default shell. Features from the TENEX and TOPS-20 operating systems (used on PDP-11s in the
1970s) that are included in this shell are responsible for the T in tcsh.
Many features of the original csh shell, such as command-line editing and its history mechanism,
are included in tcsh as well as in other shells. While you can run both csh and tcsh on most Linux
systems, both commands actually point to the same executable file. In other words, starting csh
actually runs the tcsh shell in csh compatibility mode.
Using ash
The ash shell is a lightweight version of the Berkeley UNIX sh shell. It doesn't include many of the
sh shell's basic features, and is missing such features as command histories. Kenneth Almquist cre-
ated the ash shell.
The ash shell is a good shell for embedded systems that have fewer system resources available. The
ash shell is about one-seventh the size of bash (about 100K versus 712K for bash). Because of
cheaper memory prices these days, however, many embedded and small bootable Linux systems
have enough space to include the full bash shell.
Using ksh
The ksh shell was created by David Korn at AT&T Bell Labs and is the successor to the sh shell. It
became the default and most commonly used shell with UNIX System V systems. The open source
version of ksh was originally available in many rpm-based systems (such as Fedora and Red Hat
Enterprise Linux) as part of the pdksh package. Now, however, David Korn has released the origi-
nal ksh shell as open source, so you can look for it as part of a ksh software package in most Linux
systems (see
www·kornshell·com
).
Using zsh
The zsh shell is another clone of the sh shell. It is POSIX-compliant (as is bash), but includes some
different features, such as spell checking and a different approach to command editing. The first
Mac OS X systems used zsh as the default shell, although now bash is used by default.
Exploring the Shell
Once you have access to a shell in Linux, you can begin by typing some simple commands. The
section "Using the Shell in Linux" later in this chapter provides more details about options, argu-
ments, and environment variables. For the time being, the following sections will help you poke
around the shell a bit.
40
Linux First Steps
Part I
