Bourne Shell Scripting/Parameters

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[edit] Debugging Flags

Its highly recommended that the first line of any bourne-shell script is :

#!/bin/sh

( see earlier section TBD )

To switch on debugging add in the '-x' flag to this line; for example:

#!/bin/sh -x
echo hello

Running this gives:

+ echo hello
hello

Before the 'echo' command is run by the shell, it outputs the command and arguments preceded with the '+' symbol, then the actual result of the command can be seen.

This second simple example, shows how environment variables are dealt with in this mode:

#!/bin/sh -x
MYVAR=hello
echo $MYVAR

Running this gives:

MYVAR=hello
+ echo hello
hello

Note that when the 'echo' is run, the variable 'MYVAR' is resolved as 'hello' in the output; and you will not be able to tell from this alone that there was an enviroment variable involved here.

Note earlier on , that you can see the variable being set (note also that there is no preceding '+' symbol on that line).

Another flag that is sometimes useful to combine (although it can be run on its own as well) with the '-x' flag is the '-v' flag, which simply shows the 'source' lines of the script before they are run:

#!/bin/sh -xv

Reading the output from a complicated shell script can be tricky - you have to separate out the debug output from the normal output; so sometimes you might want to redirect the debug information to a separate file like this:

#!/bin/sh -x
exec 2> /tmp/debuginfo.txt

Note that other outputs (like errors generated from commands) will also be placed in the '/tmp/debug.info.txt' file.

[edit] Breaking out of a script

When debugging a shell script, its sometimes to useful to exit out and certain points. Use the 'exit' built-in to do this - you can specify an output value with the command as well, which can be useful to 'trap' specific conditions (ie use different error conditions for different parts of the script).

#!/bin/sh -x
echo hello
exit 1

If you run this script, then you can also test the output status - using the "$?" built-in variable:

echo $?
1