The POSIX standard for the shell scripting language specifies a command named test that can be used to test something, and return an exit status to reflect the result.
Which kind of expression to evaluate is specified with the arguments to test. A straightforward and useful test is string equality; are two strings the same? We can try this out with an equals sign between two other shell words:
$ myshell=bash $ test "$myshell" = 'sh' && printf 'Match!\n' $ test "$myshell" = 'bash' && printf 'Match!\n' Match!
Note that the equals sign needs to be a separate word for the test arguments, with spaces before and after, unlike the variable assignment, where we cannot have spaces.
Many other tests are possible:
- test -e 'myfile': Whether a file or myfile directory exists
- test -n "$myvar": Whether a myvar variable is set and not empty
- test -d 'mydir': Whether a directory with the name mydir exists
- test -s 'myfile': Whether a file myfile file is not empty
You can see a complete list of these with help test. You are likely to refer to this particular help page many times during your career as a Bash programmer!