Conditional Substitution of Variables

• ${VAR:-VALUE}

  Use VAR if set, otherwise use VALUE

• ${VAR:=VALUE}

  Use VAR if set, otherwise use VALUE and assign it to VAR

• ${VAR:?MESSAGE}

  Use VAR if set, otherwise print VAR followed by MESSAGE. If MESSAGE isn't supplied then print paramter null or not set

• ${VAR:+VALUE}

  Use VALUE if VAR is set, otherwise use nothing

• ${VAR-VALUE}

  Use VALUE if VAR is unset

• ${VAR+VALUE}

  Use VALUE if VAR is set

• ${VAR=VALUE}

  Use VALUE if VAR is unset and assign VALUE to VAR

• ${VAR?MESSAGE}

  Print MESSAGE to stderr and stop the script if VAR is unset

Examples

• ${VAR:-VALUE}
  

  $mytest=${vara:-$vala}
  $echo $mytest
  
  $vala=12
  $echo $mytest
  
  $mytest=${vara:-$vala}
  $echo $mytest
  12
  $vara=5
  $mytest=${vara:-$vala}
  $echo $mytest
  5
  $ 
  

• ${VAR:=VALUE}

  $mytest=${vara:=$vala}
  $echo $mytest
  
  $echo $vara
  
  $vala=12
  $mytest=${vara:=$vala}
  $echo $mytest
  12
  $echo $vara
  12
  $
  

• ${VAR:?MESSAGE}
  

  $ unset vara
  $ unset message
  $ unset mytest
  $ mytest=${vara:?$message}
  ksh: vara: parameter null or not set
  $ echo $mytest
  
  $ echo $vara
  
  $ echo $message
  
  $ message='help me....'
  $ mytest=${vara:?$message}
  ksh: vara: help me....
  $ echo $mytest
  
  $ echo $vara
  
  $ echo $message
  help me....
  $ vara=12
  $ mytest=${vara:?$message}
  $ echo $mytest
  12
  $ echo $vara
  12
  $ echo $message
  help me....
  $