The Basic Concepts of Shell Scripting

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shell scripting

If you want to automate regular tasks and make your life easier, using shell scripts is a good option. This article introduces you to the basic concepts that will help you to write efficient shell scripts.

Ashell script is a computer program designed to be run by the UNIX shell, a command-line interpreter. The various dialects of shell scripts are considered to be scripting languages. Typical operations performed by shell scripts include file manipulation, program execution, and printing of text. A script that sets up the environment, runs the program, and does any necessary cleanup or logging, is called a wrapper.

Identification of shell prompt
You can identify whether the shell prompt on a Linux based computer is a normal or super user by looking at the symbols of the prompt in the terminal window. The ‘#’ symbol is used for a super user and the ‘$’ symbol is used for a user with standard privileges.

Manual of date command
Figure 1: Manual of date command

Basic commands
The script comes with too many commands that can be executed on the terminal window to manage your computer. Details of each command can be found in the manual included with the command. To view the manual, you need to run the command:

$man <command>

A few frequently used commands are:

$date #display current date and time
$cal #display current month calendar
$df #displays disk usages
$free #display memory usage
$ls #List files and directories
$mkdir #Creates directory

Each command comes with several options that can be used along with it. You can refer to the manual for more details. See Figure 1 for the output of:

$man date

Redirection operators
The redirection operator is really useful when you want to capture the output of a command in a file or redirect to a file.

$ls -l /usr/bin >file
default stdout to file
$ls -l /usr/bin 2>file
redirects stderr to file
$ls -l /usr/bin > ls-output 2>&1
redirects stderr & stdout to file
$ls -l /usr/bin &> ls-output
redirects stderr & stdout to file
$ls -l /usr/bin 2> /dev/null
/dev/null bitbucket

Brace expansion
Brace expansion is one of the powerful options UNIX has. It helps do a lot of operations with minimal commands in a single line instruction. For example:

$echo Front-{A,B,C}-Back
Front-A-Back, Front-B-Back, Front-C-Back

$echo {Z..A}
Z Y X W V U T S R Q P O N M L K J I H G F E D C B A

$mkdir {2009..2011}-0{1..9} {2009..2011}-{10..12}

This creates a directory for 12 months from 2009 to 2011.

Environment variables
An environment variable is a dynamic-named value that can affect the way running processes will behave on a computer. This variable is a part of the environment in which a process runs.

printenv
Print part of all of the environment
set
set shell options
export
export environment to subsequently executed programs
alias
create an alias for command

Network commands
Network commands are very useful for troubleshooting issues on the network and to check the particular port connecting to the client.

ping
Send ICMP packets
traceroute
Print route packets to a network
netstat
print network connection, routing table, 
interface stats
ftp/lftp
Internet file transfer program
wget
Non Interactive network downloader
ssh
OpenSSH SSH Client (remote login program)
scp
secure copy
sftp
Secure File transfer program

Grep commands
Grep commands are useful to find the errors and debug the logs in the system. It is one of the powerful tools that shell has.

grep -h ‘.zip’ file.list
. is any character
grep -h ‘^zip’ file.list
starts with zip
grep -h ‘zip$’ file.list
ends with zip
grep -h ‘^zip$’ file.list
containing only zip
grep -h ‘[^bz]zip’ file.list
not containing b and z
grep -h ‘^[A-Za-z0-9]’ file.list
file containing any valid names

Quantifiers
Here are some examples of quantifiers:

?
match element zero or one time
*
match an element zero or more times
+
Match an element one or more times
{}
match an element specfic number of times

Text processing
Text processing is another important task in the current IT world. Programmers and administrators can use the commands to dice, cut and process texts.

cat -A $FILE
To find any CTRL character introduced
sort file1.txt file2.txt file3.txt > 
final_sorted_list.txt
sort all files once
ls - l | sort -nr -k 5
key field 5th column
sort --key=1,1 --key=2n distor.txt
key field 1,1 sort and second column sort 
by numeric
sort foo.txt | uniq -c
to find repetition
cut -f 3 distro.txt
cut column 3
cut -c 7-10
cut character 7 - 10
cut -d ‘:’ -f 1 /etc/password
delimiter :
sort -k 3.7nbr -k 3.1nbr -k 3.4nbr
 distro.txt
3 rd field 7 the character, 
3rd field 1 character
paste file1.txt file2.txt > newfile.txt
merge two files
join file1.txt file2.txt
join on common two fields

Hacks and tips
In Linux, we can go back to our history of commands by either using simple commands or control options.

clear
clears the screen
history
stores the history
script filename
capture all command execution in a file

 

Tips:

History    : CTRL + {R, P }
!!number : command history number
!!             : last command
!?string    : history containing last string
!string      : history containing last string
export HISTCONTROL=ignoredups
export HISTSIZE=10000

As you get familiar with the Linux commands, you will be able to write wrapper scripts. All manual tasks like taking regular backups, cleaning up files, monitoring the system usage, etc, can be automated using scripts. This article will help you to start scripting, before you move to learning advanced concepts.

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