Setting Up the Exit Status of a Script

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Introduction

In case you need a little reminder on how to use the exit status of programs and scripts this section will remind you how to do so.

How do I Know the Exit Status of a Program?

In the tcsh shell the way you check the exit status of a program is by executing the program first and then executing the command echo $?. For example take a look at the following examples:

First let's see a case in which the ls command exits normally:

> ls
addressbook  collections  config   filters     maildrop  other  scores
build        commands     drivers  index.html  misc      roles  
> echo $?
0

You see that the ls command exit status is 0.

Now you can see a case in which the same command above exits abruptly
```
> ls aaaa
ls: aaaa: No such file or directory
> echo $?
1
```
In this case the exit status was 1.

As you can see from these examples, the exit status depends on what happened during the execution. This is the way that the exit status is always used.

How Can I Use the Exit Status

You can use it in a script to determine if the completion of another command was successful or not. For example, the patch program has 3 different exit status: 0 if it applied the patch without problem, 1 and 2 if there was some problem, where 2 is a serious problem (see the manual). This could be used in a script as follows:

#!/bin/sh
PATCH="$1"
patch -f -p 1 < $PATCH  #apply the patch
if [ "$?" = "2" ]; then
   echo "Serious trouble to apply the patch $PATCH"
   echo "Check the output above"
else
   echo "No serious problems applying the patch"
fi

In this script two custom messages are printed depending on how the execution of the patch program ended. If there was a serious problem then a message indicating the name of the patch that failed is printed, otherwise a message indicating that nothing serious happened is printed.

How Can I Set the Exit Status

If you want to communicate to another program your exit status, use the code that you want to exit as an argument of the exit command. For example we can rewrite the above script so that it exits with status -1 if there was a serious error and 0 otherwise as follows:

#!/bin/sh
PATCH="$1"
patch -f -p 1 < $PATCH  #apply the patch
if [ "$?" = "2" ]; then
   echo "Serious trouble to apply the patch $PATCH"
   echo "Check the output above"
   exit -1
else
   echo "No serious problems applying the patch"
   exit 0
fi

Observe that in this way you have complete control over the exit status of a program. In other words, if a program exits with an exit status that you do not want the next program to see, you simply wrap it in a script, like above and make that script exit with the status that you like.

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