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.\" ========================================================================
.\"
.IX Title "VNL-FILTER 1"
.TH VNL-FILTER 1 "2020-12-09" "" "vnlog"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
vnl\-filter \- filters vnlogs to select particular rows, fields
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& $ 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,\*(Aqdist=sqrt(x*x + y*y + z*z)\*(Aq | 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 \*(Aqtemperature >= 35\*(Aq | 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 \*(Aq{s += temperature} END { print "mean temp: " s/NR}\*(Aq
\&
\& mean temp: 34.5
\&
\&
\& $ <run.vnl vnl\-filter \-p x,y | feedgnuplot \-\-terminal \*(Aqdumb 80,30\*(Aq \-\-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
.Ve
.SH "DESCRIPTION"
.IX Header "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.
.PP
This is a UNIX-style tool, so the input/output of this tool is strictly
\&\s-1STDIN/STDOUT.\s0 Furthermore, in its usual form this tool is a filter, so the
format of the output is \fIexactly\fR the same as the format of the input. The
exception to this is when using \f(CW\*(C`\-\-eval\*(C'\fR, in which the output is dependent on
whatever expression we're evaluating.
.PP
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 \f(CW\*(C`feedgnuplot \-\-stream\*(C'\fR
to get a realtime visualization of the incoming data.
.PP
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).
.SS "Input/output data format"
.IX Subsection "Input/output data format"
The input/output data is vnlog: a plain-text table of values. Any lines
beginning with \f(CW\*(C`#\*(C'\fR are treated as comments, and are passed through. The first
line that begins with \f(CW\*(C`#\*(C'\fR but not \f(CW\*(C`##\*(C'\fR or \f(CW\*(C`#!\*(C'\fR is a \fIlegend\fR line. After the
\&\f(CW\*(C`#\*(C'\fR, follow whitespace-separated field names. Each subsequent line is
whitespace-separated values matching this legend. For instance, this is a valid
vnlog file:
.PP
.Vb 7
\& #!/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
.Ve
.PP
\&\f(CW\*(C`vnl\-filter\*(C'\fR 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.
.PP
A string \f(CW\*(C`\-\*(C'\fR is used to indicate an undefined value, so this is also a valid
vnlog file:
.PP
.Vb 4
\& # x y z
\& 1 2 3
\& 4 \- 6
\& \- \- 7
.Ve
.SS "Filtering"
.IX Subsection "Filtering"
To select specific \fIcolumns\fR, pass their names to the \f(CW\*(C`\-p\*(C'\fR option (short for
\&\f(CW\*(C`\-\-print\*(C'\fR or \f(CW\*(C`\-\-pick\*(C'\fR, which are synonyms). In its simplest form, to grab only
columns \f(CW\*(C`x\*(C'\fR and \f(CW\*(C`y\*(C'\fR, do
.PP
.Vb 1
\& vnl\-filter \-p x,y
.Ve
.PP
See the detailed description of \f(CW\*(C`\-p\*(C'\fR below for more detail.
.PP
To select specific \fIrows\fR, we use \fImatches\fR expressions. Anything on the
\&\f(CW\*(C`vnl\-filter\*(C'\fR commandline and not attached to any \f(CW\*(C`\-\-xxx\*(C'\fR option is such
an expression. For instance
.PP
.Vb 1
\& vnl\-filter \*(Aqsize > 10\*(Aq
.Ve
.PP
would select only those rows whose \f(CW\*(C`size\*(C'\fR column contains a value > 10. See
the detailed description of matches expressions below for more detail.
.SS "Context lines"
.IX Subsection "Context lines"
\&\f(CW\*(C`vnl\-filter\*(C'\fR supports the context output options (\f(CW\*(C`\-A\*(C'\fR, \f(CW\*(C`\-B\*(C'\fR and \f(CW\*(C`\-C\*(C'\fR)
exactly like the \f(CW\*(C`grep\*(C'\fR tool. I.e to print out all rows whose \f(CW\*(C`size\*(C'\fR column
contains a value > 10 \fIbut also\fR include the 3 rows immediately before
\&\fIand\fR after such matching rows, do this:
.PP
.Vb 1
\& vnl\-filter \-C3 \*(Aqsize > 10\*(Aq
.Ve
.PP
\&\f(CW\*(C`\-B\*(C'\fR reports the rows \fIbefore\fR matching ones and \f(CW\*(C`\-A\*(C'\fR the rows \fIafter\fR
matching ones. \f(CW\*(C`\-C\*(C'\fR reports both. Note that this applies \fIonly\fR to \fImatches\fR
expressions: records skipped because they fail \f(CW\*(C`\-\-has\*(C'\fR or \f(CW\*(C`\-\-skipempty\*(C'\fR are
\&\fInot\fR included in contextual output.
.SS "Backend choice"
.IX Subsection "Backend choice"
By default, the parsing of arguments and the legend happens in perl, which then
constructs a simple awk script, and invokes \f(CW\*(C`mawk\*(C'\fR 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
\&\f(CW\*(C`mawk\*(C'\fR specifically, since this is a simpler implementation than \f(CW\*(C`gawk\*(C'\fR, and
runs much faster. If for whatever reason we want to do everything with perl,
this can be requested with the \f(CW\*(C`\-\-perl\*(C'\fR option.
.SS "Special functions"
.IX Subsection "Special functions"
For convenience we support several special functions in any expression passed on
to awk or perl (named expressions, matches expressions, \f(CW\*(C`\-\-eval\*(C'\fR strings).
These generally maintain some internal state, and vnl-filter makes sure that
this state is consistent. Note that these are evaluated \fIafter\fR
\&\f(CW\*(C`\-\-skipcomments\*(C'\fR and \f(CW\*(C`\-\-has\*(C'\fR. So any record skipped because of a \f(CW\*(C`\-\-has\*(C'\fR
expression, for instance, will \fInot\fR be considered in \f(CW\*(C`prev()\*(C'\fR, \f(CW\*(C`diff()\*(C'\fR and
so on.
.IP "\(bu" 4
\&\f(CWrel(x)\fR returns value of \f(CW\*(C`x\*(C'\fR relative to the first value of \f(CW\*(C`x\*(C'\fR. For
instance we might want to see the time or position relative to the start, not
relative to some absolute beginning. Example:
.Sp
.Vb 1
\& $ cat tst.vnl
\&
\& # time x
\& 100    200
\& 101    212
\& 102    209
\&
\&
\& $ <tst.vnl vnl\-filter \-p \*(Aqt=rel(time),x=rel(x)
\&
\& # t x
\& 0 0
\& 1 12
\& 2 9
.Ve
.IP "\(bu" 4
\&\f(CWdiff(x)\fR returns the difference between the current value of \f(CW\*(C`x\*(C'\fR and the
previous value of \f(CW\*(C`x\*(C'\fR. The first row will always be \-. Example:
.Sp
.Vb 1
\& $ <tst.vnl vnl\-filter \-p x,\*(Aqd1=diff(x),d2=diff(diff(x))\*(Aq | vnl\-align
\&
\& # x d1 d2
\&   1  \-  \-
\&   8  7  7
\&  27 19 12
\&  64 37 18
\& 125 61 24
.Ve
.IP "\(bu" 4
\&\f(CWsum(x)\fR returns the cumulative sum of \f(CW\*(C`x\*(C'\fR. As \f(CWdiff(x)\fR can be thought of as
a derivative, \f(CWsum(x)\fR can be thought of as an integral. So \f(CW\*(C`diff(sum(x))\*(C'\fR
would return the same value as \f(CW\*(C`x\*(C'\fR (except for the first row; \f(CW\*(C`diff()\*(C'\fR always
returns \- for the first row).
.Sp
Example:
.Sp
.Vb 1
\& $ <tst.vnl vnl\-filter \-p \*(Aqx,s=sum(x),ds=diff(sum(x))\*(Aq | vnl\-align
\&
\& # x  s   ds
\&   1   1   \-
\&   8   9   8
\&  27  36  27
\&  64 100  64
\& 125 225 125
.Ve
.IP "\(bu" 4
\&\f(CWprev(x)\fR returns the previous value of \f(CW\*(C`x\*(C'\fR. One could construct \f(CW\*(C`sum()\*(C'\fR and
\&\f(CW\*(C`rel()\*(C'\fR using this, if they weren't already available.
.SH "ARGUMENTS"
.IX Header "ARGUMENTS"
.SS "\-p|\-\-print|\-\-pick expr"
.IX Subsection "-p|--print|--pick expr"
These option provide the mechanism to select specific columns for output. For
instance to pull out columns called \f(CW\*(C`lat\*(C'\fR, \f(CW\*(C`lon\*(C'\fR, and any column whose name
contains the string \f(CW\*(C`feature_\*(C'\fR, do
.PP
.Vb 1
\& vnl\-filter \-p lat,lon,\*(Aqfeature_.*\*(Aq
.Ve
.PP
or, equivalently
.PP
.Vb 1
\& vnl\-filter \-\-print lat \-\-print lon \-\-print \*(Aqfeature_.*\*(Aq
.Ve
.PP
We look for exact column name matches first, and if none are found, we try a
regex. If there was no column called exactly \f(CW\*(C`feature_\*(C'\fR, then the above would
be equivalent to
.PP
.Vb 1
\& vnl\-filter \-p lat,lon,feature_
.Ve
.PP
This mechanism is much more powerful than just selecting columns. First off, we
can rename chosen fields:
.PP
.Vb 1
\& vnl\-filter \-p w=feature_width
.Ve
.PP
would pick the \f(CW\*(C`feature_width\*(C'\fR field, but the resulting column in the output
would be named \f(CW\*(C`w\*(C'\fR. When renaming a column in this way regexen are \fInot\fR
supported, and exact field names must be given. But the string to the right of
the \f(CW\*(C`=\*(C'\fR 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 \f(CW\*(C`x\*(C'\fR, \f(CW\*(C`y\*(C'\fR, and \f(CW\*(C`z\*(C'\fR you can do:
.PP
.Vb 1
\& vnl\-filter \-p \*(Aql=sqrt(x*x + y*y + z*z)\*(Aq
.Ve
.PP
A single column called \f(CW\*(C`l\*(C'\fR would be produced.
.PP
We can also \fIexclude\fR columns by preceding their name with \f(CW\*(C`!\*(C'\fR. This works like
you expect. Rules:
.IP "\(bu" 4
The pick/exclude directives are processed in order given to produce the output
picked-column list
.IP "\(bu" 4
If the first \f(CW\*(C`\-p\*(C'\fR item is an exclusion, we implicitly pick \fIall\fR the columns
prior to processing the \f(CW\*(C`\-p\*(C'\fR.
.IP "\(bu" 4
The exclusion expressions match the \fIoutput\fR column names, not the \fIinput\fR
names.
.IP "\(bu" 4
We match the exact column names first. If that fails, we match as a regex
.PP
Example. To grab all the columns \fIexcept\fR the temperature(s) do this:
.PP
.Vb 1
\& vnl\-filter \-p !temperature
.Ve
.PP
To grab all the columns that describe \fIsomething\fR about a robot (columns whose
names have the string \f(CW\*(C`robot_\*(C'\fR in them), but \fInot\fR its temperature (i.e.
\&\fInot\fR \*(L"robot_temperature\*(R"), do this:
.PP
.Vb 1
\& vnl\-filter \-p robot_,!temperature
.Ve
.SS "\-\-has a,b,c,..."
.IX Subsection "--has a,b,c,..."
Used to select records (rows) that have a non-empty value in a particular field
(column). A \fInull\fR value in a column is designated with a single \f(CW\*(C`\-\*(C'\fR. If we
want to select only records that have a value in the \f(CW\*(C`x\*(C'\fR column, we pass
\&\f(CW\*(C`\-\-has x\*(C'\fR. To select records that have data for \fIall\fR of a given set of
columns, the \f(CW\*(C`\-\-has\*(C'\fR 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 \fIboth\fR columns \f(CW\*(C`x\*(C'\fR \fIand\fR \f(CW\*(C`y\*(C'\fR we can pass in
\&\f(CW\*(C`\-\-has x,y\*(C'\fR or \f(CW\*(C`\-\-has x \-\-has y\*(C'\fR. If we want to combine multiple columns in an
\&\fIor\fR (select rows that have data in \fIany\fR of a given set of columns), use a
matches expression, as documented below.
.PP
If we want to select a column \fIand\fR pick only rows that have a value in this
column, a shorthand syntax exists:
.PP
.Vb 1
\& vnl\-filter \-\-has col \-p col
.Ve
.PP
is equivalent to
.PP
.Vb 1
\& vnl\-filter \-p +col
.Ve
.PP
Note that just like the column specifications in \f(CW\*(C`\-p\*(C'\fR the columns given to
\&\f(CW\*(C`\-\-has\*(C'\fR must match exactly \fIor\fR as a regex. In either case, a unique matching
column must be found.
.SS "Matches expressions"
.IX Subsection "Matches expressions"
Anything on the commandline not attached to any \f(CW\*(C`\-\-xxx\*(C'\fR option is a \fImatches\fR
expression. These are used to select particular records (rows) in a data file.
For each row, we evaluate all the expressions. If \fIall\fR the expressions
evaluate to true, that row is output. This expression is passed directly to the
awk (or perl) backend.
.PP
Example: to select all rows that have valid data in column \f(CW\*(C`a\*(C'\fR \fIor\fR column
\&\f(CW\*(C`b\*(C'\fR \fIor\fR column \f(CW\*(C`c\*(C'\fR you can
.PP
.Vb 1
\& vnl\-filter \*(Aqa != "\-" || b != "\-" || c != "\-"\*(Aq
.Ve
.PP
or
.PP
.Vb 1
\& vnl\-filter \-\-perl \*(Aqdefined a || defined b || defined c\*(Aq
.Ve
.PP
As with the named expressions given to \f(CW\*(C`\-p\*(C'\fR (described above), these are passed
directly to awk, so anything that can be done with awk is supported here.
.SS "\-A N|\-\-after\-context N"
.IX Subsection "-A N|--after-context N"
Output N lines following each \fImatches\fR expression, even those lines that do
not themselves match. This works just like the \f(CW\*(C`grep\*(C'\fR options of the same name.
See \*(L"Context lines\*(R"
.SS "\-B N|\-\-before\-context N"
.IX Subsection "-B N|--before-context N"
Output N lines preceding each \fImatches\fR expression, even those lines that do
not themselves match. This works just like the \f(CW\*(C`grep\*(C'\fR options of the same name.
See \*(L"Context lines\*(R"
.SS "\-C N|\-\-context N"
.IX Subsection "-C N|--context N"
Output N lines preceding and following each \fImatches\fR expression, even those
lines that do not themselves match. This works just like the \f(CW\*(C`grep\*(C'\fR options of
the same name. See \*(L"Context lines\*(R"
.SS "\-\-eval expr"
.IX Subsection "--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, \f(CW\*(C`vnl\-filter\*(C'\fR 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.
.PP
Since the expression may print \fIanything\fR 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.
.PP
In awk the expr is a full set of pattern/action statements. So to print the sum
of columns \f(CW\*(C`a\*(C'\fR and \f(CW\*(C`b\*(C'\fR in each row, and at the end, print the sum of all
values in the \f(CW\*(C`a\*(C'\fR column
.PP
.Vb 1
\& vnl\-filter \-\-eval \*(Aq{print a+b; suma += a} END {print suma}\*(Aq
.Ve
.PP
In perl the arbitrary expression fits in like this:
.PP
.Vb 5
\& while(<>) # read each line
\& {
\&   next unless matches; # skip non\-matching lines
\&   eval expression;     # evaluate the arbitrary expression
\& }
.Ve
.SS "\-\-function|\-\-sub"
.IX Subsection "--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:
.PP
.Vb 5
\& $ cat tst.vnl
\& # s
\& 1\-2
\& 3\-4
\& 5\-6
\&
\& $ < tst.vnl
\&   vnl\-filter \-\-function \*(Aqbefore(x) { sub("\-.*","",x); return x }\*(Aq \e
\&              \-\-function \*(Aqafter(x)  { sub(".*\-","",x); return x }\*(Aq \e
\&              \-p \*(Aqb=before(s),a=after(s)\*(Aq
\& # b a
\& 1 2
\& 3 4
\& 5 6
.Ve
.PP
See the \s-1CAVEATS\s0 section below if you're doing something
sufficiently-complicated where you need this.
.SS "\-\-[no]skipempty"
.IX Subsection "--[no]skipempty"
Do [not] skip records where all fields are blank. By default we \fIdo\fR skip all
empty records; to include them, pass \f(CW\*(C`\-\-noskipempty\*(C'\fR
.SS "\-\-skipcomments"
.IX Subsection "--skipcomments"
Don't output non-legend comments
.SS "\-\-perl"
.IX Subsection "--perl"
By default all procesing is performed by \f(CW\*(C`mawk\*(C'\fR, but if for whatever reason we
want perl instead, pass \f(CW\*(C`\-\-perl\*(C'\fR. Both modes work, but \f(CW\*(C`mawk\*(C'\fR is noticeably
faster. \f(CW\*(C`\-\-perl\*(C'\fR 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, \f(CW\*(C`\-\-eval\*(C'\fR strings).
Note that while variables in perl use sigils, column references should \fInot\fR
use sigils. To print the sum of all values in column \f(CW\*(C`a\*(C'\fR you'd do this in awk
.PP
.Vb 1
\& vnl\-filter \-\-eval \*(Aq{suma += a} END {print suma}\*(Aq
.Ve
.PP
and this in perl
.PP
.Vb 1
\& vnl\-filter \-\-perl \-\-eval \*(Aq{$suma += a} END {say $suma}\*(Aq
.Ve
.PP
The perl strings are evaluated without \f(CW\*(C`use strict\*(C'\fR or \f(CW\*(C`use warnings\*(C'\fR so I
didn't have to declare \f(CW$suma\fR in the example.
.PP
With \f(CW\*(C`\-\-perl\*(C'\fR, empty strings (\f(CW\*(C`\-\*(C'\fR in the vnlog file) are converted to
\&\f(CW\*(C`undef\*(C'\fR.
.SS "\-\-dumpexprs"
.IX Subsection "--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.
.SS "\-\-unbuffered"
.IX Subsection "--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.
.SS "\-\-stream"
.IX Subsection "--stream"
Synonym for \f(CW\*(C`\-\-unbuffered\*(C'\fR
.SH "CAVEATS"
.IX Header "CAVEATS"
This tool is very lax in its input validation (on purpose). As a result, columns
with names like \f(CW%CPU\fR and \f(CW\*(C`TIME+\*(C'\fR do work (i.e. you can more or less feed in
output from \f(CW\*(C`top \-b\*(C'\fR). 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:
.IP "\(bu" 4
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: \f(CW\*(C`vnl\-filter \-\-eval
\&\*(Aq{suma += a} END {print suma}\*(Aq\*(C'\fR will work fine if we \fIdo\fR have a column named
\&\f(CW\*(C`a\*(C'\fR and do \fInot\fR have a column named \f(CW\*(C`suma\*(C'\fR. But will not do the right thing
if any of those are violated. For instance, if a column \f(CW\*(C`a\*(C'\fR doesn't exist, then
\&\f(CW\*(C`awk\*(C'\fR would see \f(CW\*(C`suma += a\*(C'\fR instead of something like \f(CW\*(C`suma += $5\*(C'\fR. \f(CW\*(C`a\*(C'\fR
would be an uninitialized variable, which evaluates to 0, so the full
\&\f(CW\*(C`vnl\-filter\*(C'\fR 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.
.IP "\(bu" 4
It is natural to use vnlog as a database. You can run queries with something like
.Sp
.Vb 1
\& vnl\-filter \*(Aqkey == 5\*(Aq
.Ve
.Sp
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 \*(L"large\*(R" and \*(L"significant\*(R"
varies, and you should test it. In my experience vnlog \*(L"databases\*(R" scale
surprisingly well. But at some point, importing your data to something like
sqlite is well worth it.
.IP "\(bu" 4
When substituting column names I match \fIeither\fR a word-nonword transition
(\f(CW\*(C`\eb\*(C'\fR) \fIor\fR 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 \f(CW\*(C`\eb\*(C'\fR. This means that whitespace becomes important: \f(CW\*(C`1+%CPU\*(C'\fR
will not be parsed as expected, which is correct since \f(CW\*(C`+%CPU\*(C'\fR is also a valid
field name. But \f(CW\*(C`1+ %CPU\*(C'\fR will be parsed correctly, so if you have weird field
names, put the whitespace into your expressions. It'll make them more readable
anyway.
.IP "\(bu" 4
Strings passed to \f(CW\*(C`\-p\*(C'\fR are split on \f(CW\*(C`,\*(C'\fR \fIexcept\fR if the \f(CW\*(C`,\*(C'\fR is inside
balanced \f(CW\*(C`()\*(C'\fR. This makes it possible to say things like \f(CW\*(C`vnl\-filter \-\-function
\&\*(Aqf(a,b) { ... }\*(Aq \-p \*(Aqc=f(a,b)\*(Aq\*(C'\fR. This is probably the right behavior, although
some questionable looking field names become potentially impossible: \f(CW\*(C`f(a\*(C'\fR and
\&\f(CW\*(C`b)\*(C'\fR \fIcould\fR otherwise be legal field names, but you're probably asking for
trouble if you do that.
.IP "\(bu" 4
Currently there're two modes: a pick/print mode and an \f(CW\*(C`\-\-eval\*(C'\fR mode. Then
there's also \f(CW\*(C`\-\-function\*(C'\fR, which adds bits of \f(CW\*(C`\-\-eval\*(C'\fR to the pick/print mode,
but it feels maybe insufficient. I don't yet have strong feelings about what
this should become. Comments welcome
.SH "REPOSITORY"
.IX Header "REPOSITORY"
https://github.com/dkogan/vnlog/
.SH "AUTHOR"
.IX Header "AUTHOR"
Dima Kogan \f(CW\*(C`<dima@secretsauce.net>\*(C'\fR
.SH "LICENSE AND COPYRIGHT"
.IX Header "LICENSE AND COPYRIGHT"
Copyright 2016\-2017 California Institute of Technology
.PP
Copyright 2017\-2019 Dima Kogan \f(CW\*(C`<dima@secretsauce.net>\*(C'\fR
.PP
This library is free software; you can redistribute it and/or modify it under
the terms of the \s-1GNU\s0 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.