001-inpath.sh

#!/bin/sh
# inpath - verify that a specified program is either valid as-is,
#   or can be found in the PATH directory list.

in_path()
{
  # given a command and the PATH, try to find the command. Returns
  # 0 if found and executable, 1 if not. Note that this temporarily modifies 
  # the the IFS (input field seperator), but restores it upon completion.

  cmd=$1        path=$2         retval=1
  oldIFS=$IFS   IFS=":"

  for directory in $path
  do
    if [ -x $directory/$cmd ] ; then
      retval=0      # if we're here, we found $cmd in $directory
    fi
  done
  IFS=$oldIFS
  return $retval
}

checkForCmdInPath()
{
  var=$1
  
  # The variable slicing notation in the following conditional 
  # needs some explanation: ${var#expr} returns everything after
  # the match for 'expr' in the variable value (if any), and
  # ${var%expr} returns everything that doesn't match (in this
  # case just the very first character. You can also do this in
  # Bash with ${var:0:1} and you could use cut too: cut -c1

  if [ "$var" != "" ] ; then
    if [ "${var%${var#?}}" = "/" ] ; then
      if [ ! -x $var ] ; then
        return 1
      fi
    elif ! in_path $var $PATH ; then
      return 2
    fi 
  fi
}

if [ $# -ne 1 ] ; then
 echo "Usage: $0 command" >&2 ; exit 1
fi

checkForCmdInPath "$1"
case $? in
  0 ) echo "$1 found in PATH"			;;
  1 ) echo "$1 not found or not executable"	;;
  2 ) echo "$1 not found in PATH"		;;
esac

exit 0