Usage message

In computer programming, a usage message or help message refers to a brief message displayed by a program that utilizes a command-line interface for execution. This message usually consists of the correct command line usage for the program and includes a list of the correct command-line arguments or options acceptable to said program.

Usage messages are utilized as a quick way for a program to inform the user of proper command syntax, and should not be substituted for detailed documentation, such as a man page.

Pattern

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

• They often begin with "Usage:" , 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.
• To indicate required arguments, Angled brackets are commonly used, following the same grouping conventions as square brackets.
• Exclusive parameters can be indicated by separating them with vertical bars within groups.

Examples

Here is an 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 required at least two parameter to be run, a programmer could create a usage message using something similar to the following:

if [ $# -lt 2 ]; then
    printf 'Usage: %s parameter1 parameter2 ...\n' "$(basename "$0")" >&2
    exit 64
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. >&2 is used to redirect the output of printf to stderr
4. /usr/include/sysexits.h defines 64 as the preferable exit code for usage errors.

See also

• Command-Line Arguments Parsing