my_prog arg1 arg2 arg3 is equivalent to $0 $1 $2 $3

  • $0 - gives program name, i.e. my_prog
  • $* - gives all arguements i.e. $i , $2 and $3
  • $# - gives number of arguements, i.e. 3
  • $? - gives return code
  • $$ - gives pid, useful for assigning unique names, e.g. > /tmp/file.$$

These can be used to create a usage statement (which is always good practice), e.g.

#!/usr/bin/sh 
USAGE="usage: $0 arg1 arg2 arg3" 
if [ $# -ne 3 ] 
then 
  echo "$USAGE"
  exit 1 
fi

Parameter Subsitution

 ${variable:-string}

will show variable if set otherwise string

 ${variable:=string}

if variable is not set, use string and set variable to the value of string for

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