In computing, a command is a directive to a computer program to perform a specific task. It may be issued via a command-line interface or as input to a network service as part of a network protocol, or as an event triggered in a graphical user interface.

Specifically, the term command is used in imperative programming languages. The name arises because statements in these languages are usually written in a manner similar to the imperative mood used in many natural languages. A statement in an imperative programming language would then be a sentence in a natural language, and the command would be the predicate.

Many programs allow specifically formatted arguments, known as flags or options, which modify the default behaviour of the program, while further arguments may provide objects, such as files, to act on. As an analogy to a natural language, the flags are adverbs, while the other arguments are objects.

Distinction between command and expression, statement and function

The meaning of command is highly dependent on context. For example, some authors refer to conditionals as commands ^[1] while they are called expressions in Python^[2] or Bash^[3] and statements in JAVA.^[4] Similarly, writing to stdout is done in Bash with the builtin command printf, while it is done with the built-in function print() in Python.^[5]

Examples

Here are some commands given to a command-line interpreter (Unix shell).

The following command changes the user's working position in the directory tree to the directory /home/pete. The utility program is cd and the argument is /home/pete:

cd /home/pete

The following command prints the text Hello World on the standard output stream, which, in this case, just prints the text on the screen. The program name is echo and the argument is "Hello World". The quotes are used to prevent Hello and World being treated as separate tokens:

echo "Hello World"

The following commands are equivalent. They list files in the directory /bin. The program is ls, having three flags (l, t, r), and the argument is the directory /bin:

ls -l -t -r  /bin
ls -ltr  /bin

The following command displays the contents of the files ch1.txt and ch2.txt. The program name is cat, having two file name arguments:

cat ch1.txt ch2.txt

Here are some commands for the DOS, OS/2 and Microsoft Windows command prompt processor. The following command displays the contents of the file readme.txt. The program name is type and the argument is readme.txt.^[6]

type readme.txt

The following command lists the contents of the current directory. The program name is dir, and Q is a flag requesting that the owner of each file also be listed.^[7]

dir /Q