What is shell scripting

No programming language is perfect. There is not even a single best language; there are only languages well suited or perhaps poorly suited for particular purposes.

--Herbert Mayer

A working knowledge of shell scripting is essential to anyone wishing to become reasonably proficient 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 startup scripts is important for analyzing the behavior of a system, and possibly modifying it.

The craft of scripting is not hard to master, since scripts can be built in bite-sized sections and there is only a fairly small set of shell-specific operators and options [1] to learn. The syntax is simple -- even austere -- similar to that of invoking and chaining together utilities at the command line, and there are only a few "rules" governing their use. Most short scripts work right the first time, and debugging even the longer ones is straightforward.

In the early days of personal computing, the BASIC language enabled anyone reasonably computer proficient to write programs on an early generation of microcomputers. Decades later, the Bash scripting language enables anyone with a rudimentary knowledge of Linux or UNIX to do the same on modern machines.

We now have miniaturized single-board computers with amazing capabilities, such as the Raspberry Pi. Bash scripting provides a way to explore the capabilities of these fascinating devices.

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 script is often a useful first stage in project development. In this way, the structure of the application can be tested and tinkered with, and the major pitfalls found before proceeding to the final coding in C, C++, Java, Perl, or Python.

When not to use shell scripts

Resource-intensive tasks, especially where speed is a factor (sorting, hashing, recursion [2] ...)
Procedures involving heavy-duty math operations, especially floating point arithmetic, arbitrary precision calculations, or complex numbers (use C++ or FORTRAN instead)
Cross-platform portability required (use C or Java instead)
Complex applications, where structured programming is a necessity (type-checking of variables, function prototypes, etc.)
Mission-critical applications upon which you are betting the future of the company
Situations where security is important, where you need to guarantee the integrity of your system and protect against intrusion, cracking, and vandalism
Project consists of subcomponents with interlocking dependencies
Extensive file operations required (Bash is limited to serial file access, and that only in a particularly clumsy and inefficient line-by-line fashion.)
Need native support for multi-dimensional arrays
Need data structures, such as linked lists or trees
Need to generate / manipulate graphics or GUIs
Need direct access to system hardware or external peripherals
Need port or socket I/O
Need to use libraries or interface with legacy code
Proprietary, closed-source applications (Shell scripts put the source code right out in the open for all the world to see.)

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

The Bourne-Again Shell

His countenance was bold and bashed not.

--Edmund Spenser

We will be using Bash, an acronym [3] for "Bourne-Again shell" and a pun on Stephen Bourne's now classic Bourne shell. Bash has become a de facto standard for shell scripting on most flavors of UNIX. Most of the principles this book covers apply equally well to scripting with other shells, such as the Korn Shell, from which Bash derives some of its features, [4] and the C Shell and its variants. (Note that C Shell programming is not recommended due to certain inherent problems, as pointed out in an October, 1993 Usenet post by Tom Christiansen.)

What follows is a tutorial on shell scripting. It relies heavily on examples to illustrate various features of the shell. The example scripts work -- they've been tested, insofar as possible -- and some of them are even useful in real life. The reader can play with the actual working code of the examples in the source archive (scriptname.sh or scriptname.bash), [5] give them execute permission (chmod u+rx scriptname), then run them to see what happens. Should the source archive not be available, then cut-and-paste from the HTML or pdf rendered versions. Be aware that some of the scripts presented here 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 book wrote the example scripts that follow.

[1]These are referred to as builtins, features internal to the shell.[2]Although recursion is possible in a shell script, it tends to be slow and its implementation is often an ugly kludge.[3]An acronym is an ersatz word formed by pasting together the initial letters of the words into a tongue-tripping phrase. This morally corrupt and pernicious practice deserves appropriately severe punishment. Public flogging suggests itself.[4]Many of the features of ksh88, and even a few from the updated ksh93 have been merged into Bash.[5]By convention, user-written shell scripts that are Bourne shell compliant generally take a name with a .sh extension. System scripts, such as those found in /etc/rc.d, do not necessarily conform to this nomenclature.