Editing Linux 101 hacks (section)

='''Chapter 11: Bash Scripting'''=


=='''Hack 84. Execution Sequence of .bash_* files'''==


What is the sequence in which the following files are executed?


o /etc/profile


o ~/.bash_profile


o ~/.bashrc


o ~/.bash_login


o ~/.profile


o ~/.bash_logout


'''Execution sequence for interactive login shell'''


Following pseudo code explains the sequence of execution of these files.


execute /etc/profile


IF ~/.bash_profile exists THEN execute ~/.bash_profile


ELSE


IF ~/.bash_login exist THEN execute ~/.bash_login


ELSE


IF ~/.profile exist THEN execute ~/.profile


END IF END IF

END IF


When you logout of the interactive shell, following is the sequence of execution:


IF ~/.bash_logout exists THEN execute ~/.bash_logout

END IF


Please note that /etc/bashrc is executed by ~/.bashrc as shown below:


<nowiki># </nowiki>cat ~/.bashrc


if <nowiki>[ </nowiki>-f /etc/bashrc ]; then


. /etc/bashrc Fi


'''Execution sequence for interactive non-login shell'''


While launching a non-login interactive shell, following is the sequence of execution:


IF ~/.bashrc exists THEN execute ~/.bashrc


END IF


'''Note: '''When a non-interactive shell starts up, it looks for ENV environment''' '''variable, and executes the file-name value mentioned in the ENV variable.


=='''Hack 85. How to generate random number in bash shell'''==


Use the $RANDOM bash built-in function to generate random number between 0 – 32767 as shown below.


$ echo $RANDOM

22543


$ echo $RANDOM 25387


$ echo $RANDOM 647


=='''Hack 86. Debug a shell script'''==


To debug a shell script use set –xv inside the shell script at the top.


'''Shell script with no debug command:'''


$ cat filesize.sh #!/bin/bash

for filesize in $(ls -l . | grep "^-" | awk '{print $5}')


do


let totalsize=$totalsize+$filesize done


echo "Total file size in current directory: $totalsize"


'''Output of Shell script with no debug command:'''


$ ./filesize.sh


Total file size in current directory: 652


'''Shell script with Debug command inside:'''


Add set –xv inside the shell script now to debug the output as shown below.


$ cat filesize.sh #!/bin/bash

set -xv


for filesize in $(ls -l . | grep "^-" | awk '{print


$5}') do

let totalsize=$totalsize+$filesize done

echo "Total file size in current directory: $totalsize"


'''Output of Shell script with Debug command inside:'''


$ ./fs.sh


# ls -l . 

# grep '^-' 

# awk '{print $5}' 

# for filesize in '$(ls -l . | grep "^-" | awk '\''{print $5}'\'')' 
# let totalsize=+178 

# for filesize in '$(ls -l . | grep "^-" | awk '\''{print $5}'\'')' 
# let totalsize=178+285 

# for filesize in '$(ls -l . | grep "^-" | awk '\''{print $5}'\'')' 
# let totalsize=463+189 

# echo 'Total file size in current directory: 652' Total file size in current directory: 652 

'''Execute Shell script with debug option:'''


Instead of giving the set –xv inside the shell script, you can also provide that while executing the shell script as shown below.


$ bash -xv filesize.sh


=='''Hack 87. Quoting'''==


echo statement without any special character.


$ echo The Geek Stuff


The Geek Stuff


Echo statement with a special character ; . semi-colon is a command terminator in bash. In the following example, “The Geek” works for the echo and “Stuff” is treated as a separate Linux command and gives command not found.


$ echo The Geek; Stuff The Geek


-bash: Stuff: command not found


To avoid this you can add a \ in front of semi-colon, which will remove the special meaning of semi-colon and just print it as shown below.


$ echo The Geek\; Stuff The Geek; Stuff


'''Single Quote'''


Use single quote when you want to literally print everything inside the single quote. Even the special variables such as $HOSTNAME will be print as $HOSTNAME instead of printing the name of the Linux host.


$ echo 'Hostname=$HOSTNAME ; Current User=`whoami` ; Message=\$ is USD'


Hostname=$HOSTNAME ; Current User=`whoami` ; Message=\$ is USD


'''Double Quote'''


Use double quotes when you want to display the real meaning of special variables.


$ echo "Hostname=$HOSTNAME ; Current User=`whoami` ; Message=\$ is USD"


Hostname=dev-db <nowiki>; </nowiki>Current User=ramesh <nowiki>; </nowiki>Message=$ is


USD


Double quotes will remove the special meaning of all characters except the following:


o $ Parameter Substitution.


o ` Backquotes


o \$ Literal Dollar Sign. o \´ Literal Backquote.


o \" Embedded Doublequote. o \\ Embedded Backslashes.


=='''Hack 88. Read data file fields inside a shell script'''==


This example shows how to read a particular field from a data-file and manipulate it inside a shell-script. For example, let us assume the employees.txt file is in the format of {employee-name}:{employee-id}:{department-name}, with colon delimited file as shown below.


$ cat employees.txt


Emma Thomas:100:Marketing Alex Jason:200:Sales


Madison Randy:300:Product Development Sanjay Gupta:400:Support

Nisha Singh:500:Sales


The following shell script explains how to read specific fields from this employee.txt file.


$ vi read-employees.sh #!/bin/bash


IFS=:


echo "Employee Names:" echo "---------------" while read name empid dept


do


<nowiki>echo "$name is part of $dept department" done < ~/employees.txt</nowiki>


Assign execute privilege to the shell script and execute it.


$ chmod u+x read-employees.sh


$ ./read-employees.sh Employee Names:

<nowiki>---------------</nowiki>


Emma Thomas is part of Marketing department Alex Jason is part of Sales department


Madison Randy is part of Product Development department Sanjay Gupta is part of Support department

Nisha Singh is part of Sales department