Help understanding shell script

First, generally, $1 is the first command argument of your script. (I say, generally, because the unix set comamnd can change $1, $2, etc. I'll ignore that for the purposes of this discussion). Assume your shell script is called x.ss. Executing: x.ss -tkdiff $1 will equal the string "-tkdiff". Executing x.ss with no command line arguments, sets $1 to null or undefined. Consider this line: if [ x"$1" = x"-tkdiff" ]; then if $1 equals -tkdiff, the first part of the if statement executes; If $1 is undefined or equals some other value the else executes. You're probably wondering why the guy did this? Why not just do this: if [ "$1" = "-tkdiff" ] then echo "command line arg is -tkdiff" fi He should have. The "x" really doesn't get you anything. He probably modified a bourne shell null check. While the korn and bash shells have a string null check (-z), the bourne shell does not. This is one way of checking with the bourne shell whether $1 is null: #!/bin/sh if [ x"$1" = x ]; then echo "command line arg 1 is null" else echo "it exists" fi if $1 is null, then x equals x and the first part of the if statement identifies $1 as null. Let me know if you have any questions.

It makes sense! Your help is greatly appreciated.