The magic line causes a new shell (in this case, /bin/sh) to be called to execute the script. Without the magic line, the current shell is always used to evaluate the script, regardless of which shell the script was written for. For example, without a magic line, csh and tcsh users might not be able to get a Bourne shell (sh) script to run correctly.

Comments

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The magic first line #!/bin/sh introduces the topic of comments. A comment is a statement that is embedded in a shell script but should not be executed by the shell.

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In shell scripts, comments start with the # character. Everything between the # and end of the line are considered part of the comment and are ignored by the shell.

Adding comments to a script is quite simple: Open the script using an editor and add lines that start with the # character. For example, to add the following line to the logins shell script:

# print out the date and who's logged on

I opened the file logins with my editor and inserted this line as the second line in the file. The shell script is now as follows:

#!/bin/sh
# print out the date and who's logged on
date ; who ;

There is no change in the output of the script because comments are ignored. Also comments do not slow down a script because the shell can easily skip them.

You can also add comments to lines that contain commands by adding the # character after the commands. For example, you can add a comment to the line date ; who ; as follows:

date ; who ; # execute the date and who commands

When you are writing a shell script, make sure to use comments to explain what you are doing in case someone else has to look at your shell script. You might find that this helps you figure out what your own scripts are doing, months after you write them.

Getting Help

As you read through this book, you will want to get more information about the commands and features I discuss. This information is available by using the online help feature of UNIX.

Every version of UNIX comes with an extensive collection of online help pages called manual pages. These are often referred to as man pages. The man pages are the authoritative source about your UNIX system. They contain complete information about both the kernel and all the utilities.

Using the man Command

To access a man page you need to use the man (man as in manual) command as follows:

man command

Here, command is the name of a command that you want more information about. As an example,

$ man uptime

displays the following on a Solaris machine:

User Commands                                           uptime(1)

NAME
     uptime - show how long the system has been up

SYNOPSIS
     uptime

DESCRIPTION
     The uptime command prints the current time,  the  length  of
     time  the system has been up, and the average number of jobs
     in the run queue over the last 1, 5 and 15 minutes.  It  is,
     essentially,  the first line of a w(1) command.

EXAMPLE
     Below is an example of the output uptime provides:
          example% uptime
          10:47am  up 27 day(s), 50 mins,  1 user,
          ⇒load average: 0.18, 0.26, 0.20

SEE ALSO
     w(1), who(1), whodo(1M), attributes(5)

NOTES
     who -b gives the time the system was last booted.

As you can see this man page is divided into several sections. These sections are described in Table 2.1.