In computer programming, a one-liner program originally was textual input to the command line of an operating system shell that performed some function in just one line of input. In the present day, a one-liner can be
Certain dynamic languages for scripting, such as AWK, sed, and Perl, have traditionally been adept at expressing one-liners.Shell interpreters such as Unix shells or 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.
The concept of a one-liner program has been known since the 1960s[1] with the release of the APL programming language. With its terse syntax and powerful mathematical operators, APL allowed useful programs to be represented in a few symbols.
In the 1970s, one-liners became associated with the rise of the home computer and BASIC. Computer magazines published type-in programs in many dialects of BASIC. Some magazines devoted regular columns solely to impressive short and one-line programs.[2]
The word One-liner also 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:
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.
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.
A single line of BASIC can typically hold up to 255 characters, and one liners ranged from simple games[3] to graphical demos. One of the better-known demo one-liners is colloquially known as 10PRINT, written for the Commodore 64:
The following example is a C program (a winning entry in the "Best one-liner" category of the IOCCC).
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:
The TAPL book contains 20 examples of one-liners at the end of the book's first chapter.
Here are the very first of them:
Here are examples in J:
perl -0777 -ne '
perl -lne '
perl -p -i.bak -e '
Many one-liners are practical. For example, the following Perl one-liner will reverse all the bytes in a file:
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:
perl -e'
to print out a comma-separated list of primes in the range 2 - number.
The following Haskell program is a one-liner: it sorts its input lines ASCIIbetically.
An even shorter version:
Usable on the command line like:
The following Racket program is equivalent to the above Haskell example:
(for-each displayln (sort (port->lines) stringand this can be used on the command line as follows: racket -e '
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 "
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 (printing the last field of a unix long listing):
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:
Tcl (Tool Command Language) is a dynamic programming/scripting language based on concepts of Lisp, C, and Unix shells. It can be used interactively, or by running scripts (programs) which can use a package system for structuring.[4]
Many strings are also well-formed lists. Every simple word is a list of length one, and elements of longer lists are separated by whitespace. For instance, a string that corresponds to a list of three elements:
Strings with unbalanced quotes or braces, or non-space characters directly following closing braces, cannot be parsed as lists directly. You can explicitly split them to make a list.
The "constructor" for lists is of course called list. It's recommended to use when elements come from variable or command substitution (braces won't do that). As Tcl commands are lists anyway, the following is a full substitute for the list command:
Finding palindromes in file words.txt
Piping semantics in PowerShell help enable complex scenarios with one-liner programs. This one-liner in PowerShell script takes a list of names and counts from a comma-separated value file, and returns the sum of the counts for each name.