Verifying File Types

To test file types, a number of primitives are used (taken from [7, p. 439]):

-s

checks that the file exists and is not empty.

-f

checks that the file is an ordinary file (not a directory).

-d

checks whether the file is really a directory.

-x

checks that the file is executable.

-w

checks that the file is writeable.

-r

checks that the file is readable.

An example would be where a program needs to output something to a file, but first checks that the file exists:

    #!/bin/sh
    if test ! -s arg.file
    then
        echo "arg.file is empty or does not exist."
        ls -l > arg.file
        exit
    else
        echo "File arg.file already exists."
    fi
    exit 0

Note the exclamation mark within the test sequence. The exclamation mark means ``not".