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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 edit
The meaning of command is highly dependent on context. For example, some authors refer to conditionals as commands  while they are called expressions in Python or Bash and statements in JAVA. 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.
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"
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.
See also edit
- Maurizio Gabbrielli, Simone Martini (2010). Programming Languages - Principles and Paradigms. Springer London, 6.3.2 Conditional Commands, p. 140
- "Conditional expressions". python.org. Retrieved 23 October 2023.
- "Bash Conditional expressions". gnu.org. Retrieved 23 October 2023.
- "The if-then and if-then-else Statements". oracle.com. Retrieved 23 October 2023.
- "Built-in Functions - print". python.org. Retrieved 23 October 2023.
- "Type - Display a text file - Windows CMD". SS64.com. Retrieved 14 March 2019.
- "DIR - list files and folders - Windows CMD". SS64.com. Retrieved 14 March 2019.