The Internet Encyclopedia (Volume 3)

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502 UNIXOPERATINGSYSTEM

Bourne Shell C Shell

Korn Shell Bash Shell Z Shell TC Shell

Figure 1: Shell lineage.

determine what shell her or she is currently running by

issuing a command such as

echo $SHELL

The above command will print out the user’s default

shell, something like “/bin/sh,” indicating the original

Bourne shell, or “/usr/local/bin/bash,” indicating the Bash

shell. An example of this command is given in Figure 2.

Although different shells support different features,

several significant features generally appear in one form

or another. Among these features are redirection, com-

mand history, command line editing, command line com-

pletion, and aliases.

Entering Commands

Users can enter commands to run programs at the shell’s

command prompt. Every unix command has a command

name, which the user can use to run the program. For

example, the program to copy one file to another is called

“cp.” To execute the copy command, the user would enter

cp file1 file2

which would copy the contents of file1 into another file

called file2 (creating that file if necessary).

The execution of commands can be tailored by the user

by issuing defined options (called “flags”) or supplying

arguments to the command. Options are preceded by

a single dash character ‘-’. For example, the standard

command to list out the files in a directory is the “ls”

command. By default, the “ls” command with no op-

tions will just list the names of the files in the directory.

By giving the ‘-l’ flag or option, however, more detail is

give about each file, including the files owner, group, ac-

cess permissions, file size, number of links, and so on.

Figure 2: Discovering a default login shell.