Introduction to Unliner
  A day in the life
    ... of a unix plumber.

    Let's say you have a huge access log file in a typical Apache-like
    format like this:

        10.9.2.1 - - [10/Oct/2012:03:53:11 -0700] "GET /report.cgi HTTP/1.0" 200 724083

    However, you notice that report.cgi is chewing up lots of system
    resources. Who is responsible? Let's find out the IP addresses that are
    hitting this URL the most so we can track them down.

    The first step is to extract out the requests for report.cgi so we'd
    probably do something like this:

        $ grep "GET /report.cgi" access.log

    Now we'll extract the IP address:

        $ grep "GET /report.cgi" access.log | awk '{print $1}'

    Next we add the standard "sort | uniq -c | sort -rn" tallying pipeline:

        $ grep "GET /report.cgi" access.log | awk '{print $1}' | sort | uniq -c | sort -rn

    Oops, the important bit scrolled off the screen. Let's add a "head"
    process to limit the output:

        $ grep "GET /report.cgi" access.log | awk '{print $1}' | sort | uniq -c | sort -rn | head -n 5

    And we finally get our nice report:

        3271039 10.3.0.29
            912 10.9.2.7
            897 10.9.2.1
            292 10.9.2.3
            101 10.9.2.4

    Looks like we've found our culprit.

  Installing unliner
    If you want to follow along with this tutorial, or start coding right
    away, the easiest way to install unliner is with cpanminus:

        curl -sL https://raw.github.com/miyagawa/cpanminus/master/cpanm | sudo perl - App::Unliner

  You want it to do *what*?
    Usually one-liners entered in your shell are thrown away after they are
    used because it's so easy to re-create them as necessary. That's one
    reason why unix pipes are so cool.

    Besides, as soon as your pipelines reach a full line or two of text they
    start to become very hard to work with (though I confess I've gotten a
    lot of use out of crazy long pipelines before). At this point, usually
    the one-liner is re-written as a "real" program.

    The point of unliner is to provide an intermediate stage between a
    one-liner and a real program. And you might even find that there is no
    need to make it a real program after all.

    To turn your one-liner into an unliner just wrap a "def main { }" around
    it like this:

        def main {
          grep "GET /report.cgi" access.log | awk '{print $1}' | sort | uniq -c | sort -rn | head -n 5
        }

    If you save this in the file "log-report" then your unliner program can
    be invoked with this command:

        $ unliner log-report < input.txt

    You could also put a shebang line
    <https://en.wikipedia.org/wiki/Shebang_(Unix)> at the top of your
    script:

        #!/usr/bin/env unliner

    Now if you "chmod +x log-report" you can run it directly:

        $ ./log-report < input.txt

  Defs
    The "def main { }" isn't a special type of def except that it happens to
    be what is called when your program is invoked. You can create other
    defs and they can be invoked by your main def and other defs, kind of
    like subroutines.

    For example, we could move the "awk" command into a "ip-extractor" def,
    and the tallying logic into a "tally" def:

        def main {
          grep "GET /report.cgi" access.log | ip-extractor | tally | head -n 5
        }

        def ip-extractor {
          awk '{print $1}'
        }

        def tally {
          sort | uniq -c | sort -rn
        }

    The same sequences of processes will be created with this program as
    with the previous. However, defs let you organize and re-use pipeline
    components better.

  Arguments
    The unliner program shown so far is not very flexible. For instance, the
    "access.log" filename is hard-coded.

    To fix this the arguments passed in to our log-report program are
    available in the variable $@, just like in a shell script:

        def main {
          grep "GET /report.cgi" $@ | ip-extractor | tally | head -n 5
        }

    Now we can pass in a log file argument to our program (otherwise it will
    read input from standard input):

        $ unliner log-report access.log

    Note that $@ escapes whitespace like bourne shell's "$@". Actually it
    just passes the argument array untouched through to the process (grep in
    this case) so the arguments can contain any characters. The bourne
    equivalent of unquoted $@ and $* are not supported because they cause
    way too many bugs (use templates if you need to do this).

    We can parameterise other aspects of the unliner program too. For
    example, suppose you wanted to control the number of lines that are
    included in the report. To do this add a "prototype":

        def main(head|h=i, junkarg=s) {
          grep "GET /report.cgi" $@ | ip-extractor | tally | head -n $head
        }

    The prototype indicates that the main def requires arguments. Since the
    main def is the entry-point, these arguments must come from the command
    line:

        $ unliner log-report access.log --head 5

    "head|h=i" is a Getopt::Long argument definition. It means that the
    official name of this argument is "head", that there is a single-dash
    alias "h", and that the argument's "type" is required to be an integer.
    Because "h" is an alias we could also use that as the argument:

        $ unliner log-report access.log -h 5

    However, if you forget to add one of these arguments, the head process
    will die with an error like "head: : invalid number of lines".

    Other common GetOpt::Long argument types are string (ie "hostname|h=s")
    and boolean on/off switches that require no argument (ie "flag|f").

    In order to have a default value for a parameter, you put parentheses
    around the argument definition followed by the default value (just like
    lisp):

        def main((head|h=i 5)) {
          grep "GET /report.cgi" $@ | ip-extractor | tally | head -n $head
        }

    None of these variables need to be quoted. They are always passed
    verbatim to the underlying command. If you do quote them, be aware that
    string interpolation is not implemented (use templates for that).

    Defs internal to your program accept arguments in exactly the same way.
    You can think of internal defs as being their own mini command-line
    programs:

        def main {
          grep "GET /report.cgi" $@ | ip-extractor | tally | my-head -n 5
        }

        def my-head((n=i 10)) {
          head -n $n
        }

  Argument pass-through and environment variables
    Normally if you pass an argument into a def (from the command line or
    from another def) that isn't listed in the prototype, an "Unknown
    option" error will be thrown. This is the default Getopt::Long
    behaviour. If you wish to suppress this error and leave unknown options
    in the argument list, you can use the "pass-through" def modifier like
    so:

        def main : pass-through {
          my-head $@
        }

        def my-head(count=i) {
          head -n $count
        }

    "pass-through" simply sets the "pass_through" option of Getopt::Long.

    Environment variables that were given to the unliner process are present
    in your scripts as variables too. For example, this does what you'd
    expect:

        def main {
          echo $PATH
        }

    But note that interpolating variables isn't (yet?) supported so "echo
    "$PATH:/opt/bin"" won't work (use templates for that -- see below).

    There is a def modifier called "env" that allows you to install
    arguments into environment variables while invoking the def. This is
    useful for languages like "perl" where access to environment variables
    is easier than parsing an argument list:

        def main((name=s 'Anonymous')) : perl : env {
          print "Hello, $ENV{name}\n";
        }

  Def Modifiers
    The contents of all the defs we've seen so far are in a custom unliner
    language called Shell. You can add it if you want, but the ": sh" def
    modifier is redundant because Shell is the default language.

    Shell is mostly like bourne shell/bash but a little bit different. The
    differences are described in the distribution's TODO file. Some
    differences are deliberate and some are just features that haven't been
    implemented yet. One difference is that unliner uses perl-style
    backslashed single quotes in single quoted string literals, not bourne
    shell-style. If you don't know what the bourne shell-style is, consider
    yourself lucky.

    Def modifiers can be used to change how the def body is interpreted by
    changing the language to something other than Shell. Modifiers go in
    between the def name/prototype and the body. One language modifier that
    can be used is "perl". It causes the def body to be interpreted as perl
    code. For example:

        def body-size-extractor : perl {
          while (<STDIN>) {
            ## body size is the last field in the log
            print "$1\n" if /(\d+)$/;
          }
        }

    This def could also have been written in sh, but dealing with shell
    escapes is sometimes annoying:

        def body-size-extractor {
          perl -e 'while(<STDIN>) { ... }'
        }

    Def modifiers themselves sometimes take arguments. For example, perl
    defs can take the "-n" switch which implicitly adds a loop (just like
    the perl binary):

        def body-size-extractor : perl -n {
          print "$1\n" if /(\d+)$/;
        }

    Another supported language is python:

        def wrap-in-square-brackets : python {
          import sys

          for line in sys.stdin:
            line = line[:-1] # chop newline
            print "[" + line + "]"
        }

    Note that python is very noisy when it receives a SIGPIPE so polite
    pipeline components should manually catch it and then exit silently.

    A general-purpose "language" is exec. It is useful for running any
    command on your system, even when there are no such custom languages. As
    an example of exec usage, the following defs are equivalent:

        def second-column {
          awk -F, '{ print $2 }'
        }

        def second-column : exec awk -F, -f {
          { print $2 }
        }

    Note that the "-f" is required because awk doesn't follow the common
    scripting language convention where a program path is the first
    argument.

    Github pull requests for new languages appreciated.

  Templates
    Another def modifier is "template". This modifier processes your def
    body with Template Toolkit <http://template-toolkit.org/> before it
    passes it on to whatever language type is specified. Because the
    template has access to the def's arguments, this lets you conditionally
    include pipeline components.

    Let's say we wanted to add a "filter-localhost" switch to our log-report
    unliner that will exclude requests from localhost (127.0.0.1) from the
    tally. This can be accomplished with templates:

        def main((head|h=i 5), filter-localhost) : template {
          grep "GET /report.cgi" $@ |
          ip-extractor |

          [% IF filter_localhost %]  ## Note: - changes to _
            grep -v '^127\.0\.0\.1$' |
          [% END %]

          tally |
          head -n $head
        }

        def ip-extractor {
          awk '{print $1}'
        }
 
        def tally {
          sort | uniq -c | sort -rn
        }

    We can now enable this option from the command line:

        $ unliner log-report access.log --filter-localhost

    A grep process will only be created if the "--filter-localhost" option
    is passed in.

    Remember that templates are processed as strings before the language
    even sees them. For example, here is how you could take advantage of the
    head "negative number" trick:

        def my-head((n=i 5)) : template {
          head -[% n %]
        }

    When using templates always be careful about escaping or sanitising
    values.

    The above example is OK because "n" is guaranteed to be an integer.

  Debugging
    In order to see the actual pipeline being run, you can set the
    environment variable "UNLINER_DEBUG" and it will print some information
    to standard error:

        $ UNLINER_DEBUG=2 unliner log-report access.log --filter-localhost
        unliner: TMP: Not cleaning up temp directory /tmp/GPtXapOfib because UNLINER_DEBUG >= 2
        unliner: CMD: grep 'GET /report.cgi' access.log | perl /tmp/GPtXapOfib/56ba8ad7a6431cbe6b64835c97e248d27a4234a0 | sort | uniq -c | sort -rn | head -n 5

    Note that when you write defs in languages like perl and python, scripts
    will be created in a temporary directory and executed from there.

  Optimisation
    Unliner does pipeline optimisation by default. Currently only spurious
    cat processes are optimised away.

    *   Leading cats

        If a pipeline begins with a cat of no arguments, that cat is removed
        and no cat process is created. If a pipeline begins with a cat of
        exactly one argument, then that file is opened and dup2()ed to
        STDIN.

    *   Trailing cats

        If a pipeline ends with a trailing cat, that cat is removed unless
        STDOUT is a terminal. Trailing cats are useful to prevent a program
        from doing special terminal formatting things like adding ANSI
        colours.

    *   Internal cats

        All internal cats with no arguments are removed. Such cats aren't as
        silly as they sound. Sometimes pipeline components have leading or
        trailing cats for some reason. When these components are used in
        pipelines, internal cats result. This optimisation will stop any
        unnecessary cat processes from being created.

    Consider the following unliner script:

        def main {
          cat $@ | cat | cat | cat | wc -l | cat | cat
        }

    Because of the spurious cat optimisations, running it like so won't
    start a single cat process:

        unliner lots-of-cats.unliner file.txt > output.txt

    It will be optimised to this equivalent command:

        wc -l < file.txt > output.txt

SEE ALSO
    App::Unliner

    unliner

    Unliner github repo <https://github.com/hoytech/unliner>

AUTHOR
    Doug Hoyte, "<doug@hcsw.org>"

COPYRIGHT & LICENSE
    Copyright 2012-2014 Doug Hoyte.

    This module is licensed under the same terms as perl itself.

