Options Processing Pattern

groan options pattern

groan uses a pattern for processing command line options that works very well. Custom sub-commands can adopt a similar approach by borrowing code from examples in the codebase.

The general pattern

set -o allexport ensures that all variables are automatically exported to sub-commands.

All of the variables that are to be set by options are given their initial values.

#default flags

DEBUG=false
LOUD=true
THEME=default

Options are processed by a loop surrounding a case statement. Below you see that multiple matches can be provided for each case, and the value of the named parameter theme is easily extracted.

args=()
 
for arg in "$@"
do
    case "$arg" in
      --debug | -d | -D)
            DEBUG=true           
            LOUD=true
      ;;
      --quiet | -q)
            LOUD=false
            DEBUG=false
      ;;
      --theme=*)
            THEME=${arg##--theme=}
      ;;
      --*)
          args+=($arg)
      ;;
      *)
          if [[ "$subcommand" = "" ]]; then
             subcommand=$arg
          else
             args+=($arg)
          fi
      ;;
    esac
done

After the options for this script have been read into their variables, the remaining options and arguments are collected in the $args array to be passed on to the sub-command. Note how the first unprocessed argument is used as the sub-command name.

Additional elements

If an option wants the value of the next parameter, this is the pattern we use. In the case statement the option sets the GETEXEC variable to true. On the following iteration the if $GETEXEC collects the next parameter whatever it is.

GETEXEC=false
what="env"
for arg in "$@"  
do
    if $GETEXEC; then
        what=$arg
        GETEXEC=false
        continue
    fi

    case $arg in
    --eval | -e)
        GETEXEC=true
    ;;
    esac
done

This alternative arrangement gets the next non-option parameter.

GETEXEC=false
what="env"
for arg in "$@"  
do
    case $arg in
    --eval | -e)
        GETEXEC=true
    ;;
    --*)  
    ;;
    *)
      if $GETEXEC; then
        what=$arg
        GETEXEC=false
        continue
      fi
    ;;
    esac
done

Miscellaneous

For case-insensitive matching of options the following command is available in bash. This affects case, == and ~=. shopt -s nocasematch