Chapter 1. Why Shell Programming?

The shell is a command interpreter. It is the insulating layer between the operating system kernel and the user. Yet, it is also a fairly powerful programming language. A shell program, called a script , is an easy-to-use tool for building applications by "gluing" together system calls, tools, utilities, and compiled binaries. Virtually the entire repertoire of UNIX commands, utilities, and tools is available for invocation by a shell script. If that were not enough, internal shell commands, such as testing and loop constructs, give additional power and flexibility to scripts. Shell scripts lend themselves exceptionally well to to administrative system tasks and other routine repetitive jobs not requiring the bells and whistles of a full-blown tightly structured programming language.

A working knowledge of shell scripting is essential to everyone wishing to become reasonably adept at system administration, even if they do not anticipate ever having to actually write a script. Consider that as a Linux machine boots up, it executes the shell scripts in /etc/rc.d to restore the system configuration and set up services. A detailed understanding of these scripts is important for analyzing the behavior of a system, and possibly modifying it.

Writing shell scripts is not hard to learn, since the scripts can be built in bite-sized sections and there is only a fairly small set of shell-specific operators and options to learn. The syntax is simple and straightforward, similar to that of invoking and chaining together utilities at the command line, and there are only a few "rules" to learn. Most short scripts work right the first time, and debugging even the longer ones is straightforward.

A shell script is a "quick and dirty" method of prototyping a complex application. Getting even a limited subset of the functionality to work in a shell script, even if slowly, is often a useful first stage in project development. This way, the structure of the application can be tested and played with, and the major pitfalls found before proceeding to the final coding in C, C++, Java, or Perl.

Shell scripting hearkens back to the classical UNIX philosophy of breaking complex projects into simpler subtasks, of chaining together components and utilities. Many consider this a better, or at least more esthetically pleasing approach to problem solving than using one of the new generation of high powered all-in-one languages, such as Perl, which attempt to be all things to all people, but at the cost of forcing you to alter your thinking processes to fit the tool.

When not to use shell scripts

• resource-intensive tasks, especially where speed is a factor

• complex applications, where structured programming is a necessity

• file handling (Bash is limited to serial file access, and that only in a particularly clumsy and inefficient line-by-line fashion)

• need to generate or manipulate graphics or GUIs

• need direct access to system hardware

• need port or socket I/O

• need to use libraries or interface with legacy code

If any of the above applies, consider a more powerful scripting language, perhaps Perl, Tcl, Python, or even a high-level compiled language such as C, C++, or Java. Even then, prototyping the application as a shell script might still be a useful development step.

We will be using Bash, an acronym for "Born-Again Shell" and a pun on Stephen Bourne's now classic Bourne Shell. Bash has become the de facto standard for shell scripting on all flavors of UNIX. Most of the principles dealt with in this document apply equally well to scripting with other shells, such as the Korn Shell, from which Bash derives some of its features, and the C Shell and its variants. (Note that C Shell programming is not recommended due to certain inherent problems, as pointed out in a news group posting by Tom Christiansen in October of 1993).

The following is a tutorial in shell scripting. It relies heavily on examples to illustrate features of the shell. As far as possible, the example scripts have been tested, and some of them may actually be useful in real life. The reader should cut out and save the examples, assign them appropriate names, give them execute permission (chmod u+x scriptname), then run them to see what happens. Note that some of the scripts below introduce features before they are explained, and this may require the reader to temporarily skip ahead for enlightenment.

Unless otherwise noted, the author of this document wrote the example scripts that follow.