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Chapter 7. bash: The Bourne-Again Shell

Contents:

Invoking the Shell
Syntax
Variables
Arithmetic Expressions
Command History
Job Control
Built-in Commands

bash is the GNU version of the standard Bourne shell—the original Unix shell—and incorporates many popular features from other shells such as csh, tcsh, and the Korn shell (ksh). Both tcsh, which is described in the following chapter, and ksh, which offers many of the features in this chapter, are also available on most distributions of Linux. But bash is the standard Linux shell, loaded by default when most user accounts are created.

If executed as part of the user's login, bash starts by executing any commands found in /etc/profile. Then it executes the commands found in ~/.bash_profile, ~/.bash_login, or ~/.profile (searching for each file only if the previous file is not found). Many distributions change shell defaults in /etc/profile for all users, even changing the behavior of common commands like ls.

In addition, every time it starts (as a subshell or a login shell), bash looks for a file named ~/.bashrc. Many system administration utilities create a small ~/.bashrc automatically, and many users create quite large startup files. Any commands that can be executed from the shell can be included. A small sample file may look like this (each feature can be found either in this chapter or in Chapter 3):

# Set bash variable to keep 50 commands in history.
HSTSIZE=50
#
# Set prompt to show current working directory and history number of 
# command.
PS1='\w: Command \!$ '
#
# Set path to search for commands in my directories, then standard ones.
PATH=~/bin:~/scripts:$PATH
#
# Keep group and others from writing my newly created files.
umask 022
#
# Show color-coded file types.
alias ls='ls --color=yes'
#
# Make executable and .o files ugly yellow so I can find and delete them.
export LS_COLORS="ex=43:*.o=43"
#
# Quick and dirty test of a single-file program.
function gtst ( ) {
    g++ -o $1 $1.C && ./$1
}
#
# Remove .o files.
alias clean='find ~ -name \*.o -exec rm {  } \;'

bash provides the following features:

7.1. Invoking the Shell

The command interpreter for bash can be invoked as follows:

bash [options] [arguments]

bash can execute commands from a terminal (when -i is specified), from a file (when the first argument is an executable script), or from standard input (if no arguments remain or if -s is specified).

7.1.1. Options

Options that appear here with double hyphens also work when entered with single hyphens, but using double hyphens is standard coding procedure.

-, --
Treat all subsequent strings as arguments, not options.

-D, --dump-strings
For execution in non-English locales, dump all strings that bash translates.

--dump-po-strings
Same as --dump-strings, but uses the GNU gettext po (portable object) format suitable for scripting.

-c str
Read commands from string str.

--help
Print usage information and exit.

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

-l, --login
Behave like a login shell; try to process /etc/profile on startup. Then process ~/.bash_profile, ~/.bash_login, or ~/.profile (searching for each file only if the previous file is not found).

--noediting
Disable line editing with arrow and control keys.

--noprofile
Do not process /etc/profile, ~/.bash_profile, ~/.bash_login, or ~/.profile on startup.

--norc
Do not process ~/.bashrc on startup.

--posix
Conform to POSIX standard.

-r, --restricted
Restrict users to a very secure, limited environment; for instance, they cannot change out of the startup directory or use the > sign to redirect output.

--rcfile file
Substitute file for .bashrc on startup.

-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.

-v, --verbose
Print each line as it is executed (useful for tracing scripts).

--version
Print information about which version of bash is installed.

-x
Turn on debugging, as described under the -x option to the set built-in command later in this chapter.

The remaining options to bash are listed under the set built-in command.

7.1.2. Arguments

Arguments are assigned, in order, to the positional parameters $1, $2, and so forth. If the first argument is an executable script, it is assigned to $0; then commands are read from it, and remaining arguments are assigned to $1, $2, and so on.

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