bash_quick_reference.pdf

bash Quick

Reference

By Arnold Robbins

Contents

History ........................................................2

Overview of Features.................................2

Invoking the Shell ......................................3

Syntax..........................................................4

Functions ..................................................10

Variables ...................................................10

Arithmetic Expressions ...........................19

Command History ...................................20

Job Control...............................................25

Shell Options ............................................26

Command Execution ...............................28

Restricted Shells.......................................29

Built-in Commands..................................29

Resources ..................................................64

ISBN: 0596527764

In this quick reference, you'll find

everything you need to know about the

bash shell. Whether you print it out or

read it on the screen, this book gives

you the answers to the annoying

questions that always come up when

you're writing shell scripts: What

characters do you need to quote? How

do you get variable substitution to do

exactly what you want? How do you

use arrays? It's also helpful for

interactive use.

If you're a Unix user or programmer,

or if you're using bash on Windows,

you'll find this quick reference

indispensable.

Find more at pdfs.oreilly.com

CHAPTER 1

The Bash Shell

This reference covers Bash, which is the primary shell for GNU/Linux and Mac OS X. In

particular, it covers version 3.1 of Bash. Bash is available for Solaris and can be easily com-

piled for just about any other Unix system. This reference presents the following topics:

•

Histor y

•

Overvie w of features

•

Invoking the shell

•

Syntax

•

Functions

•

Variables

•

Arithmetic expressions

•

Command history

•

Job control

•

Shell options

•

Command execution

•

Restricted shells

•

Built-in commands

•

Resources

Histor y

The original Bourne shell distributed with V7 Unix in 1979 became the standard shell for

writing shell scripts. The Bourne shell is still be found in /bin/sh on many commercial

Unix systems. The Bourne shell itself has not changed that much since its initial release,

although it has seen modest enhancements over the years. The most notable new features

were the CDPATH variable and a built-in test command with System III (circa 1980),

command hashing and shell functions for System V Release 2 (circa 1984), and the addition

of job control features for System V Release 4 (1989).

Because the Berkeley C shell ( csh ) offered features that were more pleasant for interactive

use, such as command history and job control, for a long time the standard practice in the

Unix world was to use the Bourne shell for programming and the C shell for daily use. David

Korn at Bell Labs was the ﬁrst developer to enhance the Bourne shell by adding csh -like fea-

tures to it: history, job control, and additional programmability. Eventually, the Korn shell’s

feature set surpassed both the Bourne shell and the C shell, while remaining compatible with

the Bourne shell for shell programming. Today, the POSIX standard deﬁnes the “standard

shell” language and behavior based on the System V Bourne shell, with a selected subset of

features from the Korn shell.

The Free Software Foundation, in keeping with its goal to produce a complete Unix work-

alike system, developed a clone of the Bourne shell, written from scratch, named “Bash,” the

Bourne-Again SHell. Over time, Bash has become a POSIX-compliant version of the shell,

with many additional features. A large part of these additional features overlap the features of

the Korn shell, but Bash is not an exact Korn shell clone.

Over view of Features

The Bash shell provides the following features:

•

Input/output redirection

•

Wildcard characters (metacharacters) for ﬁlename abbreviation

•

Shell variables and options for customizing your environment

•

A built-in command set for writing shell programs

•

Shell functions, for modularizing tasks within a shell program

•

Job control

•

Command-line editing (using the command syntax of either vi or Emacs)

•

Access to previous commands (command history)

•

Integer arithmetic

•

Arrays and arithmetic expressions

•

Command-name abbreviation (aliasing)

•

Upwards compliance with POSIX

2 Chapter 1 – The Bash Shell

•

Internationalization facilities

•

An arithmetic for loop

Invoking the Shell

The command interpreter for the Bash shell ( bash ) can be invoked as follows:

bash [ options ][ arguments ]

Bash can execute commands from a terminal, from a ﬁle (when the ﬁrst argument is a script),

or from standard input (if no arguments remain or if -s is speciﬁed). The shell automatically

prints prompts if standard input is a terminal, or if -i is given on the command line.

On many systems, /bin/sh is a link to Bash. When invoked as sh , Bash acts more like the

traditional Bourne shell: login shells read /etc/profile and ˜/.profile , and regular

shells read $ENV, if it’s set. Full details are available in the bash (1) manpage.

Options

-c str

Read commands from string str .

-D , --dump-strings

Print all $"..." strings in the program.

-i Create an interactive shell (prompt for input).

-O option

Enable shopt option option .

-p Start up as a privileged user. Don’t read $ENV or $BASH_ENV, don’t import functions

from the environment, and ignore the value of $SHELLOPTS. The normal ﬁxed-

name startup ﬁles (such as $HOME/.bash_profile ) are read.

-r , --restricted

Create a restricted shell.

-s Read commands from standard input. Output from built-in commands goes to ﬁle

descriptor 1; all other shell output goes to ﬁle descriptor 2.

--debugger

Read the debugging proﬁle at startup, turn on the extdebug option to shopt , and

enable function tracing. For use by the Bash debugger (see http://bashdb.sourceforge.net ).

--dump-po-strings

Same as -D , but output in GNU gettext format.

--help

Print a usage message and exit successfully.

--init-file file , --rcfile file

Use ﬁle as the startup ﬁle instead of ˜/.bashrc for interactive shells.

Invoking the Shell 3

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