This ppt is about BASH shell , introduction to bash, how commands are executed in bash , command types etc

1. *** BASH (Bourne Again SHell) ***
1) What is shell ?
Shell is tool/utility by which USER interacts with OS (kernel). Shell read (get) commands from
keyboard (entered by user on comand line)
Shell is the command­line INTERPRETER that executes (run) commands.
Linux OS
========================================================
# ­­­­­­­­­­­­­­­­­ ­­­­­­­­­­­­­ #
# | ls | ­­­­­­­ | | #
# | pwd |<­­­>| BASH | <­­­> | KERNEL | #
# | ifconfig | ­­­­­­­ | | #
# |­­­­­­­­­­­­­­­| ­­­­­­­­­­­­­­ #
# linux commands #
#=======================================================
2) How to get list of SHELLs ­
/etc/shells is a text file which contains the full pathnames of valid login shells on your linux
system!
root@mylaptop:~# cat /etc/shells
# /etc/shells: valid login shells
/bin/csh
/bin/sh
/bin/ksh
/bin/tcsh
/bin/bash
/bin/rbash
...
root@mylaptop:~#
sh ­ Standard shell available on linux
ksh ­ Korn shell
csh ­ C ­ shell
tcsh ­ Turbo C­shell
bash ­ Bourne Again Shell (BASH)
rbash ­ Restricted bash shell
*** Shortcut for bash command line ­
ctrl+c ­ intrupt command execution (stop command)
ctrl+l ­ clear screen OR clear command
ctrl+d ­ close/exit from shell OR exit command
ctrl+a ­ go to begining of command line
ctrl+e ­ go to end of command line
ctrl+w ­ delete/erase one previous word
altr+d ­ delete/erase next one word
ctrl+u ­ delete/erase text from the current cursor position to the begining of the line

2. ctrl+k ­ delete/erase text from the current cursor position to the end of the line
ctrl+y ­ undo changes
3) Pipe and Output redirection
* Pipe ­ A pipe is a way to connect the output of one program to the input of another program
without any temporary file
root@mylaptop:/var# ls | wc ­l
root@mylaptop:/var# ls | sort
root@mylaptop:/var# pwd | ls ­l
* Redirections ­ Before a command is executed, its input and output may be redirected using a
special notation interpreted by the shell. Redirection may also be used to open and close files for the
current shell execution environment. Redirections are processed in the order they appear, from left
to right.
Device File Descriptor
standard input 0
standard output 1
standard error 2
root@mylaptop:~# ls ­l /dev/stdin
lrwxrwxrwx 1 root root 15 2008­10­05 12:42 /dev/stdin ­> /proc/self/fd/0
root@mylaptop:~# ls ­l /dev/stdout
lrwxrwxrwx 1 root root 15 2008­10­05 12:42 /dev/stdout ­> /proc/self/fd/1
root@mylaptop:~# ls ­l /dev/stderr
lrwxrwxrwx 1 root root 15 2008­10­05 12:42 /dev/stderr ­> /proc/self/fd/2
root@mylaptop:~#
** redirects both standard output (file descriptor 1) and standard error (file descriptor 2) to the file
dirlist
# ls > dirlist 2>&1
** redirects only the standard output to file dirlist, because the standard error was duplicated as
standard output before the standard output was
redirected to dirlist.
# ls 2>&1 > dirlist
root@mylaptop:~# ls ­l > /tmp/mylist ( redirect only standard output to file '/tmp/mylist' )
root@mylaptop:~# ls ­lMM > /tmp/mylist
ls: invalid option ­­ M
Try `ls ­­help' for more information.
root@mylaptop:~# ls ­lMM 2> /tmp/mylist ( redirect only standard error to file '/tmp/mylist' )
root@mylaptop:~#
** Appending Redirected Output
root@mylaptop:~# ls ­l >> /tmp/mylist
root@mylaptop:~#

3. root@mylaptop:~# ls ­l >> /tmp/mylist 2>&1
root@mylaptop:~#
** NOTE :: There are two formats for redirecting standard output and standard error
arun@mylaptop:~$ find / ­name quot;*.confquot; > /tmp/mylist
find: /etc/ssl/private: Permission denied
find: /etc/chatscripts: Permission denied
find: /etc/jabberd2: Permission denied
...
arun@mylaptop:~$ find / ­name quot;*.confquot; &> /tmp/mylist
arun@mylaptop:~$
arun@mylaptop:~$ find / ­name quot;*.confquot; >& /tmp/mylist
arun@mylaptop:~$
==>> First is preferred. This is semantically equivalent to
arun@mylaptop:~$ find / ­name quot;*.confquot; > /tmp/mylist 2>&1
arun@mylaptop:~$
** Redirecting Input ­ Redirection of input causes the file to be opened for reading on file
descriptor n, or the standard input (file descriptor 0) if n is not specified...
# command_name [n]< /path_of_file
root@mylaptop:~# while read line
> do
> echo $line
> done < /tmp/mylist
Desktop
etc_ORIG
ftp_download.sh
tataindicom.dev
root@mylaptop:~#
root@mylaptop:~# wc ­l < /tmp/mylist
6
root@mylaptop:~#
4) Types of variables in Bash ­
There are two types of variables in Bash shell...
A) System variables ­ Created and maintained by Linux (BASH) itself. This type of variable
defined in CAPITAL LETTERS.
root@mylaptop:~# env
ORACLE_HOSTNAME=arun.yahoo.com

4. ORACLE_BASE=/u01/app
SHELL=/bin/bash
USERNAME=arun
LOGNAME=root
HOME=/root
SESSION_MANAGER=local/mylaptop.com:/tmp/.ICE­unix/6670
MAIL=/var/mail/root
DESKTOP_SESSION=default
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
PWD=/root
...
root@mylaptop:~#
** env command gives list of all system and user defined variables
root@mylaptop:~# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
root@mylaptop:~#
root@mylaptop:~# echo $PWD
/root
root@mylaptop:~# echo $USER
root
root@mylaptop:~#
HOME ­ The current user's home directory; the default for the cd builtin command. The value of
this variable is also used by tilde (~) expansion
IFS ­ (Internal Field Separater) A list of characters that separate fields; used when the shell splits
words as part of expansion.
MAIL ­ If this parameter is set to a filename and the MAILPATH variable is not set, Bash informs
to user of arrival of mail in the specified file.
PATH ­ A colon­separated list of directories in which the shell looks for commands.
PS1 ­ The primary prompt string. The default value is ‘s­v$ ’.
PS2 ­ The secondary prompt string. The default value is ‘> ’.
HOSTNAME ­ The name of the current host.
PPID ­ The process ID of the shell's parent process.
B) User defined variables ­ Created and maintained by user. This type of variable defined in lower
LETTERS.
root@mylaptop:~# num1=10
root@mylaptop:~# num2=30
root@mylaptop:~# expr $num1 + $num2
40
root@mylaptop:~# echo $num1
10
root@mylaptop:~# echo $num2
30
root@mylaptop:~#

5. ** NOTE ­ above user defined variables are available in current shell only!!
5
5) User Profile
/bin/bash ­ The bash executable
/etc/profile ­ The systemwide initialization file, executed for login shells
/etc/bash.bashrc ­ The systemwide per­interactive­shell startup file
/etc/bash.logout ­ The systemwide login shell cleanup file, executed when a login shell exits
~/.bash_profile ­ The personal initialization file, executed for login shells
~/.bashrc ­ The individual per­interactive­shell startup file
~/.bash_logout ­ The individual login shell cleanup file, executed when a login shell exits
6
6) Types of Linux (bash) commands ­
There are 4 types of commands.
a) Functions
b) Aliases
c) Buildin commands (bash)
d
d) Linux (OS) commands
When you type any command on command prompt, Bash looks for function then Aliases if
both not found then only it will check for buildin command and finally it will search for command
executable binary file in directories list provided by $PATH system variable from Lelt to Right
o
order. $PATH variable contains directories separated by quot;:quot;
root@mylaptop:~# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
r
root@mylaptop:~#
** How to check the command type ­ use quot;typequot; command to check whether command is function ,
a
alias , buildin and linux command
root@mylaptop:~# type pwd
pwd is a shell builtin
root@mylaptop:~# type ls
ls is aliased to `ls ­­color=auto'
root@mylaptop:~#
arun@mylaptop:~$ type chmod
chmod is /bin/chmod
arun@mylaptop:~$
*
** How to list all aliases , functions , system and user defined variables
root@mylaptop:~# alias ­ list all aliases defined
root@mylaptop:~# set ­ list all functions , user and variables
** How to define and undefine function and how to check the order of command execution ­

6. root@mylaptop:~# pwd
/root
root@mylaptop:~# function pwd()
> {
> echo quot;Hey buddy, I am function NOT Aliasquot;
> }
root@mylaptop:~#
root@mylaptop:~# pwd
Hey buddy, I am function NOT Alias
root@mylaptop:~#
root@mylaptop:~# unset pwd
root@mylaptop:~#
root@mylaptop:~# pwd
/root
r
root@mylaptop:~#
** How to create/remove alias ­
*
r
root@mylaptop:~# netstat ­nlp | head ­n 30
r
root@mylaptop:~# alias check_port='netstat ­nlp | head ­n 30'
root@mylaptop:~# check_port
...
....
root@mylaptop:~#
r
root@mylaptop:~# unalias check_port