Shell Programming in Unix, Linux and OS X, Fourth Edition

7. Decisions, Decisions

This chapter introduces the conditional statement, a construct that is present in almost every programming language: if. It enables you to test a condition and then change the flow of program execution based on the result of the test.

The general format of the if command is

if command_t then         command         command         ... fi

where command_t is executed and its exit status is tested. If the exit status is zero, the commands that follow between then and fi are executed; otherwise, they’re skipped.

Exit Status

To understand how conditional tests work, it’s important to know about how Unix works with what’s called the exit status. Whenever any program completes execution, it returns an exit status code to the shell. ...