How Linux Works

Conditionals

The Bourne shell has special constructs for conditionals, such as if/then/else and case statements. Here is a simple script with an if conditional that checks to see if the script's first argument is hi:

#!/bin/sh
if [ $1 = hi ]; then
   echo 'The first argument was "hi"'
else
   echo -n 'The first argument was not "hi" -- '
   echo It was '"'$1'"'
fi

The words if, then, else, and fi in the preceding script are shell keywords. Everything else is a command. This is an extremely important point, because one of the commands is [ $1 = "hi" ]. The [ character is an actual program on a Unix system; it is not special shell syntax. All Unix systems have a command called [ that performs tests for shell script conditionals. This program is also known ...