Bash wizardry: Command Line Switches

If you’re like me (and God help you if you are) You write a lot of bash scripts… When something comes up bash is a VERY handy language to use because it’s a) portable (between almost all *nixes), b) lightweight, and c) flexible (thanks to the plethora of linux commands which can be piped together) One large reason people prefer perl (or some other language) is because they’re more flexible. And one of those cases is processing command line switches. Commonly bash scripts are coded in a way which makes it necessary to give certain switches as a certain argument to the script. This makes the script brittle, and you CANNOT leave out switch $2 if you plan to use switch $3. Allow me to help you get around this rather nasty little inconvenience! (note: this deals with SWITHCES ONLY! *NOT* switches with arguments!) 

check_c_arg() {

  count=0

  for i in $@

    do

      if [ $count -eq 0 ]

        then

          count=1

        else

          if [ "$i" = "$1" ]

            then

              return 1

          fi

      fi

  done

  return 0

}

This beautiful little bit of code will allow you to take switches in ANY order. Simply setup a script like this:

#!/bin/bash

host="$1"



check_c_arg() {

  count=0

  for i in $@

    do

      if [ $count -eq 0 ]

        then

          count=1

        else

          if [ "$i" = "$1" ]

            then

              return 1

          fi

      fi

  done

  return 0

}



check_c_arg "-v" $@

cfg_verbose=$?

check_c_arg "-d" $@

cfg_dry_run=$?

check_c_arg "-h" $@

cfg_help=$?





if [ $cfg_help -eq 1 ]

  then

    echo -e "Usage: $0 [-v] [-h]"

    echo -e "\t-v\tVerbose Mode"

    echo -e "\t-d\tDry run (echo command, do not run it)"

    echo -e "\t-h\tPrint this help message"

    exit 1

fi



if [ $cfg_dry_run -eq 1 ]

  then

    echo "ping -c 4 $host"

  else

    if [ $cfg_verbose -eq 1 ]

      then

        ping -c 4 $host

      else

        ping -c 4 $host 1>/dev/null 2>/dev/null

    fi

fi

In the above all of the following are valid:

  • 127.0.0.1 -v -d
  • 127.0.0.1 -d -v
  • 127.0.0.1 -v
  • 127.0.0.1 -d
  • 127.0.0.1 -h
  • 127.0.0.1 -h -v -d
  • 127.0.0.1 -h -d -v
  • 127.0.0.1 -h -v
  • 127.0.0.1 -h -d
  • 127.0.0.1 -v -h -d
  • 127.0.0.1 -d -h -v
  • 127.0.0.1 -v -h
  • 127.0.0.1 -d -h
  • 127.0.0.1 -v -d -h
  • 127.0.0.1 -d -v -h

I hope this helps inspire people to take the easy (and often times more correct) path when faced with a problem which requires a solution, but not necessarily a terribly complex one.

Cheers!
DK

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