Classic Shell Scripting

A Simple Script

Let's start with a simple script. Suppose that you'd like to know how many users are currently logged in. The who command tells you who is logged in:

$ who
george     pts/2        Dec 31 16:39    (valley-forge.example.com)
betsy      pts/3        Dec 27 11:07    (flags-r-us.example.com)
benjamin   dtlocal      Dec 27 17:55    (kites.example.com)
jhancock   pts/5        Dec 27 17:55    (:32)
camus      pts/6        Dec 31 16:22
tolstoy    pts/14       Jan  2 06:42

On a large multiuser system, the listing can scroll off the screen before you can count all the users, and doing that every time is painful anyway. This is a perfect opportunity for automation. What's missing is a way to count the number of users. For that, we use the wc (word count) program, which counts lines, words, and characters. In this instance, we want wc -l, to count just lines:

$ who | wc -l                       
            Count users
      6

The | (pipe) symbol creates a pipeline between the two programs: who's output becomes wc's input. The result, printed by wc, is the number of users logged in.

The next step is to make this pipeline into a separate command. You do this by entering the commands into a regular file, and then making the file executable, with chmod, like so:

$ cat > nusers                      
            Create the file, copy terminal input with cat
            who | wc -l                         
            Program text
            ^D                                  
            Ctrl-D is end-of-file
$ chmod +x nusers                   
            Make it executable
$ ./nusers                          
            Do a test run
      6                             Output is what we expect

This shows the typical development cycle for small one- or two-line shell scripts: first, you experiment directly at the command line. ...