One-liner program

A one-liner is textual input to the command-line of an operating system shell that performs some function in just one line of input.

The one liner can be

1. An expression written in the language of the shell.
2. The invocation of an interpreter together with program source for the interpreter to run.
3. The invocation of a compiler together with source to compile and instructions for executing the compiled program.

Certain dynamic scripting languages such as AWK, sed, and Perl have traditionally been adept at expressing one-liners. Specialist shell interpreters such as these Unix shells or the Windows PowerShell, allow for the construction of powerful one-liners.

The use of the phrase one-liner has been widened to also include program-source for any language that does something useful in one line.

History

The word One-liner has two references in the index of the book The AWK Programming Language (the book is often referred to by the abbreviation TAPL). It explains the programming language AWK, which is part of the Unix operating system. The authors explain the birth of the One-liner paradigm with their daily work on early Unix machines:

The 1977 version had only a few built-in variables and predefined functions. It was designed for writing short programs [...] Our model was that an invocation would be one or two lines long, typed in and used immediately. Defaults were chosen to match this style [...] We, being the authors, knew how the language was supposed to be used, and so we only wrote one-liners.

Notice that this original definition of a One-liner implies immediate execution of the program without any compilation. So, in a strict sense, only source code for interpreted languages qualifies as a One-liner. But this strict understanding of a One-liner was broadened in 1985 when the IOCCC introduced the category of Best One Liner for C, which is a compiled language.

Examples

The TAPL book contains 20 examples of One-liners (A Handful of Useful awk One-Liners^{[dead link]}) at the end of the book's first chapter.

Here are the very first of them:

1. Print the total number of input lines:

   END { print NR }
   

2. Print the tenth input line:

   NR == 10
   

3. Print the last field of every input line:

   { print $NF }
   

Here are examples in J:

1. A function avg to return the average of a list of numbers:

   avg=: +/ % #
   

2. Quicksort:

   quicksort=: (($:@(<#[) , (=#[) , $:@(>#[)) ({~ ?@#)) ^: (1<#)
   

Here are examples in Perl:

• Look for duplicate words

  perl -0777 -ne 'print "$.: doubled $_\n" while /\b(\w+)\b\s+\b\1\b/gi'
  

• Find Palindromes in /usr/dict/words

  perl -lne 'print if $_ eq reverse' /usr/dict/words
  

• in-place edit of *.c files changing all foo to bar

  perl -p -i.bak -e 's/\bfoo\b/bar/g' *.c
  

Many one-liners are practical. For example, the following Perl one-liner will reverse all the bytes in a file:

perl -0777e 'print scalar reverse <>' filename

One-liners are also used to show off the differential expressive power of programming languages. Frequently, one-liners are used to demonstrate programming ability. Contests are often held to see who can create the most exceptional one-liner.

The following example is a C program (a winning entry in the "Best one-liner" category of the IOCCC, here split to two lines for presentation).

main(int c,char**v){return!m(v[1],v[2]);}m(char*s,char*t){return
*t-42?*s?63==*t|*s==*t&&m(s+1,t+1):!*t:m(s,t+1)||*s&&m(s+1,t);}

This one-liner program is a glob pattern matcher. It understands the glob characters `*' meaning `zero or more characters' and `?' meaning exactly one character, just like most Unix shells.

Run it with two args, the string and the glob pattern. The exit status is 0 (shell true) when the pattern matches, 1 otherwise. The glob pattern must match the whole string, so you may want to use * at the beginning and end of the pattern if you are looking for something in the middle. Examples:

$ prog foo 'f??'; echo $?
$ prog 'best short program' '??st*o**p?*'; echo $?

One-liner in functional programming

Haskell

The following Haskell program is a one-liner: it sorts its input lines asciibetically.

 main = (mapM_ putStrLn . List.sort . lines) =<< getContents -- In ghci a qualified name like List.sort will work, although as a standalone executable you'd need to import List.

An even shorter version:

 main = interact (unlines . List.sort . lines) -- Ditto.

Racket

The following Racket program is equivalent to the above Haskell example:

#lang racket
(for-each displayln (sort (port->lines) string<?))

and this can be used on the command line as follows:

racket -e '(for-each displayln (sort (port->lines) string<?))'

Perl

While most Perl one-liners are imperative, Perl's support for anonymous functions, closures, map, filter (grep) and fold (List::Util::reduce) allows the creation of 'functional' one-liners.

This one liner creates a function that can be used to return a list of primes up to the value of the first parameter:

my $z = sub { grep { $a=$_; !grep { !($a % $_) } (2..$_-1)} (2..$_[0]) }

It can be used on the command line, like this:

perl -e'$,=",";print sub { grep { $a=$_; !grep { !($a % $_) } (2..$_-1)} (2..$_[0]) }->(shift)' number

to print out a comma-separated list of primes in the range 2 - number.

Python

Performing one liners directly on the Unix command line can be accomplished by using python's -cmd flag (-c for short), and typically requires the import of one or more modules. Statements are separated using ";" instead of newlines. For example, to print the last field of unix long listing:

ls -l | python -c "import sys;[sys.stdout.write(' '.join([line.split(' ')[-1]])) for line in sys.stdin]"

Executable libraries

The python CGIHTTPServer module for example is also an executable library that performs as a web server with CGI. To start the web server enter:

python -m CGIHTTPServer

It returns with:

Serving HTTP on 0.0.0.0 port 8000 ...

Python wrappers

Several open-source scripts have been developed to facilitate the construction of python one liners. Scripts such as pyp or Pyline import commonly used modules and provide more human-readable variables in an attempt to make python functionality more accessible on the command line. Here is a redo of the above example:

ls -l | pyp "whitespace[-1]"                      #"whitespace" represents each line split on white space
ls -l | pyline "words[-1]"                        #"words"      represents each line split on white space

External links

• awk one-liners
• sed one-liners
• Awk and Sed One-Liners Explained
• Perl One Liners, practical examples
• More Perl One Liners
• Oneliner Contest 2007, the longest thread ever about one liner development for Sinclair machines
• Oneliner Contest 2008, ongoing thread about one liner development for Sinclair machines
• International Obfuscated C Code Contest, IOCCC
• Lambda One Liners in Python
• Pyline:a grep-like, sed-like command-line tool (Python recipe)
• pyp: Python Power at the Prompt