Bourne Shell Scripting/Running Commands

Before we can start any kind of examination of the abilities of the Bourne Shell and how you can tap into its power, we have to cover some basic ground first: we have to discuss how to enter commands into the shell for execution by that shell.

Taking another look at what you've probably already seen
If you have access to a Unix-based machine (or an emulator on another operating system), you've probably been using the Bourne Shell -- or one of its descendants -- already, possibly without realising. Surprise: you've been doing shell scripting for a while already!

In your Unix environment, go to a terminal; either a textual logon terminal, or a terminal-in-a-window if you're using the X Window System (look for something called xterm or rxvt or just terminal, if you have actually not ever done this yet). You'll probably end up looking at a screen looking something like this:

Ben_Tels:Local_Machine:~>_

or The admin says: everybody, STOP TRYING TO CRASH THE SYSTEM Have a lot of fun! bzt:Another_Machine:~>_

or even something as simple as

$_

That's it. That's your shell: your direct access to everything the system has to offer.

Using the shell in interactive mode
Specifically, the program you accessed a moment ago is your shell, running in interactive mode: the shell is running in such a way that it displays a prompt and a cursor (the little, blinking line) and is waiting for you to enter a command for it to execute. You execute commands in interactive mode by typing them in, followed by a press of the Enter key. The shell then translates your command to something the operating system understands and passes off control to the operating system so that it can actually carry out the task you have sent it. You'll notice that your cursor will disappear momentarily while the command is being carried out, and you cannot type anymore (at this point, the Bourne Shell program is no longer in control of your terminal -- the other program that you started  by executing your command is). At some point the operating system will be finished working on your command and the shell will bring up a new prompt and the cursor as well and will then start waiting again for you to enter another command. Give it a try: type the command


 * ls enter

After a short time, you'll see a list of files in the working directory (the directory that your shell considers the "current" directory), a new prompt and the cursor.

This is the simplest way of executing shell commands: typing them in one at a time and waiting for each to complete in order. The shell is used in this way very often, both to execute commands that belong to the Bourne Shell programming language and simply to start running other programs (like the ls program from the example above).

A useful tidbit
Before we move on, we'll mention two useful key combinations when using the shell: the command to interrupt running programs and shell commands and the command to quit the shell (although, why you would ever want to stop using the shell is beyond me....).

To interrupt a running program or shell command, hit the Control and C keys at the same time. We'll get back to what this does exactly in a later chapter, but for now just remember this is the way to interrupt things.

To quit the shell session, hit Control+d. This key combination produces the Unix end-of-file character -- we'll talk more later about why this also terminates your shell session. Some modern shells have disabled the use of Control+d in favor of the "exit" command (shame on them). If you're using such a shell, just type the word "exit" (like with any other command) and press Enter (from here on in, I'll leave the "Enter" out of examples).

The only slightly less easy way: the script
As we saw in the last section, you can very easily execute shell commands for all purposes by starting an interactive shell session and typing your commands in at the prompt. However, sometimes you have a set of commands that you have to repeat regularly, even at different times and in different shell sessions. Of course, in the programming-centric environment of a Unix system, you can write a program to get the same result (in the C language for instance). But wouldn't it be a lot easier to have the convenience of the shell for this same task? Wouldn't it be more convenient to have a way to replay a set of commands? And to be able to compose that set as easily as you can write the single commands that you type into the shell's interactive sessions?

The shell script
Fortunately, there is such a way: the Bourne Shell's non-interactive mode. In this mode, the shell doesn't have a prompt or wait for your commands. Instead, the shell reads commands from a text file (which tells the shell what to do, kind of like an actor gets commands from a script -- hence, shell script). This file contains a sequence of commands, just as you would enter them into the interactive session at the prompt. The file is read by the shell from top to bottom and commands are executed in that order.

A shell script is very easy to write; you can use any text-editor you like (or even any wordprocessor or other editor, as long as you remember to save your script in plain text format). You write commands just as you would in the interactive shell. And you can run your script the moment you have saved it; no need to compile it or anything.

Running a shell script
To run a shell script (to have the shell read it and execute all the commands in the script), you enter a command at an interactive shell prompt as you would when doing anything else (if you're using a graphical user interface, you can probably also execute your scripts with a click of the mouse). In this case, the program you want to start is the shell program itself. For instance, to run a script called MyScript, you'd enter this command in the interactive shell (assuming the script is in your working directory):

Running a script

Starting the shell program from inside the shell program may sound weird at first, but it makes perfect sense if you think about it. After all, you're typing commands in an interactive mode shell session. To run a script, you want to start a shell in non-interactive mode. That's what's happening in the above command. You'll note that the Bourne Shell executable takes a single parameter in the example above: the name of the script to execute.

If you happen to be using a POSIX 1003.1-compliant shell, you can also execute a single command in this new, non-interactive session. You have to use the -c command-line switch to tell the shell you're passing in a command instead of the name of a script:

Running a command in a new shell

We'll get to why you would want to do this (rather than simply enter your command directly into the interactive shell) a little further down.

There is also another way to run a script from the interactive shell: you type the execute command (a single period) followed by the name of the script:

Sourcing a script

The difference between that and using the sh command is that the sh command starts a new process and the execute command does not. We'll look into this (and its importance) in the next section. By the way, this notation with the period is commonly referred to as sourcing a script.

Running a shell script the other way
There is also another way to execute a shell script, by making more direct use of a feature of the Unix operating system: the executable mode.

In Unix, each and every file has three different permissions (read, write and execute) that can be set for three different entities: the user who owns the file, the group that the file belongs to and "the world" (everybody else). Give the command

in the interactive shell to see the permissions for all files in the working directory (the column with up to nine letters, r, w and x for read write and execute, the first three for the user, the middle ones for the group, the right ones for the world). Whenever one of those entities has the "execute" permission, that entity can simply run the file as a program. To make your scripts executable by everybody, use the command

as in

Making MyScript executable

You can then execute the script with a simple command like so (assuming it is in a directory that is in your PATH, the directories that the shell looks in for programs when you don't tell it exactly where to find the program):

Running a command in a new shell

If this fails then the current directory is probably not in your PATH. You can force the execution of the script using

Making the shell look for your script in the current directory

At this command, the operating system examines the file, places it in memory and allows it to run like any other program. Of course, not every file makes sense as a program; a binary file is not necessarily a set of commands that the computer will recognize and a text file cannot be read by a computer at all. So to make our scripts run like this, we have to do something extra.

As we mentioned before, the Unix operating system starts by examining the program. If the program is a text file rather than a binary one (and cannot simply be executed), the operating system expects the first line of the file to name the interpreter that the operating system should start to interpret the rest of the file. The line the Unix operating system expects to find looks like this:

In our case, the following line should work pretty much everywhere:

The Bourne Shell executable, to be found in the bin directory, which is right under the top of the filesystem tree. For example:

Bourne shell script with an explicit interpreter

Executing shell scripts like this has several advantages. First it's less cumbersome than the other notations (it requires less typing). Second, it's an extra safety if you're going to pass your scripts around to others. Instead of relying on them to have the right shell, you can simply specify which shell they should use. If Bourne Shell is enough, that's what you ask for. If you absolutely need ksh or bash, you specify that instead (mind you, it's not foolproof &mdash; other people can ignore your interpreter specification by running your script with one of the other commands that we discussed above, even if the script probably won't work if they do that).

Just as a sidenote, Unix doesn't limit this trick to shell scripts. Any script interpreter that expects its scripts to be plain-text can be specified in this way. You can use this same trick to make directly executable Perl scripts or Python, Ruby, etc. scripts as well as Bourne Shell scripts.

Note also that with the distributions using bash as their default shell, you can use the #!/bin/sh shebang and have typical bash syntax in your script. It will work. But for the same script to work with a distribution not using bash as its default shell (as example Debian), you will have to modify the script or to change its shebang to #!/bin/bash.

Why you want to know about multiprocessing
While this is not directly a book about Unix, there are some aspects of the Unix operating system that we must cover to fully understand why the Bourne Shell works the way it does from time to time.

One of the most important aspects of the Unix operating system – in fact, the main aspect that sets it apart from all other main-stream operating systems – is that the Unix Operating System is and always has been a multi-user, multi-processing operating system (this in contrast with other operating systems like MacOS and Microsoft's DOS/Windows operating systems). The Unix OS was always meant to run machines that would be used simultaneously by several users, who would all want to run at least one but possibly several programs at the same time. The ability of an operating system to divide the time of a machine's processor among several programs so that it seems to the user that they are all running at the same time is called multiprocessing. The Unix Operating System was designed from the core up with this possibility in mind and it has an effect on the way your shell sessions behave.

Whenever you start a new process (by running a program, for instance) on your Unix machine, the operating system provides that process with its very own operating environment. That environment includes some memory for the process to play in and it can also include certain predefined settings for all processes. Whenever you run the shell program, it is running in its own environment.

Whenever you start a new process from another process (for instance by issuing a command to your shell program in interactive mode), the new process becomes what is called a child process of the first process (the ls program runs as a child process of your shell, for instance). This is where it becomes important to know about multiprocessing and process interaction: a child process always starts with a copy of the environment of the parent process. This means two things:


 * 1) A child process can never make changes to the operating environment of its parent—it only has access to a copy of that environment;
 * 2) If you actually do want to make changes in the environment of your shell (or specifically want to avoid it), you have to know when a command runs as a child process and when it runs within your current shell; you might otherwise pick a variant that has the opposite effect of that which you want.

What does what
We have seen several ways of running a shell command or script. With respect to multiprocessing, they run in the following way:

A useful thing to know: background processes
With the above, it may seem like multiprocessing is just a pain when doing shell scripting. But if that were so, we wouldn't have multiprocessing—Unix doesn't tend to keep things that aren't useful. Multiprocessing is a valuable tool in interacting with the rest of the system and one that you can use to work more efficiently. There are many books available on the benefits of multiprocessing in program development, but from the point of view of the Bourne Shell user and scripter the main one is the ability to hand off control of a process to the operating system and still keep on working while that child process is running. The way to do this is to run your process as a background process.

Running a process as a background process means telling the operating system that you want to start a process, but that it should not attach itself to any of the interactive devices (keyboard, screen, etc.) that its parent process is using. And more than that, it also tells the operating system that the request to start this child process should return immediately and that the parent process should then be allowed to continue working without having to wait for its child process to end.

This sounds complicated, but you have to keep in mind that this ability is completely ingrained in the Unix operating system and that the Bourne Shell was intended as an easy interface to the power of Unix. In other words: the Bourne Shell includes the ability to start a child process as a simple command of its own. Let's demonstrate how to do this and how useful the ability is at the same time, with an example. Give the following (rather pointless but still time consuming) command at the prompt:


 * N=0 && while [ $N -lt 10000 ]; do date >> scriptout; N=`expr $N + 1`; done

We'll get into what this says in later chapters; for now, it's enough to know that this command asks the system for the date and time and writes the result to a file named "scriptout". Since it then repeats this process 10000 times, it may take a little time to complete.

Now give the following command:


 * N=0 && while [ $N -lt 10000 ]; do date >> scriptout; N=`expr $N + 1`; done&

You'll notice that you can immediately resume using the shell (if you don't see this happening, hit Control+C and check that you have the extra ampersand at the end). After a while the background process will be finished and the scriptout file will contain another 10000 time reads.

The way to start a background process in Bourne Shell is to append an ampersand (&amp;) to your command.

Remarks
Actually, you can force a child process here as well -- we'll see how when we talk about command grouping