Linux in a Nutshell, Fifth Edition

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Bash and Korn

This is the Title of the Book, eMatter Edition

Copyright © 2006 O’Reilly & Associates, Inc. All rights reserved.

Restricted shells are not used much in practice, as they are difficult to set up

correctly.

Built-in Commands (Bash and Korn Shells)

Examples to be entered as a command line are shown with the $ prompt. Other-

wise, examples should be treated as code fragments that might be included in a

shell script. For convenience, some of the reserved words used by multiline

commands are also included.

!

! pipeline

Not ksh88. Negate the sense of a pipeline. Returns an exit status of

0 if the pipeline exited nonzero, and an exit status of 1 if the pipe-

line exited zero. Typically used in if and while statements.

Example

This code prints a message if user jane is not logged on:

if ! who | grep jane > /dev/null

then

echo jane is not currently logged on

fi

#

#

Ignore all text that follows on the same line. # is used in shell

scripts as the comment character and is not really a command.

#!shell

#!shell [option]

Used as the first line of a script to invoke the named shell. Anything

given on the rest of the line is passed as a single argument to the

named shell. This feature is typically implemented by the kernel,

but may not be supported on some older systems. Some systems

have a limit of approximately 32 characters on the maximum

length of shell. For example:

#!/bin/sh

:

:

Null command. Returns