Linux Pocket Guide

What’s a Command?

A Linux command typically consists of a program name followed by options and arguments, typed within a shell. The program name refers to a program somewhere on disk (which the shell will locate and run). Options, which usually begin with a dash, affect the behavior of the program, and arguments usually represent inputs and outputs. For example, this command to count the lines in a file:

$ wc -l myfile

consists of a program (wc, the “word count” program), an option (-l) saying to count lines, and an argument (myfile) indicating the file to read. (The dollar sign is a prompt from the shell, indicating that it is waiting for your command.) Options may be given individually:

$ myprogram -a -b -c myfile           Three individual options

or combined behind a single dash:

$ myprogram -abc myfile               Same as -a -b -c

though some programs are quirky and do not recognize combined options.

Commands can also be much more complex than running a single program:

• They can run several programs at once, either in sequence (one after the other) or connected into a “pipeline” with the output of one command becoming the input of the next.

• Options are not standardized. The same option (say, -l) may have different meanings to different programs: in wc -l it means “count lines of text,” but in ls -l it means “produce longer output.” In the other direction, two programs might use different options to mean the same thing, such as -q for “run quietly” versus -s for “run silently.”

• Likewise, arguments are not standardized. ...