webpointmorpheus Linux Info
The BASH and Other Shells

     Introduction      Command I/O      BASH Filter Commands      Variables      Shell Scripting      BASH Builtins
     SED Stream Editor      The awk Command      BASH Resources      Documents in this Series

©2005 - material compiled by Bob Carnaghi, www.webpointmorpheus.com

 

Introduction     Top of Page

A Linux shell is a command line interpreter, the method of user interaction with the operating sytem. Everyting that happens in Linux between the user and the operating system must take place in a shell of some kind. There are several shells available, such as BASH (BOURNE again shell), csh (C Shell), zsh (Z shell), KORN shell, and others. Each shell has its strong and weak points, and the BASH shell is the most popular. This document considers the BASH shell exclusively.

Listed in the table immediately below are some of the most basic commands that a BASH shell user will find useful. If more information about any of the commands is necessary, type man command to view the man page for the command. Addional system commands are listed further along in this document. For more information on Linux system interaction, see the resource section below.

Basic BASH Functionality
Command	Description
clear	Clears the terminal screen of previous commands and history.
reset	Resets the terminal to the default settings.
finger	Displays information about system users. Must enter the username.
who	Displays the users who are currently logged into the system.
w	Displays the users who are currently logged into the system along with their tasks.
whoami	Displays your login name.
id	Displays your UIDs, GID's.
date	Displays the current date and time.
cal	Displays the current month in calendar form. Note that the command is not limited to only the current month: enter another month and/or year for further details.
exit	Will exit the current shell.

 

Command Input & Output     Top of Page

Entering commands is what accomplishes tasks in the system, but what is to be done with the output of the commands? Command redirection allows the output of a command to be sent to a file and saved on the hard drive or elsewhere. The common redirection characters are listed in the table below. Another common method of moving output of commands around is to send the output of one command to another command using pipes (|). There can be several pipes in one command line entry that moves the output of command one to the input of command two which moves the output of command two to the input of command three, etc. It's possible to finally redirect the output of the last command to a file or elsewhere.

Command Input/Output (I/O) is approached from a threefold viewpoint: Standard Input (stdin), Standard Output (sdtout), and Standard Error (stderr). Each command that is entered has available all three streams of data, and the individual streams can be redirected. See the table below for more details.

BASH Redirection Character Sequences
Command & Character Sequence	Description
command 1>file   command >file	command STDOUT is redirected to a file (not to the terminal screen)
command 2>file	command STDERR is redirected to a file (not to the terminal screen)
command 1>fileA 2>fileB   command >fileA 2>fileB	command STDOUT is redirected to fileA (not to the terminal screen) & command STDERR is redirected to fileB (not to the terminal screen)
command 1>file 2>&1   command >file 2>&1   command 1>&2 2>file   command >&2 2>file	both command STDOUT & STDERR are sent to the same file (not to the terminal screen)
command 1>>file   command 1>>file	command STDOUT is redirected and appended to fileA (not to the terminal screen)
command 2>>file	command STDERR is redirected and appended to fileA (not to the terminal screen)
command 0<file   command <file	command STDIN is taken from a file
Notes: The default action of command redirection to a file is to create the file if it does not exist, and to overwrite the file if it does exist. When redirection is indicated on the command line, the BASH shell will clear the contents of the file. For command sequences that utilize complex redirection and pipes, this may cause unexpected results.

 

BASH Filter Commands     Top of Page

Filter commands are designed to be used in a pipe sequence to alter the output from a command before moving it along to another command or printing it to the terminal screen. Typically, filter commands do not stand alone, as they depend on input from another command or file. Listed below are come of the more common filter commands.

BASH Useful Filter Commands
Command (-flag)	Description
sort	sorts the lines in a file by alphanumeric character
sort -r	reverse the output of the sort command
wc	(word count) - counts number of characters, words, or lines in a file
wc -c	(word count) - counts number of characters in a file
wc -w	(word count) - counts number of words in a file
wc -l	(word count) - counts number of lines in a file
pr	(print) - formats a file for printing, has many options
pr -d	(print) - formats a file for printing with double-spaces
tr	(translate) - command that will 'translate' certain characters in a file
nl	(number line) - numbers the lines in a file
grep *	searches the text within a file and returns the lines that match a regular expression
awk *	command that will return a formatted result from a text file based upon pattern matching and specific action statements
sed *	(stream editor) - command that can search and replace text in a file and return a formatted result
Notes: *This is an extremely useful and versatile BASH command. There are entire books as well as extensive online documentation about this command.

 

Environment & User-Defined Variables     Top of Page

The BASH shell, indeed the Linux operating system itself, has as one of its attributes, the concept of variables, typically associated with the shell (environment variables), or the user (user variables). There are additional system variables that are set by various methods and utilities. To see the shell variables, type the set command. Environment variables can be viewed by entering the command env at the command prompt of the shell. Individual variables can be viewed by typing echo $VARIABLE at the command prompt.

Listed in the table below is a list of BASH shell variables. Note that all of these variables may not be set in any given shell environment, some may need to be configured per user login, or manually. For more information about these variables, type man bash. For more information about setting, accessing, and using variables, type man set or man env.

BASH SHELL Variables
Variable	Description
PPID	The process ID of the shell's parent.
PWD	The current working directory as set by the cd command.
OLDPWD	The previous working directory as set by the cd command.
REPLY	The line of input read by the read builtin command when no arguments are supplied.
UID	The user ID of the current user.
EUID	The effective user ID of the current user.
BASH	The full pathname used to invoke the current instance of bash.
BASH_VERSION	The version number of the current instance of bash.
SHLVL	A counter that is incremented by one each time an instance of bash is started.
RANDOM	A random number integer generator. May be initialized by assigning a value to RANDOM. If RANDOM is unset, it loses its special properties, even when reset.
SECONDS	The number of seconds since the current shell invocation. If a value is assigned to SECONDS, the value returned upon subsequent references is the number of seconds since the assignment plus the value assigned. If SECONDS is unset, it loses its special properties, even when reset.
LINENO	A decimal number representing the current sequential line number (starting with 1) within a script or function. When not in a script or function, the value substituted is not guaranteed to be meaningful. When in a function the value is an approximately the number of simple commands executed in the current function. If LINENO is unset, it loses its special properties, even when reset.
HISTCMD	The history number of the current command. If HISTCMD is unset, it loses its special properties, even if it is subsequently reset.
OPTARG	The value of the last option argument processed by the getopts builtin command.
OPTIND	The index of the next argument to be processed by the getopts builtin command.
HOSTTYPE	Set to a string that uniquely describes the type of machine on which bash is executing. The default is system-dependent.
IFS	The Internal Field Separator that is used for word splitting after expansion, as well as to split lines into words with the read builtin command. The default value is ``<space><tab><>newline>''.
PATH	A colon-separated list of directories in which the shell looks for commands. The default path is system-dependent, and is typically set during installation, or by the administrator who installs bash.
HOME	The home directory of the current user.
CDPATH	A colon-separated list of directories that are the search path for the cd command.
MAIL	If this parameter is set to a filename, and the MAILPATH variable is not set, bash informs the user of the arrival of mail in the specified file.
MAILCHECK	Specifies how often (in seconds) bash checks for mail.
MAILPATH	A colon-separated list of pathnames to be checked for mail.
MAIL_WARNING	A configurable mail read warning message.
PS1	The primary prompt string used at the command line.
PS2	The secondary prompt string used at the command line.
PS3	The select prompt string used at the command line.
PS4	The prompt string used at the command line during execution trace. The first character of PS4 is replicated multiple times, as necessary, to indicate multiple levels of indirection.
HISTSIZE	The number of commands to remember in the command history. The default value is 500.
HISTFILE	The name of the file in which command history is saved. The default value is ~/.bash_history. If unset, the command history is not saved when an interactive shell exits.
HISTFILESIZE	The maximum number of lines contained in the history file.
PROMPT_COMMAND	If set, the value is executed as a command prior to issuing each primary prompt.
IGNOREEOF	Controls the action of the shell on receipt of an EOF character as the sole input.
TMOUT	If set to a value greater than zero, the value is interpreted as the number of seconds to wait for input after issuing the primary prompt. Bash terminates after waiting for that number of seconds if input does not arrive.
FCEDIT	The default editor for the fc builtin command.
FIGNORE	A colon-separated list of suffixes to ignore when performing filename completion.
INPUTRC	The filename for the readline startup file which will override the default of ~/.inputrc.
HISTCONTROL	Variable that controls what is entered into the HISTORY list, and how it is entered.
command_oriented_history	If set, bash attempts to save all lines of a multiple-line command in the same history entry. This allows easy re-editing of multi-line commands.
glob_dot_filenames	If set, bash includes filenames beginning with a `.' in the results of pathname expansion.
allow_null_glob_expansion	If set, bash allows pathname patterns which match no files to expand to a null string rather than themselves.
histchars	The two or three characters which control history expansion and tokenization.
nolinks	An option that can be set such that the shell does not follow symbolic links when executing commands that change the current working directory. It uses the physical directory structure instead.
HOSTFILE	Contains the name of a file in the same format as /etc/hosts that should be read when the shell needs to complete a hostname. The file may be changed interactively; the next time hostname completion is attempted bash adds the contents of the new file to the already existing database.
noclobber	If set, bash does not overwrite an existing file with the >, >&, and <> redirection operators. This variable may be overridden when creating output files by using the redirection operator >| instead of >. For more info, see the -C option to set.
auto_resume	This variable controls how the shell interacts with the user and job control. See the man page for the complexities of this variable.
no_exit_on_failed_exec	If this variable has been assigned, it renders a non-interactive shell such that it will not exit if it cannot execute the file specified in the exec builtin command. An interactive shell does not exit if exec fails.
cdable_vars	A variable that can be assigned as an argument to the cd builtin command. The assigned value is assumed to be a value that is the directory to change to.
NOTES: For a list of all BASH options and their settings, type set -o.

 

Shell Scripting     Top of Page

The bash shell is capable of execuing files that have been created and configured as scripts to complete admistrative and other tasks. These files typically contain a series of commands, functions, and other constructs that make it possible to do almost anything on the Linux system. The programming constructs included in the bash capability are, but are not limited to, if constructs, loops, case construct, and more. See the resources section below for more information.

During script execution, interactive reading of input from the user is often necessary for proper command execution. The following escape sequences may be necessary to the echo -e command.

BASH SHELL Variables
Variable	Description
PPID	The process ID of the shell's parent.
\0xxx	the character whose ASCII code is xxx in octal format
\\	backslash
\a	alert (beep)
\b	backspace
\c	suppress trailing newline
\f	form feed
\n	new line
\r	carriage return
\t	horizontal tab
\v	vertical tab

BASH Builtin Commands     Top of Page

As a sophisticated and complex Linux utility, the BASH shell has within its capability several built-in commands. Listed below are most of these commands. Please note that the command options and many details are not included in this document. The BASH built-in commands are simply defined here for convenience, see man bash for details.

BASH Built-in Commands
Command	Description
alias	Alias will create a substitute name for another command or script. Alias with no arguments prints the list of aliases in the form name=value on standard output. The Alias command has several options available.
bg	Place a specified job in the background, as if it had been started wit &.
bind	Display current readline key and function bindings, or bind a key sequence to a readline function or macro.
break	Exit from within a for, while, or until loop.
builtin shell-builtin	Execute the specified shell builtin, passing it arguments, and return its exit status.
cd	Change the current directory to the argument given, or to the user's home directory if no argument is given.
command	Run command with the arguments supplied, suppressing the normal shell function lookup.
continue	Resume the next iteration of the enclosing for, while, or until loop.
declare  & typeset	Declare variables and/or give them attributes.
dirs	Display the list of currently remembered directories. Directories are added to the list with the pushd command; the popd command moves back up through the list.
echo	Output the arguments supplied, separated by spaces.
enable	Enable and disable builtin shell commands.
eval	The arguments supplied are read and concatenated together into a single command.
exec	If command is specified, it replaces the shell. No new process is created.
exit	Cause the shell to exit with a status of n.
export	The supplied arguments are marked for automatic export to the environment of subsequently executed commands.
fc	Fix Command. A range of commands from first to last is selected from the his-tory list.
fg	Place the specified job in the foreground, and make it the current job.
getopts	getopts is used by shell procedures to parse positional parameters.
hash	Supplies a list (hash table) of commands and number of times used (hits) during the current login session.
help	Display helpful information about builtin commands.
history	Displays the command history list with line numbers.
jobs	Lists the active jobs.
kill	Terminate processes. Note that there are levels of kill signals.
let	Permforms mathematical operations and assignment.
local	Create a local variable, and assign it value.
logout	Exit a login shell.
popd	Removes entries from the directory stack.
pushd	Adds a directory to the top of the directory stack, or rotates the stack, making the new top of the stack the current working directory.
pwd	Print the absolute pathname of the current working directory.
read	Used for shell script interaction, prompts and reads user input.
readonly	The given arguments are marked readonly and the values of these names may not be changed by subsequent assignment.
return	Causes a function to exit with the return value specified by the argument given.
set	Displays, sets, or unsets shell attributes.
shift	Shifts command arguments.
suspend	Suspend the execution of this shell until it receives a SIGCONT signal.
test	Return a status of 0 (true) or 1 (false) depending on the evaluation of the conditional expression that is supplied as an argument.
trap	Expects a command as an argument, and the command is to be read and executed when the shell receives signal(s) sigspec.
type	Indicates how each supplied argument would be interpreted if used as a command name. type returns true if any of the arguments are found, false if none are found.
ulimit	Ulimit provides control over the resources available to the shell and to processes started by it on systems that allow such control.
umask	The user file-creation mask. Sets default file and directory permissions that are given to new entities when created.
unalias	Remove names from the list of defined aliases.
unset	Takes a name or list of names as an argument. For each name, remove the corresponding variable or, given the -f option, function.
wait	Wait for the specified process and return its termination status.

 

SED Stream Editor     Top of Page

sed is a popular stream editor that works on a file line by line. sed acts like a filter by executing a group of editing instructions for the text file. sed is the program of choice for making changes to a text file. For a sed tutorial or book, see the resources section below.

List of Useful sed commands.
Command & Character Sequence	Number of Arguments	Description
a	1	append text
blabel	2	branch to label
c	2	change lines
d	2	delete lines
g	2	get contents of hold area
h	2	hold pattern space (in a hold buffer)
i	1	insert lines
l	2	list lines
n	2	next line
p	2	print
q	1	quit
r file	1	read the contents of file
s/RE/replacement/[flags]	2	substition
tlabel	2	test substitutions and branch if successful
w file	2	write to file
x	2	exchange buffer space with pattern space
y/list1/list2/	2	translates list1 into list2
D	2	delete first part of the pattern space
G	2	append contents of hold area
H	2	append pattern space on buffer
N	2	append next line
P	2	print first part of the pattern space
!cmd	2	taken to mean "Don't apply to specified addresses"
#	0	comment
:label	0	place a label
=	1	display line number
{	2	group commands

 

The awk Programming Language     Top of Page

awk is a programming language that can be applied to data-manipulation and computing tasks on a Linux operating system. For more information on awk, consult the resources immediately below.

BASH Resources     Top of Page

• Learning the bash Shell by Cameron Newham, Bill Rosenblatt, listed on Amazon.
• BASH Reference Manual
• Linux Shell Scripting with Bash by Ken O. Burtch
• BASH How-To #1
• BASH How-To #2
• Linux System Introduction
• sed & awk by Arnold Robbins, Dale Dougherty, listed on Amazon.
• sed tutorial
• awk tutorial

1. Introduction and History
2. Installation, Advanced Installation, and Usage
3. The Linux Kernel and the Boot Process
4. Filesystems - Management & Administration
5. The BASH and Other Shells
6. System Initialization and the X Environment
7. Linux Processes
8. Linux Administration, Peripherals, and Hardware
9. Software Installation and Management
10. Backups and Log Files
11. Performance and Problems
12. Network Configuration
13. Security
14. Key Linux Commands
15. Essential Linux Definitions