Using shell functions is a simple way to group and store a set of commands for later execution. The general form of a shell function definition is:

	name() { list; }

where name is the command name you will use to execute the function and list is a list of commands or pipelines, separated by semi-colons, which specify what task the function will perform.

For example, if you enter the following function definition:

	$ ll() { ls -l; }

you will then be able to use ll as a command to perform the same task as ls -l:

	$ ll
	drwxr-xr-x	3 PC	book	1024 May 28 23:30 book

You might imagine that if you wanted to execute the command ls -al, given that the definition for ll already exists, you could achieve this with the command ll -a. Sadly, however, it isn't quite that simple. It is possible to do what we want, but in order to do it the definition for the ll function needs to be changed as follows:

	$ ll() { ls -l $*; }

The addition of the $* characters means that any parameters or command line switches entered after the ll command are to be inserted into the ls -l command in place of the $* characters. Using the new definition for ll gives the expected results:

	$ ll -a
	-rw-r--r--	1 pc	book	3392	May	29	07: 55	.bash_history
	-rw-------	1 pc	book	25	Mar	26      1994	.profue
	drwxr-xr-x	3 PC	book	1024	May	28	23:30	book

A more complete discussion of $* and related topics will be given in another tutorial (possibly upload tomorrow), when we look at shell programming.

The body of a shell function (between { and }) can consist of several commands or pipelines, as long as they are separated by semi-colons. Here is an example (coded in a rather inefficient way just for demonstration purposes) to display the number of files contained in a given directory. The function is called lswc as it combines the actions of these two commands:

	$ lswc() { ls $* >/tmp/dir; wc -w </tmp/dir; rm /tmp/dir; }

The use of the $* characters in the definition allows lswc to take a parameter which should be the pathname of the directory upon which the command is to operate:

	$ lswc /usr/bin
	    459

Next: Questions

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