1.6 3.1 42
$ <run.vnl vnl-filter --eval '{s += temperature} END { print "mean temp: " s/NR}'
mean temp: 34.5
$ <run.vnl vnl-filter -p x,y | feedgnuplot --terminal 'dumb 80,30' --unset grid --domain --lines --exit
2.3 +---------------------------------------------------------------------+
| + + *************** + |
| ************** |
| *******|
2.2 |-+ ************|
| ******** |
| ******** |
2.1 |-+ ********* +-|
| ******** |
| ******** |
| **** |
2 |-+ * +-|
| * |
| * |
| * |
1.9 |-+ * +-|
| * |
| * |
| * |
1.8 |-+ * +-|
| * |
| * |
1.7 |-+ * +-|
| * |
| * |
| * + + + + |
1.6 +---------------------------------------------------------------------+
0.98 1 1.02 1.04 1.06 1.08 1.1
DESCRIPTION
This tool is largely a frontend for awk to operate on vnlog files. Vnlog is both an input and an output.
This tool makes it very simple to select specific rows and columns for output and to manipulate the data
in various ways.
This is a UNIX-style tool, so the input/output of this tool is strictly STDIN/STDOUT. Furthermore, in its
usual form this tool is a filter, so the format of the output is exactly the same as the format of the
input. The exception to this is when using "--eval", in which the output is dependent on whatever
expression we're evaluating.
This tool is convenient to process both stored data or live data; in the latter case, it's very useful to
pipe the streaming output to "feedgnuplot --stream" to get a realtime visualization of the incoming data.
This tool reads enough of the input file to get a legend, at which point it constructs an awk program to
do the main work, and execs to awk (it's possible to use perl as well, but this isn't as fast).
Input/output data format
The input/output data is vnlog: a plain-text table of values. Any lines beginning with "#" are treated as
comments, and are passed through. The first line that begins with "#" but not "##" or "#!" is a legend
line. After the "#", follow whitespace-separated field names. Each subsequent line is whitespace-
separated values matching this legend. For instance, this is a valid vnlog file:
#!/usr/bin/something
## more comments
# x y z
-0.016107 0.004362 0.005369
-0.017449 0.006711 0.006711
-0.018456 0.014093 0.006711
-0.017449 0.018791 0.006376
"vnl-filter" uses this format for both the input and the output. The comments are preserved, but the
legend is updated to reflect the fields in the output file.
A string "-" is used to indicate an undefined value, so this is also a valid vnlog file:
# x y z
1 2 3
4 - 6
- - 7
Filtering
To select specific columns, pass their names to the "-p" option (short for "--print" or "--pick", which
are synonyms). In its simplest form, to grab only columns "x" and "y", do
vnl-filter -p x,y
See the detailed description of "-p" below for more detail.
To select specific rows, we use matches expressions. Anything on the "vnl-filter" commandline and not
attached to any "--xxx" option is such an expression. For instance
vnl-filter 'size > 10'
would select only those rows whose "size" column contains a value > 10. See the detailed description of
matches expressions below for more detail.
Context lines
"vnl-filter" supports the context output options ("-A", "-B" and "-C") exactly like the "grep" tool. I.e
to print out all rows whose "size" column contains a value > 10 but also include the 3 rows immediately
before and after such matching rows, do this:
vnl-filter -C3 'size > 10'
"-B" reports the rows before matching ones and "-A" the rows after matching ones. "-C" reports both. Note
that this applies only to matches expressions: records skipped because they fail "--has" or "--skipempty"
are not included in contextual output.
Backend choice
By default, the parsing of arguments and the legend happens in perl, which then constructs a simple awk
script, and invokes "mawk" to actually read the data and to process it. This is done because awk is
lighter weight and runs faster, which is important because our data sets could be quite large. We default
to "mawk" specifically, since this is a simpler implementation than "gawk", and runs much faster. If for
whatever reason we want to do everything with perl, this can be requested with the "--perl" option.
Special functions
For convenience we support several special functions in any expression passed on to awk or perl (named
expressions, matches expressions, "--eval" strings). These generally maintain some internal state, and
vnl-filter makes sure that this state is consistent. Note that these are evaluated after "--skipcomments"
and "--has". So any record skipped because of a "--has" expression, for instance, will not be considered
in prev(), diff() and so on.
• rel(x) returns value of "x" relative to the first value of "x". For instance we might want to see the
time or position relative to the start, not relative to some absolute beginning. Example:
$ cat tst.vnl
# time x
100 200
101 212
102 209
$ <tst.vnl vnl-filter -p 't=rel(time),x=rel(x)
# t x
0 0
1 12
2 9
• diff(x) returns the difference between the current value of "x" and the previous value of "x". The
first row will always be "-". Example:
$ <tst.vnl vnl-filter -p x,'d1=diff(x),d2=diff(diff(x))' | vnl-align
# x d1 d2
1 - -
8 7 7
27 19 12
64 37 18
125 61 24
• sum(x) returns the cumulative sum of "x". As diff(x) can be thought of as a derivative, sum(x) can be
thought of as an integral. So "diff(sum(x))" would return the same value as "x" (except for the first
row; diff() always returns "-" for the first row).
Example:
$ <tst.vnl vnl-filter -p 'x,s=sum(x),ds=diff(sum(x))' | vnl-align
# x s ds
1 1 -
8 9 8
27 36 27
64 100 64
125 225 125
• prev(x) returns the previous value of "x". One could construct sum() and rel() using this, if they
weren't already available.
• latestdefined(x) returns the most recent value of "x" that isn't "-". If "x" isn't "-", this simply
returns "x".
ARGUMENTS
Matches expressions
Anything on the commandline not attached to any "--xxx" option is a matches expression. These are used to
select particular records (rows) in a data file. For each row, we evaluate all the expressions. If all
the expressions evaluate to true, that row is output. This expression is passed directly to the awk (or
perl) backend.
Example: to select all rows that have valid data in column "a" or column "b" or column "c" you can
vnl-filter 'a != "-" || b != "-" || c != "-"'
or
vnl-filter --perl 'defined a || defined b || defined c'
As with the named expressions given to "-p" (described above), these are passed directly to awk, so
anything that can be done with awk is supported here.
-p|--print|--pick expr
These option provide the mechanism to select specific columns for output. For instance to pull out
columns called "lat", "lon", and any column whose name contains the string "feature_", do
vnl-filter -p lat,lon,'feature_.*'
or, equivalently
vnl-filter --print lat --print lon --print 'feature_.*'
We look for exact column name matches first, and if none are found, we try a regex. If there was no
column called exactly "feature_", then the above would be equivalent to
vnl-filter -p lat,lon,feature_
This mechanism is much more powerful than just selecting columns. First off, we can rename chosen fields:
vnl-filter -p w=feature_width
would pick the "feature_width" field, but the resulting column in the output would be named "w". When
renaming a column in this way regexen are not supported, and exact field names must be given. But the
string to the right of the "=" is passed on directly to awk (after replacing field names with column
indices), so any awk expression can be used here. For instance to compute the length of a vector in
separate columns "x", "y", and "z" you can do:
vnl-filter -p 'l=sqrt(x*x + y*y + z*z)'
A single column called "l" would be produced.
We can also exclude columns by preceding their name with "!". This works like you expect. Rules:
• The pick/exclude directives are processed in order given to produce the output picked-column list
• If the first "-p" item is an exclusion, we implicitly pick all the columns prior to processing the
"-p".
• The exclusion expressions match the output column names, not the input names.
• We match the exact column names first. If that fails, we match as a regex
Example. To grab all the columns except the temperature(s) do this:
vnl-filter -p !temperature
To grab all the columns that describe something about a robot (columns whose names have the string
"robot_" in them), but not its temperature (i.e. not "robot_temperature"), do this:
vnl-filter -p robot_,!temperature
--has a,b,c,...
Used to select records (rows) that have a non-empty value in a particular field (column). A null value in
a column is designated with a single "-". If we want to select only records that have a value in the "x"
column, we pass "--has x". To select records that have data for all of a given set of columns, the
"--has" option can be repeated, or these multiple columns can be given in a whitespace-less comma-
separated list. For instance if we want only records that have data in both columns "x" and "y" we can
pass in "--has x,y" or "--has x --has y". If we want to combine multiple columns in an or (select rows
that have data in any of a given set of columns), use a matches expression, as documented below.
If we want to select a column and pick only rows that have a value in this column, a shorthand syntax
exists:
vnl-filter --has col -p col
is equivalent to
vnl-filter -p +col
Note that just like the column specifications in "-p" the columns given to "--has" must match exactly or
as a regex. In either case, a unique matching column must be found.
-l|list-columns
Instead of doing any processing, parse the input to get the available columns, print those out, and exit
-A N|--after-context N
Output N lines following each matches expression, even those lines that do not themselves match. This
works just like the "grep" options of the same name. See "Context lines"
-B N|--before-context N
Output N lines preceding each matches expression, even those lines that do not themselves match. This
works just like the "grep" options of the same name. See "Context lines"
-C N|--context N
Output N lines preceding and following each matches expression, even those lines that do not themselves
match. This works just like the "grep" options of the same name. See "Context lines"
--eval expr
Instead of printing out all matching records and picked columns, just run the given chunk of awk (or
perl). In this mode of operation, "vnl-filter" acts just like a glorified awk, that allows fields to be
accessed by name instead of by number, as it would be in raw awk.
Since the expression may print anything or nothing at all, the output in this mode is not necessarily
itself a valid vnlog stream. And no column-selecting arguments should be given, since they make no sense
in this mode.
In awk the expr is a full set of pattern/action statements. So to print the sum of columns "a" and "b" in
each row, and at the end, print the sum of all values in the "a" column
vnl-filter --eval '{print a+b; suma += a} END {print suma}'
In perl the arbitrary expression fits in like this:
sub evalexpr
{
eval expression; # evaluate the arbitrary expression
}
while(<>) # read each line
{
chomp;
next unless matches; # skip non-matching lines
evalexpr();
}
--function|--sub
Evaluates the given expression as a function that can be used in other expressions. This is most useful
when you want to print something that can't trivially be written as a simple expression. For instance:
$ cat tst.vnl
# s
1-2
3-4
5-6
$ < tst.vnl
vnl-filter --function 'before(x) { sub("-.*","",x); return x }' \
--function 'after(x) { sub(".*-","",x); return x }' \
-p 'b=before(s),a=after(s)'
# b a
1 2
3 4
5 6
See the CAVEATS section below if you're doing something sufficiently-complicated where you need this.
--function-abs|--sub-abs
Convenience option to add an absolute-value abs() function. This is only useful for awk programs (the
default, no "--perl" given) since perl already provides abs() by default.
--begin|--BEGIN
Evaluates the given expression in the BEGIN {} block of the generated awk (or perl) program.
--end|--END
Evaluates the given expression in the END {} block of the generated awk (or perl) program.
--[no]skipempty
Do [not] skip records where all fields are blank. By default we do skip all empty records; to include
them, pass "--noskipempty"
--skipcomments
Don't output non-legend comments
--perl
By default all procesing is performed by "mawk", but if for whatever reason we want perl instead, pass
"--perl". Both modes work, but "mawk" is noticeably faster. "--perl" could be useful because it is more
powerful, which could be important since a number of things pass commandline strings directly to the
underlying language (named expressions, matches expressions, "--eval" strings). Note that while
variables in perl use sigils, column references should not use sigils. To print the sum of all values in
column "a" you'd do this in awk
vnl-filter --eval '{suma += a} END {print suma}'
and this in perl
vnl-filter --perl --eval '{$suma += a} END {say $suma}'
The perl strings are evaluated without "use strict" or "use warnings" so I didn't have to declare $suma
in the example.
With "--perl", empty strings ("-" in the vnlog file) are converted to "undef".
--dumpexprs
Used for debugging. This spits out all the final awk (or perl) program we run for the given commandline
options and given input. This is the final program, with the column references resolved to numeric
indices, so one can figure out what went wrong.
--unbuffered
Flushes each line after each print. This makes sure each line is output as soon as it is available, which
is crucial for realtime output and streaming plots.
--stream
Synonym for "--unbuffered"
CAVEATS
This tool is very lax in its input validation (on purpose). As a result, columns with names like %CPU and
"TIME+" do work (i.e. you can more or less feed in output from "top -b"). The downside is that shooting
yourself in the foot is possible. This tradeoff is currently tuned to be very permissive, which works
well for my use cases. I'd be interested in hearing other people's experiences. Potential
pitfalls/unexpected behaviors:
• All column names are replaced in all eval strings without regard to context. The earlier example that
reports the sum of values in a column: vnl-filter --eval '{suma += a} END {print suma}' will work
fine if we do have a column named "a" and do not have a column named "suma". But will not do the
right thing if any of those are violated. For instance, if a column "a" doesn't exist, then "awk"
would see "suma += a" instead of something like "suma += $5". "a" would be an uninitialized variable,
which evaluates to 0, so the full "vnl-filter" command would not fail, but would print 0 instead.
It's the user's responsibility to make sure we're talking about the right columns. The focus here was
one-liners so hopefully nobody has so many columns, they can't keep track of all of them in their
head. I don't see any way to resolve this without seriously impacting the scope of the tool, so I'm
leaving this alone.
• It is natural to use vnlog as a database. You can run queries with something like
vnl-filter 'key == 5'
This works. But unlike a real database this is clearly a linear lookup. With large data files, this
would be significantly slower than the logarithmic searches provided by a real database. The meaning
of "large" and "significant" varies, and you should test it. In my experience vnlog "databases" scale
surprisingly well. But at some point, importing your data to something like sqlite is well worth it.
• When substituting column names I match either a word-nonword transition ("\b") or a whitespace-
nonword transition. The word boundaries is what would be used 99% of the time. But the keys may have
special characters in them, which don't work with "\b". This means that whitespace becomes important:
"1+%CPU" will not be parsed as expected, which is correct since "+%CPU" is also a valid field name.
But "1+ %CPU" will be parsed correctly, so if you have weird field names, put the whitespace into
your expressions. It'll make them more readable anyway.
• Strings passed to "-p" are split on "," except if the "," is inside balanced "()". This makes it
possible to say things like vnl-filter --function 'f(a,b) { ... }' -p 'c=f(a,b)'. This is probably
the right behavior, although some questionable looking field names become potentially impossible:
"f(a" and "b)" could otherwise be legal field names, but you're probably asking for trouble if you do
that.
• Currently there're two modes: a pick/print mode and an "--eval" mode. Then there's also "--function",
which adds bits of "--eval" to the pick/print mode, but it feels maybe insufficient. I don't yet have
strong feelings about what this should become. Comments welcome
REPOSITORY
https://github.com/dkogan/vnlog/
AUTHOR
Dima Kogan "<dima@secretsauce.net>"
LICENSE AND COPYRIGHT
Copyright 2016-2017 California Institute of Technology
Copyright 2017-2019 Dima Kogan "<dima@secretsauce.net>"
This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser
General Public License as published by the Free Software Foundation; either version 2.1 of the License,
or (at your option) any later version.
2025-02-15 VNL-FILTER(1)