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Tip

In general, you should not have the current directory in your path at all. It represents a security problem. (See Chapter 15 for more information.) We describe empty components only so that you understand how path searching works.

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[5] Interestingly enough, the current version of the standard has echo being essentially the same as the System V version, which processes escape sequences in its arguments and does not treat -n specially.

[6] "Standard I/O," as used here, should not be confused with the C library's standard I/O library, whose interface is defined in , although that library's job is to provide this abstraction to C programs.

[7] stty is possibly the most baroque and complicated Unix command in existence. See the stty(1) manpage for the gory details, or Unix in a Nutshell.

Accessing Shell Script Arguments

The so-called positional parameters represent a shell script's command-line arguments. They also represent a function's arguments within shell functions. Individual arguments are named by integer numbers. For historical reasons, you have to enclose the number in braces if it's greater than nine:

echo first arg is $1

echo tenth arg is ${10}

Special "variables" provide access to the total number of arguments that were passed, and to all the arguments at once. We provide the details later, in Section 6.1.2.2.

Suppose you want to know what terminal a particular user is using. Well, once again, you could use a plain who command and manually scan the output. However, that's difficult and error prone, especially on systems with lots of users. This time what you want to do is search through who's output for a particular user. Well, anytime you want to do searching, that's a job for the grep command, which prints lines matching the pattern given in its first argument. Suppose you're looking for user betsy because you really need that flag you ordered from her:

$ who | grep betsy

Where is betsy?

betsy pts/3 Dec 27 11:07 (flags-r-us.example.com)

Now that we know how to find a particular user, we can put the commands into a script, with the script's first argument being the username we want to find:

$ cat > finduser

Create new file

#! /bin/sh

# finduser --- see if user named by first argument is logged in

who | grep $1

^D

End-of-file

$ chmod +x finduser

Make it executable

$ ./finduser betsy

Test it: find betsy

betsy pts/3 Dec 27 11:07 (flags-r-us.example.com)

$ ./finduser benjamin

Now look for good old Ben

benjamin dtlocal Dec 27 17:55 (kites.example.com)

$ mv finduser $HOME/bin

Save it in our personal bin

The line beginning with # finduser ... is a comment. The shell ignores everything from the # to the end of the line. (This is serendipitous; the special #! line described earlier acts as a comment when the shell reads a script.) Commenting your programs is always a good idea. It will help someone else, or you a year from now, to figure out what you were doing and why. Once we see that the program works, we move it to our personal bin directory.

This program isn't perfect. What happens if we don't give it any arguments?

$ finduser

Usage: grep [OPTION]... PATTERN [FILE]...

Try 'grep --help' for more information.

We will see in Section 6.2.4, how to test the number of command-line arguments and take appropriate action when the right number isn't supplied.

Simple Execution Tracing

Because program development is a human activity, there will be times when your script just doesn't do what you want it to do. One way to get some idea of what your program is doing is to turn on execution tracing. This causes the shell to print out each command as it's executed, preceded by "+ "—that is, a plus sign followed by a space. (You can change what gets printed by assigning a new value to the PS4 shell variable.) For example:

$ sh -x nusers

Run with tracing on

+ who Traced commands

+ wc -l

7 Actual output

You can turn execution tracing on within