Better Command Execution with bash (
Page 1 of 5 )
In this second part of a two-part series on executing commands with the bash shell, you will learn how to use fewer if statements, display error messages when failures occur, and more. This article is excerpted from chapter four of the bash Cookbook, Solutions and Examples for bash Users, written by Carl Albing, JP Vossen and Cameron Newham (O'Reilly, 2007; ISBN: 0596526784). Copyright © 2007 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.
4.6 Using Fewer if Statements
Problem
As a conscientious programmer, you took to heart what we described in the previous recipe, Recipe 4.5. "Deciding Whether a Command Succeeds." You applied the concept to your latest shell script, and now you find that the shell script is unreadable, if with all those if statements checking the return code of every command. Isn’t there an alternative?
Solution
Use the double-ampersand operator in bash to provide conditional execution:
$ cd mytmp && rm *
Discussion
Two commands separated by the double ampersands tells bash to run the first command and then to run the second command only if the first command succeeds (i.e., its exit status is 0). This is very much like using an if
statement to check the exit status of the first command in order to protect the running of the second command:
cd mytm
p
if (( $? )); then rm * ; fi
The double ampersand syntax is meant to be reminiscent of the logical and operator in C Language. If you know your logic (and your C) then you’ll recall that if you are evaluating the logical expression
A AND B
, then the entire expression can only be true if both (sub)expression
A
and (sub)expression
B
evaluate to true. If either one is false, the whole expression is false. C Language makes use of this fact, and when you code an expression like
if (A && B) { ... }
, it will evaluate expression
A
first. If it is false, it won’t even bother to evaluate
B
since the overall outcome (false) has already been determined (by
A
being false).
So what does this have to do with bash? Well, if the exit status of the first command (the one to the left of the
&&
) is non-zero (i.e., failed) then it won’t bother to evaluate the second expression—i.e., it won’t run the other command at all.
If you want to be thorough about your error checking, but don’t want
if
statements all over the place, you can have bash exit any time it encounters a failure (i.e., a non-
zero exit status) from every command in your script (except in
while
loops and
if
statements where it is already capturing and using the exit status) by setting the
-e
flag.
set -
e
cd mytmp
rm *
Setting the -e flag will cause the shell to exit when a command fails. If the cd fails, the script will exit and never even try to execute the rm * command. We don’t recommend doing this on an interactive shell, because when the shell exits it will make your shell window go away.
See Also
• Recipe 4.8, "Displaying Error Messages When Failures Occur" for an explanation of the || syntax, which is similar in some ways, but also quite different from the && construct
