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.
Shell scripting hearkens back to the classic 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.
According to Herbert Mayer, "a useful language needs arrays, pointers, and a generic mechanism for building data structures." By these criteria, shell scripting falls somewhat short of being "useful." Or, perhaps not. . . .
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When not to use shell scripts
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Resource-intensive tasks, especially where speed is a factor (sorting, hashing, recursion [2] ...)
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Procedures involving heavy-duty math operations, especially floating point arithmetic, arbitrary precision calculations, or complex numbers (use C++ orFORTRAN instead)
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Cross-platform portability required (use C or Java instead)
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Complex applications, where structured programming is a necessity (type-checking of variables, function prototypes, etc.)
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Mission-critical applications upon which you are betting the future of the company
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Situations where security is important, where you need to guarantee the integrity of your system and protect against intrusion, cracking, and vandalism
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Project consists of subcomponents with interlocking dependencies
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Extensive file operations required (Bash is limited to serial file access, and that only in a particularly clumsy and inefficient line-by-line fashion.)
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Need native support for multi-dimensional arrays
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Need data structures, such as linked lists or trees
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Need to generate / manipulate graphics or GUIs
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Need direct access to system hardware or external peripherals
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Need port or socket I/O
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Need to use libraries or interface with legacy code
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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.
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.