Usage message

In computer programming, the usage message of a program with command-line interface refers to the brief message prompted for the list of command-line arguments or options acceptable to the program.

The usage message is a quick way for the users to learn or remember how to use a program or command before reading the detailed document or seeking help from people.

Pattern

On the Unix-like platforms, usage messages usually follows the same common pattern:

• They often begin with "Usage:" (hence possibly the name), the command, followed by a list of arguments.
• To indicate optional arguments, Square brackets are commonly used, and can also be used to group parameters that must be specified together.
• Exclusive parameters can be indicated by separating them with vertical bars within groups.

Examples

Here is a thorough example based on the NetBSD source code style guide:

Usage: program [-aDde] [-f | -g] [-n number] [-b b_arg | -c c_arg] req1 req2 [opt1 [opt2]]

This would indicate that "program" should be called with:

• options without operands: a, D, d, e (any of which may be omitted). Note that in this case some parameters are case-sensitive
• exclusive options: f, g (denoted by the vertical bar)
• options with operands: n
• exclusive options with operands: b, c
• required arguments: req1, req2
• optional argument opt1, which may be used with or without opt2 (marked optional within the group by using another set of square brackets)
• optional argument opt2, which requires opt1

Implementation

For example, if a shell script called "myscript" required at least two parameter to be run, a programmer could create a usage message using something similar to the following:

if [ $# -lt 2 ]; then
    echo Usage: `basename $0` parameter1 parameter2 ... 1>&2
    exit 1
fi

Explanation of the code:

1. $# is number of parameter passed on the command line to the script
2. `basename $0` is the output of basename, to strip any path away from the script's filename
3. 1>&2 is used to redirect the output of echo to stderr

See also

• Command-Line Arguments Parsing