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In computing, a command is a directive to a computer program acting as an interpreter of some kind to perform a specific task. Most commonly a command is either a directive to some kind of command-line interface, such as a shell, or an event in a graphical user interface triggered by the user selecting an option in a menu.
Specifically, the term command is used in imperative computer languages. These languages are called this, because statements in these languages are usually written in a manner similar to the imperative mood used in many natural languages. If one views a statement in an imperative language as being like a sentence in a natural language, then a command is generally like a verb in such a language.
Many programs allow specially formatted arguments, known as flags or options, which modify the default behaviour of the command, while further arguments describe what the command acts on. Comparing to a natural language: the flags are adverbs, whilst the other arguments are objects.
The following command prints the text Hello World out to the standard output stream, which, in this case, will just print the text out on the screen. The command is echo and the argument is "Hello World". The quotes are used to prevent Hello and World being treated as separate arguments:
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 command is cat, and ch1.txt and ch2.txt are both arguments.
cat ch1.txt ch2.txt
Here are some commands given to a different command-line interpreter (the DOS, OS/2 and Microsoft Windows command prompt). Notice that the flags are identified differently but that the concepts are the same:
The following command lists all the contents of the current directory. The command is dir, and "A" is a flag. There is no argument.
The following command displays the contents of the file readme.txt. The command is type. The argument is "readme.txt". "P" is a parameter.
type /P readme.txt