Chapter 1. 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.
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
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
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++,
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. . . .
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 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,
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
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
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.
His countenance was bold and bashed not.