Using double dash to specify pesky command line parameters

Many Linux commands accept command-line options and positional parameters. The grep command searches for a pattern in a given file. Its 2 positional parameters are the pattern and the filename. It accepts options such as -i which specifies that the search is case insensitive.

An interesting scenario is when a positional parameter has a value starting with a dash ('-'). This makes the parameter indistinguishable from an option. For example, you try to grep the string -tea in a given file.

$ grep -i '-tea' test.txt
grep: invalid option -- 't'
Usage: grep [OPTION]... PATTERN [FILE]...
Try 'grep --help' for more information.

The -tea is interpreted as an option. Consequently, the command failed.

To solve the problem, insert the double dash (--) on the command line to mark the end of all options. Everything that come after the -- marker are interpreted as positional parameters.

$ grep -i -- '-tea' test.txt
Beverages -tea and coffee- are served.

Anther example scenario for using the double dash is the deleting of a file with a pesky file name. Suppose you want to delete a file named '-a'.

$ rm -a
rm: invalid option -- 'a'
Try `rm --help' for more information.

Enter -- to mark where the positional parameters start.

$ rm -- -a