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Shell tips 1: Options

December 4, 2010

The shell has many options: two of them are particularly useful for programme development.
Although they can be set on invocation like other options, it is generally more useful to use the “set” command which
can be placed within a script.
In most cases these lines should be added at the top of the file
To make your life easier, add the line

set -u

to the beginning of you scripts. This is the rough equivalent of
PERL’s “use strict”. It will give you an error message if you
attempt to use an undefined variable. Undefined variables are
a common headache and cause all sorts of weird behaviour. For instance

if [ -f $MyFile ]

will evaluate to

if [ -f ]

if MyFile is unassigned, which is a *syntax error*.
Another example:

cat $MyFile


will *hang on inpu* if myfile is undefined, since that is what

cat

by itself does. And you don’t get any error message, the script just stops!

If you find you have mysterious hanging problems or othe puzzling
behaviour another global option you can use is

set -x

which basically prints each line (on stderr) as it is executed.
We do not recommend you leave
this switched on all the time.

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