One of the most frustrating things about bash scripts is how challenging it is to create unix style executables. You know, the ones where you can pass in -h or –help and see the set of options for the program. Up until now this has been a very manual process in bash, but no longer. Enter the shflags project from Kate Ward where a bash library takes care of all the nasty work and producing an elegant way to add option (or argument) support to your scripts.

shflags is a powerful (and simple) tool for handling command line args in bash. It works on both Gnu and Bsd systems without any tricky handling which makes it a unique solution.

Usage

Shflags syntax is easy to understand an use. It comprises three sections, including shflags, a definition section and an initialization section. As in everything with bash, this needs to be written in order, as bash executes top down.

Including shflags

    # source shflags
    . ./shflags 
      or
    source /path/to/shflags

The first line is a shortcut to sourcing a file in the same directory which makes this quite portable.
If the shflags library exists in another place on the drive, but it needs to be specified and isn’t all that portable.

Defining options

shFlags supports boolean, float, integer and string. Since shell doesn’t natively support float, they are technically strings handled like floats and as such regular string comparison operators ( =, != ) should be used.

Here’s the syntax:

    DEFINE_type   'longoption' 'default value' 'description' 'one letter option'

    DEFINE_string 'name' 'world' 'name to say hello to' 'n'
    DEFINE_boolean 'force' false 'force overwriting' 'f'
    DEFINE_integer 'limit' 10 'number of items retuned' 'l'
    DEFINE_float 'time' '10.5' 'number of seconds to run' 't'

Initialization

This section is the exact same every time. Just copy the boilerplate and enjoy.

    # parse the command-line
    FLAGS "$@" || exit 1
    eval set -- "${FLAGS_ARGV}"

Putting it all together

    #!/bin/sh

    # source shflags
    . ./shflags

    # define a 'name' command-line string flag
    DEFINE_string 'name' 'world' 'name to say hello to' 'n'

    # parse the command-line
    FLAGS "$@" || exit 1
    eval set -- "${FLAGS_ARGV}"

    # say Hello!
    echo "Hello, ${FLAGS_name}!"