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Previous: 44.7 Exit Status of UNIX Processes Chapter 44
Shell Programming for the Uninitiated
Next: 44.9 Testing Your Success
 

44.8 Test Exit Status with the if Statement

If you are going to write a shell script of any complexity at all, you need some way to write "conditional expressions." Conditional expressions are nothing more than statements that have a value of "true" or "false": like "Have I gotten dressed today?" or "Is it before 5 p.m.?" or "Does the file indata exist?" or "Is the value of $aardvark greater than 60?"

The UNIX shell is a complete programming language. Therefore, it allows you to write "if" statements with conditional expressions - just like C, Basic, Pascal, or any other language. Conditional expressions can also be used in several other situations; but most obviously, they're the basis for any sort of if statement. Here's the syntax of an if statement for the Bourne shell:

if 

conditional


then
    # do this if 

conditional

 returns a zero ("true") status
    

one-or-more-commands


else
    # do this if 

conditional

 returns non-zero ("false") status
    

one-or-more-commands


fi

You can omit the else and the block of code following it. However, you can't omit the then or the fi . If you want to omit the then (i.e., if you want to do something special when condition is false, but nothing when it is true), write the statement like this:

if 

conditional


then
    :    # do nothing
else
    # do this if 

conditional

 returns non-zero ("false") status
    

one-or-more-commands


fi

Note that this uses a special null command, a colon ( : ) ( 45.9 ) . There's another, more useful way of expressing the inverse of a condition (do something if conditional is not "true"), the || operator ( 44.9 ) (two vertical bars).

Don't forget the fi terminating the statement. This is a surprisingly common source of bugs. (At least for me.)

Another common debugging problem: the manual pages that discuss this material imply that you can smash the if , the then , and the else onto one line. Well, it's true, but it's not always easy. Do yourself a favor: write your if statements exactly like the one above. You'll rarely be disappointed, and you may even start writing programs that work correctly the first time.

Here's a real-life example: a shell script named bkedit that makes a backup copy of a file before editing it. If cp returns a zero status, the script edits the file; otherwise, it prints a message. (The $1 is replaced with the first filename from the command line - see article 44.15 .)







1>&2
 

#!/bin/sh
if cp "$1" "$1.bak"
then
    vi "$1"
else
    echo "bkedit quitting: can't make backup?" 1>&2
fi

You can try typing in that shell script and running it. Or, just type in the lines (starting with the if ) on a terminal running the Bourne shell; use a real filename instead of $1 .

The if statement is often used with a command named test ( 44.20 ) . The test command does a test and returns an exit status of 0 or 1.

- ML , JP


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