Why Shell Programming?


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.
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.
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 [1] 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 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 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.
When not to use shell scripts
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Resource-intensive tasks, especially where speed is a factor (sorting, hashing, etc.)
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Procedures involving heavy-duty math operations, especially floating point arithmetic, arbitrary precision calculations, or complex numbers (use C++ or FORTRAN 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 (need type-checking of variables, function prototypes, etc.)
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Mission-critical applications upon which you are betting the ranch, or 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 or manipulate graphics or GUIs
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Need direct access to system hardware
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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 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 "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 all flavors of UNIX.