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 proper error messages or 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
To print a usage statement in a shell script, one might write:
case "$arg" in
...
h) printf 'Usage: %s parameter1 parameter2 ...\n' "$(basename "$0")"
exit $0
;;
...
esac
Anti-patterns
A usage statement is not an error message, but is often used as a lazy way to avoid printing a useful error message. A properly written command line program will print a succinct error message that describes the exact error made by the caller rather than printing the usage statement and requiring the user to figure out what the mistake was. If a user fails to pass the correct number of arguments, for example, a single line stating that an argument is missing is far more useful than several pages of output providing a general usage. As a corollary, a usage statement should only be printed when specifically requested by the user (via --help, or -h, or -?, or some similar flag or argument) and should be written to stdout .