Unix Power Tools, 3rd Edition

Chapter 35. Shell Programming for the Uninitiated

Writing a Simple Shell Program

A shell script need be no more than a command line saved in a file. For example, let’s assume that you’d like a compact list of all the users who are currently logged in on the system.

A command like this might do the trick:

% who | cut -c1-8 | sort -u | pr -l1 -8 -w78 -t

A list of logged-in users should come out in columns, looking something like this:

abraham  appleton biscuit  charlie  charlott fizzie   howard   howie
hstern   jerry    kosmo    linda    ocshner  peterson root     ross
sutton   yuppie

We used four Unix commands joined with pipes:

1. who (Section 2.8) gives a list of all users.

2. cut -c1-8 (Section 21.14) outputs columns 1-8 of the who output — the usernames.

3. sort -u (Section 22.6) puts names in order and takes out names of users who are logged on more than once.

4. pr -l1 -8 -w78 -t (Section 21.15, Section 45.6) takes the list of usernames, one per line, and makes it into 8 columns on 78-character-wide lines. (The -l1 is the lowercase letter L followed by the digit 1.)

If you wanted to do this frequently, wouldn’t it be better if all you had to do was type something like:

% loggedin

to get the same result? Here’s how:

1. Start a text editor on a new file named loggedin.

2. If your system supports the special #! notation (Section 36.2) (and it probably does), the first line of the script file should be:

   #!/bin/sh

   Otherwise, leave the first line blank. (When the first line of a script is blank, most shells will start a Bourne shell to ...