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 &amp;/' ^ 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 ; \
  &gt; 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

Is this section needed?

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 $

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