Shell Error Handling

1. Shell Error Handling

2. Error Handling • What do you do when your shell script encounters an error? • Exit the script • Ignore the error and keep on running • Assume the user messed up so don't worry about it • None of these is particularly attractive, especially for the user of your script

3. Handling Error Conditions • There are two major types of errors your scripts should check for and handle • Internal errors within your script • External errors caused by something not associated with your script • If you want to be known as someone who produces high quality robust scripts that are a pleasure to use, then you need to check for these types of errors and respond in an appropriate manner

4. Internal Errors • Generally caused by: • User not supplying needed information to the script or supplying invalid information • Some command failing to perform as expected • User error conditions are unique and need to be tested for on a case-by-case basis • You need to ensure that the use is not allowed to provide invalid input • You should also allow the user to correct the input if at all possible

5. To test if a command performed as expected, you need to test the return status of the command • Every time a shell command is executed the shell variable $? is set to the return status the command • Any time you execute a critical command in your script you should test the return status to ensure the command completed successfully • If it did not complete successfully then your script should take some reasonable corrective action

6. External Errors • Certain actions by the user or the system can cause errors which will interrupt your script, usually at the most inopportune time • These errors are usually captured by the Unix system and communicated to your script via signals • The system is telling your script that something has happened and some action may need to be taken • This action may be any valid shell command or commands or it may be no action at all! • You control how you script will respond by using the trap command

7. trap • Syntax: • trap "cmd(s)" signal_list • Example • trap "rm /tmp/krf*$$; exit 0" 1 2 3 14 15 • The trap command let's your script decide how to respond to signals passed from the OS • It is particularly useful to allow your script to clean up after itself in case of abnormal termination

8. Common Signals

9. Other trap Examples • trap "echo ${filename} > stopfile; exit 0" 1 2 3 14 • stopfile will contain the name of the last file used by your script • trap "" 1 2 3 • Your script will ignore signals 1, 2 and 3, the common ways users attempt to stop a running script • trap "" 1 • Keep running even if the user logs out • trap signal_number resets the trap for signal_number to the default action

10. By setting a trap for signal number 0, you can specify some action to take whenever the script exits with a 0 status (successful completion) • This can be used for cleanup routines or simply to log a message that the script ran properly • trap "echo "scriptname completed successfully at `date`" >> logfile" 0 • Executing trap with no arguments displays any traps that you have changed