The Bash Shell
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If you only use Bash, you can put your definitions in .bashrc. This ensures that the defi-
nitions take effect whenever you start any new shell. If you start a shell of a different type,
then the two bash files (.bashrc and .bash_profile) do not execute. However, there are other,
similar, files for other shells. For instance, as .bashrc runs whenever
a new Bash shell is
started, .cshrc runs whenever a new C-shell (csh) is started.
Note that making changes to one of these files will not immediately impact your current
session(s). These files are only read when you log in or start a new bash session. In order for
you to force the changes to take effect, you can use the instruction
source
on the given
file, as in
source ~/.bashrc
. This causes the interpreter to read the file,
executing the
statements therein.
2.5 OTHER SHELLS
Bash is only one shell available in Linux and Unix operating systems. Linux and Unix
usually come with other shells that the user can select either from within a bash shell or
the system administrator can specify as their default shell. Let us briefly compare the Bash
shell to other shells.
The earliest shell was called the Thompson shell, introduced in 1971 in Unix. This was
replaced in 1977 by the Bourne shell, again in Unix.
Both shells were denoted by sh, the
name of the program that would provide the shell environment and interpreter.
The primary features of the Bourne shell were that shell scripts (programs) could be run
by name as if they were any other program in Unix. Programs run in a Bourne shell could
be interactive or run in a batch processing mode. Redirection and pipes were made avail-
able. Variables were typeless, meaning that any variable could change the type of entity
it stored at any time. Quotation marks were introduced for use in commands. Variables
could be exported.
In addition, the Thompson shell defined a number of programming
instructions sep-
arate from the Unix kernel for use in shell scripts. The Bourne shell defined additional
instructions such as nested if–then–else constructs, a case (switch) statement, and a while
loop. The syntax for the Bourne shell programming constructs was loosely based on Algol.
In the late 1970s, Bill Joy wrote a new Unix shell that he called C shell (csh). The main
differences between the Bourne shell and the C shell are that the syntax is largely based
on the C programming language and the C shell contains a number of useful editing and
shortcut features. In fact, because of
the utility of these features, they have been incorpo-
rated into many shells since C shell. Among the editing and shortcut features introduced
in C shell were:
• History
• Command line editing
• Aliases
• Directory stack (we visit this in Chapter 3)
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Linux with Operating System Concepts
• Tilde completion
• Escape completion (in Bash, it is tab completion)
• Job control (covered in Chapter 4)
A variation of the C shell was created between 1975 and 1983, known as TC shell (tcsh)
where the ‘T’ indicates the TENEX operating system. The TC shell, or the T shell for short,
added greater operability to accessing commands in the history. Table 2.7
shows the varia-
tions made available in TC shell.
The KornShell (ksh) was developed by David Korn of Bell Labs and released in 1983. It
shares many of the same features as C Shell but includes WYSIWYG style command line
editing features, unlike the emacs-style command line features of Bash, C shell, and TC
shell. It also includes floating point variables and operators.
The next shell released was the Almquist shell, known as ash. This was released in
1989. It is a variation of the Bourne shell written for computers with modest resources
because it was smaller and faster than the Bourne shell. Later versions began to incorpo-
rate features found in C shell such as command line editing and history. The ash shell has
largely
been replaced by dash, a newer version available in Debian Linux, or discarded
altogether.
In 1989, Brian Fox released the Bourne Again Shell (Bash) for the GNU project. As we
have already explored the Bash shell through this chapter, we will not go over the details
here. However, as you have seen, it shares many features with C Shell and TC Shell.
Finally, in 1990, the Z shell (zsh) was released as an extension of both the Bourne shell
and the Korn shell. In addition to the features obtained
from the previous shells, it adds
spelling correction, customizable prompts, and modules that directly support both net-
working and mathematical computations. In addition, some shells have adopted previous
shells and added features to support specific programming languages, whether they be Java
(Rhino, BeanShell), Perl (psh), SQL (sqsh), or Scheme (scsh).
Most users today will either use a variation of Bourne/Bash or C Shell (often TC shell).
So here, we conclude with a comparison between TC Shell and Bash.
The differences, rel-
evant to the discussion of this chapter, are illustrated in Table 2.8. If features are not listed
TABLE 2.7
TC Shell History Commands
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