Table of Contents
Ultimate Differences Amongst UNIX® Shells
This tutorial is a work in progress (WIP).
Introduction
The UNIX shells described in this tutorial include: osh (a port of the Thompson shell), sh (and sh-like shells), csh (and csh-like shells), and others.
What are the critical differences and/or similarities among the various UNIX shells? The differences are generally command-line syntax issues.
In the following sections, each example command line is noted by “Ecl [123]:”. You can get an idea of how the shells differ by comparing the “Ecl [123]:” from each section against its corresponding “Ecl [123]:” in the other sections.
The similarities among the UNIX shells described in this tutorial are more important than the differences. This is because when you learn how to use one shell, you in effect also learn how to use the others. In essence, each of the shells plays the same role from the user's perspective. That role is to act as an interface to the functionality which is available in the UNIX operating system.
Thompson Shell
In UNIX history, the Thompson shell preceded both the Bourne shell and the C shell, and its command language is a subset of both of these shells. Control flow in the Thompson shell is implemented via if(1) and goto(1), which are external shell utilities. However, flow control in the Bourne and C shells is built into each shell.
An enhanced port of the Thompson shell is available on SDF as /usr/pkg/bin/osh. You can read its manual by doing a man osh at the command prompt. You can also read its manual online if you prefer to do so.
Ecl 1:
% setenv PATH /usr/pkg/bin:/usr/bin:/bin ; printenv PATH /usr/pkg/bin:/usr/bin:/bin
Ecl 2:
% which osh if goto ^ ( tr '\n' ' ' ; echo ) ^ sed 's/.*/ls -l &/' ^ osh -r-xr-xr-x 1 root wheel 7880 Jul 18 21:37 /usr/pkg/bin/goto -r-xr-xr-x 1 root wheel 14160 Jul 18 21:37 /usr/pkg/bin/if -r-xr-xr-x 1 root wheel 38216 Jul 18 21:37 /usr/pkg/bin/osh
Ecl 3:
% if -r /netbsd echo /netbsd: Is readable ; \ if -w /netbsd echo /netbsd: Is writable /netbsd: Is readable
…
Notice that the setenv command above is not available in the original Thompson shell. This is just one of the osh enhancements. Do we want to say anything about the PWB shell?
Bourne Shell
This includes the original Bourne shell, its derivatives, and workalikes. A true Bourne shell or a Bourne-like shell typically serves as the standard command interpreter on most UNIX systems.
A Bourne-shell workalike is available on SDF as /bin/sh. You can read its manual by doing a man sh at the command prompt. You can also read its manual online if you prefer to do so.
Ecl 1:
$ PATH=/usr/pkg/bin:/usr/bin:/bin ; export PATH ; printenv PATH /usr/pkg/bin:/usr/bin:/bin
Ecl 2:
$ ls -l `which sh ksh bash` -r-xr-xr-x 1 root wheel 298431 Oct 24 2005 /bin/ksh -r-xr-xr-x 1 root wheel 199717 Oct 24 2005 /bin/sh -rwxr-xr-x 1 root wheel 963696 Jun 4 08:57 /usr/pkg/bin/bash
Ecl 3:
$ if test -r /netbsd ; then echo /netbsd: Is readable ; fi ; \ > if test -w /netbsd ; then echo /netbsd: Is writable ; fi /netbsd: Is readable
Bourne-Again Shell
The Bourne-Again shell (Bash) was written for the GNU project as a free software replacement for the Bourne shell. Bash can execute the vast majority of Bourne shell scripts without modification. However, its syntax has many extensions that are not present in the Bourne shell.
The Bourne-Again shell is available on SDF as /usr/pkg/bin/bash. You can read its manual by doing a man bash at the command prompt.
Ecl 1:
$ PATH=/usr/pkg/bin:/usr/bin:/bin ; export PATH ; printenv PATH /usr/pkg/bin:/usr/bin:/bin
Ecl 2:
$ ls -l `which sh ksh bash` -r-xr-xr-x 1 root wheel 298431 Oct 24 2005 /bin/ksh -r-xr-xr-x 1 root wheel 199717 Oct 24 2005 /bin/sh -rwxr-xr-x 1 root wheel 963696 Jun 4 08:57 /usr/pkg/bin/bash
Ecl 3:
$ if test -r /netbsd ; then echo /netbsd: Is readable ; fi ; \ > if test -w /netbsd ; then echo /netbsd: Is writable ; fi /netbsd: Is readable
Z Shell
The Z shell (Zsh) is an extended Bourne shell with many improvements. Paul Falstad wrote the first version of Zsh in 1990 at Princeton University.
The Z shell is available on SDF as /usr/pkg/bin/zsh. You can read its manual by doing a man zsh at the command prompt.
Ecl 1:
% PATH=/usr/pkg/bin:/usr/bin:/bin ; export PATH ; printenv PATH /usr/pkg/bin:/usr/bin:/bin
Ecl 2:
% ls -l `which csh tcsh zsh` -r-xr-xr-x 1 root wheel 214333 Oct 24 2005 /bin/csh -r-xr-xr-x 1 root wheel 459408 Jun 29 2005 /usr/pkg/bin/tcsh -r-xr-xr-x 2 root wheel 6352 May 11 2007 /usr/pkg/bin/zsh
Ecl 3:
% if test -r /netbsd ; then echo /netbsd: Is readable ; fi ; \ > if test -w /netbsd ; then echo /netbsd: Is writable ; fi /netbsd: Is readable
C Shell
This includes the original C shell, its derivatives, and workalikes.
The C shell is available on SDF as /bin/csh. You can read its manual by doing a man csh at the command prompt. You can also read its manual online if you prefer to do so.
Ecl 1:
% setenv PATH /usr/pkg/bin:/usr/bin:/bin ; printenv PATH /usr/pkg/bin:/usr/bin:/bin
Ecl 2:
% ls -l `which csh tcsh zsh` -r-xr-xr-x 1 root wheel 214333 Oct 24 2005 /bin/csh -r-xr-xr-x 1 root wheel 459408 Jun 29 2005 /usr/pkg/bin/tcsh -r-xr-xr-x 2 root wheel 6352 May 11 2007 /usr/pkg/bin/zsh
Ecl 3:
% if (-r /netbsd) echo /netbsd: Is readable ; \ if (-w /netbsd) echo /netbsd: Is writable /netbsd: Is readable
Other Shells
This section will include information about the Z shell, rc (AT&T Plan 9 shell), and any other UNIX shell which does not neatly fall into one category..?
Conclusion
Expand on the differences/similarities in light of the preceding content. One important thing to notice is that though there are differences… The similarities are more important IMO. After all, it is the similarities that allow the user to switch from one shell to another with little difficulty.
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$Id: shells.html,v 1.1 2008/12/19 16:35:57 jwodder Exp $ Ultimate Differences Amongst UNIX® Shells - traditional link (using RCS)