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Chapter 2

In this part of tutorial you are introduce to shell programming, how to write script, execute them etc. We will getting started with writing small shell script, that will print "Knowledge is Power" on screen. Before starting with this you should know

 

How to write shell script

Following steps are required to write shell script:

(1) Use any editor like vi or mcedit to write shell script.

(2) After writing shell script set execute permission for your script as follows
syntax:
chmod permission your-script-name

Examples:
$ chmod +x your-script-name
$ chmod 755 your-script-name

Note: This will set read write execute(7) permission for owner, for group and other permission is read and execute only(5).

(3) Execute your script as
syntax:
bash your-script-name
sh your-script-name
./your-script-name

Examples:
$ bash bar
$ sh bar
$ ./bar

NOTE In the last syntax ./ means current directory, But only . (dot) means execute given command file in current shell without starting the new copy of shell, The syntax for . (dot) command is as follows
Syntax:
. command-name

Example:
$ . foo

Now you are ready to write first shell script that will print "Knowledge is Power" on screen. See the common vi command list , if you are new to vi.

$ vi first
#
# My first shell script
#
clear
echo "Knowledge is Power"

After saving the above script, you can run the script as follows:
$ ./first

This will not run script since we have not set execute permission for our script first; to do this type command
$ chmod 755 first
$ ./first

First screen will be clear, then Knowledge is Power is printed on screen.

Script Command(s)
Meaning
$ vi first Start vi editor
#
# My first shell script
#
# followed by any text is considered as comment. Comment gives more information about script, logical explanation about shell script.
Syntax:
# comment-text
clear clear the screen
echo "Knowledge is Power" To print message or value of variables on screen, we use echo command, general form of echo command is as follows
syntax:
echo "Message"

How Shell Locates the file (My own bin directory to execute script)

Tip: For shell script file try to give file extension such as .sh, which can be easily identified by you as shell script.

Exercise:
1)Write following shell script, save it, execute it and note down the it's output.

$ vi ginfo
#
#
# Script to print user information who currently login , current date & time
#
clear
echo "Hello $USER"
echo "Today is \c ";date
echo "Number of user login : \c" ; who | wc -l
echo "Calendar"
cal
exit 0

Future Point: At the end why statement exit 0 is used? See exit status for more information.

 

Variables in Shell

To process our data/information, data must be kept in computers RAM memory. RAM memory is divided into small locations, and each location had unique number called memory location/address, which is used to hold our data. Programmer can give a unique name to this memory location/address called memory variable or variable (Its a named storage location that may take different values, but only one at a time).

In Linux (Shell), there are two types of variable:
(1) System variables - Created and maintained by Linux itself. This type of variable defined in CAPITAL LETTERS.
(2) User defined variables (UDV) - Created and maintained by user. This type of variable defined in lower letters.

You can see system variables by giving command like $ set, some of the important System variables are:

System Variable
Meaning
BASH=/bin/bash Our shell name
BASH_VERSION=1.14.7(1) Our shell version name
COLUMNS=80 No. of columns for our screen
HOME=/home/vivek Our home directory
LINES=25 No. of columns for our screen
LOGNAME=students students Our logging name
OSTYPE=Linux Our Os type
PATH=/usr/bin:/sbin:/bin:/usr/sbin Our path settings
PS1=[\u@\h \W]\$ Our prompt settings
PWD=/home/students/Common Our current working directory
SHELL=/bin/bash Our shell name
USERNAME=vivek User name who is currently login to this PC

NOTE that Some of the above settings can be different in your PC/Linux environment. You can print any of the above variables contains as follows:
$ echo $USERNAME
$ echo $HOME

Exercise:
1) If you want to print your home directory location then you give command:
a) $ echo $HOME

OR

(b) $ echo HOME

Which of the above command is correct & why? Click here for answer.

Caution: Do not modify System variable this can some time create problems.

 

How to define User defined variables (UDV)

To define UDV use following syntax
Syntax:
variable name=value

' value' is assigned to given ' variable name' and Value must be on right side = sign.

Example:
$ no=10# this is ok
$ 10=no# Error, NOT Ok, Value must be on right side of = sign.
To define variable called 'vech' having value Bus
$ vech=Bus
To define variable called n having value 10
$ n=10

 

Rules for Naming variable name (Both UDV and System Variable)

(1) Variable name must begin with Alphanumeric character or underscore character (_), followed by one or more Alphanumeric character. For e.g. Valid shell variable are as follows
HOME
SYSTEM_VERSION
vech
no

(2) Don't put spaces on either side of the equal sign when assigning value to variable. For e.g. In following variable declaration there will be no error
$ no=10
But there will be problem for any of the following variable declaration:
$ no =10
$ no= 10
$ no = 10

(3) Variables are case-sensitive, just like filename in Linux. For e.g.
$ no=10
$ No=11
$ NO=20
$ nO=2

Above all are different variable name, so to print value 20 we have to use $ echo $NO and not any of the following
$ echo $no # will print 10 but not 20
$ echo $No# will print 11 but not 20
$ echo $nO# will print 2 but not 20

(4) You can define NULL variable as follows (NULL variable is variable which has no value at the time of definition) For e.g.
$ vech=
$ vech=""
Try to print it's value by issuing following command
$ echo $vech
Nothing will be shown because variable has no value i.e. NULL variable.

(5) Do not use ?,* etc, to name your variable names.

 

How to print or access value of UDV (User defined variables)

To print or access UDV use following syntax
Syntax:
$variablename

Define variable vech and n as follows:
$ vech=Bus
$ n=10

To print contains of variable 'vech' type
$ echo $vech
It will print 'Bus',To print contains of variable 'n' type command as follows
$ echo $n

Caution: Do not try $ echo vech, as it will print vech instead its value 'Bus' and $ echo n, as it will print n instead its value '10', You must use $ followed by variable name.

Exercise
Q.1.How to Define variable x with value 10 and print it on screen.
Q.2.How to Define variable xn with value Rani and print it on screen
Q.3.How to print sum of two numbers, let's say 6 and 3?
Q.4.How to define two variable x=20, y=5 and then to print division of x and y (i.e. x/y)
Q.5.Modify above and store division of x and y to variable called z
Q.6.Point out error if any in following script

$ vi variscript
#
#
# Script to test MY knowledge about variables!
#
myname=Vivek
myos = TroubleOS
myno=5
echo "My name is $myname"
echo "My os is $myos"
echo "My number is myno, can you see this number"

For Answers Click here

 

echo Command

Use echo command to display text or value of variable.

echo [options] [string, variables...]
Displays text or variables value on screen.
Options
-n Do not output the trailing new line.
-e Enable interpretation of the following backslash escaped characters in the strings:
\a alert (bell)
\b backspace
\c suppress trailing new line
\n new line
\r carriage return
\t horizontal tab
\\ backslash

For e.g. $ echo -e "An apple a day keeps away \a\t\tdoctor\n"

How to display colorful text on screen with bold or blink effects, how to print text on any row, column on screen, click here for more!

 

Shell Arithmetic

Use to perform arithmetic operations.

Syntax:
expr op1 math-operator op2

Examples:
$ expr 1 + 3
$ expr 2 - 1
$ expr 10 / 2
$ expr 20 % 3
$ expr 10 \* 3
$ echo `expr 6 + 3`

Note:
expr 20 %3 - Remainder read as 20 mod 3 and remainder is 2.
expr 10 \* 3 - Multiplication use \* and not * since its wild card.

For the last statement not the following points

(1) First, before expr keyword we used ` (back quote) sign not the (single quote i.e. ') sign. Back quote is generally found on the key under tilde (~) on PC keyboard OR to the above of TAB key.

(2) Second, expr is also end with ` i.e. back quote.

(3) Here expr 6 + 3 is evaluated to 9, then echo command prints 9 as sum

(4) Here if you use double quote or single quote, it will NOT work
For e.g.
$ echo "expr 6 + 3" # It will print expr 6 + 3
$ echo 'expr 6 + 3' # It will print expr 6 + 3

See Parameter substitution - To save your time.

 

More about Quotes

There are three types of quotes

Quotes
Name
Meaning
" Double Quotes "Double Quotes" - Anything enclose in double quotes removed meaning of that characters (except \ and $).
' Single quotes 'Single quotes' - Enclosed in single quotes remains unchanged.
` Back quote
`Back quote` - To execute command

Example:
$ echo "Today is date"
Can't print message with today's date.
$ echo "Today is `date`".
It will print today's date as, Today is Tue Jan ....,Can you see that the `date` statement uses back quote?

 

Exit Status

By default in Linux if particular command/shell script is executed, it return two type of values which is used to see whether command or shell script executed is successful or not.

(1) If return value is zero (0), command is successful.
(2) If return value is nonzero, command is not successful or some sort of error executing command/shell script.

This value is know as Exit Status .

But how to find out exit status of command or shell script?
Simple, to determine this exit Status you can use $? special variable of shell.

For e.g. (This example assumes that unknow1file doest not exist on your hard drive)
$ rm unknow1file
It will show error as follows
rm: cannot remove `unkowm1file': No such file or directory
and after that if you give command
$ echo $?
it will print nonzero value to indicate error. Now give command
$ ls
$ echo $?

It will print 0 to indicate command is successful.

Exercise
Try the following commands and not down the exit status:
$ expr 1 + 3
$ echo $?

$ echo Welcome
$ echo $?

$ wildwest canwork?
$ echo $?

$ date
$ echo $?

$ echon $?
$ echo $?

$? useful variable, want to know more such Linux variables click here to explore them!

 

The read Statement

Use to get input (data from user) from keyboard and store (data) to variable.
Syntax:
read variable1, variable2,...variableN

Following script first ask user, name and then waits to enter name from the user via keyboard. Then user enters name from keyboard (after giving name you have to press ENTER key) and entered name through keyboard is stored (assigned) to variable fname.

$ vi sayH
#
#Script to read your name from key-board
#
echo "Your first name please:"
read fname
echo "Hello $fname, Lets be friend!"

Run it as follows:
$ chmod 755 sayH
$ ./sayH

Your first name please: vivek
Hello vivek, Lets be friend!

 

Wild cards (Filename Shorthand or meta Characters)

 

Wild card /Shorthand Meaning Examples
* Matches any string or group of characters. $ ls * will show all files
$ ls a* will show all files whose first name is starting with letter 'a'
$ ls *.c   will show all files having extension .c
$ ls ut*.c will show all files having extension .c but file name must begin with 'ut'.
? Matches any single character. $ ls ?   will show all files whose names are 1 character long 
$ ls fo? will show all files whose names are 3 character long and file name begin with fo
[...] Matches any one of the enclosed characters $ ls [abc]*  will show all files beginning with letters a,b,c

Note:
[..-..] A pair of characters separated by a minus sign denotes a range.

Example:
$ ls /bin/[a-c]*

Will show all files name beginning with letter a,b or c like

   /bin/arch           /bin/awk           /bin/bsh     /bin/chmod           /bin/cp
   /bin/ash           /bin/basename   /bin/cat      /bin/chown           /bin/cpio
   /bin/ash.static   /bin/bash          /bin/chgrp   /bin/consolechars  /bin/csh

But
$ ls /bin/[!a-o]
$ ls /bin/[^a-o]

If the first character following the [ is a ! or a ^ ,then any character not enclosed is matched i.e. do not show us file name that beginning with a,b,c,e...o, like

   /bin/ps            /bin/rvi              /bin/sleep /bin/touch      /bin/view
   /bin/pwd           /bin/rview        /bin/sort   /bin/true        /bin/wcomp
   /bin/red           /bin/sayHello     /bin/stty   /bin/umount   /bin/xconf
   /bin/remadmin  /bin/sed           /bin/su      /bin/uname     /bin/ypdomainname
   /bin/rm            /bin/setserial    /bin/sync   /bin/userconf  /bin/zcat
   /bin/rmdir         /bin/sfxload      /bin/tar    /bin/usleep
   /bin/rpm           /bin/sh            /bin/tcsh    /bin/vi

 

More command on one command line

Syntax:
command1;command2
To run two command with one command line.

Examples:
$ date;who
Will print today's date followed by users who are currently login. Note that You can't use
$ date who
for same purpose, you must put semicolon in between date and who command.

 

Command Line Processing

Try the following command (assumes that the file " grate_stories_of" is not exist on your system)
$ ls grate_stories_of
It will print message something like - grate_stories_of: No such file or directory.

ls is the name of an actual command and shell executed this command when you type command at shell prompt. Now it creates one more question What are commands? What happened when you type $ ls grate_stories_of ?

The first word on command line is, ls - is name of the command to be executed.
Everything else on command line is taken as arguments to this command. For e.g.
$ tail +10 myf
Name of command is tail, and the arguments are +10 and myf.

Exercise
Try to determine command and arguments from following commands
$ ls foo
$ cp y y.bak
$ mv y.bak y.okay
$ tail -10 myf
$ mail raj
$ sort -r -n myf
$ date
$ clear

Answer:

Command No. of argument  to this command (i.e $#) Actual Argument
ls 1 foo
cp 2 y  and   y.bak
mv 2 y.bak and  y.okay
tail 2 -10  and  myf
mail 1 raj
sort 3 -r, -n, and myf
date 0  
clear 0  

NOTE:
$# holds number of arguments specified on command line. And $* or $@ refer to all arguments passed to script.

 

Why Command Line arguments required

  1. Telling the command/utility which option to use.
  2. Informing the utility/command which file or group of files to process (reading/writing of files).

Let's take rm command, which is used to remove file, but which file you want to remove and how you will tail this to rm command (even rm command don't ask you name of file that you would like to remove). So what we do is we write command as follows:
$ rm [file-name]
Here rm is command and filename is file which you would like to remove. This way you tail rm command which file you would like to remove. So we are doing one way communication with our command by specifying filename Also you can pass command line arguments to your script to make it more users friendly. But how we access command line argument in our script.

Lets take ls command
$ Ls -a /*
This command has 2 command line argument -a and /* is another. For shell script,
$ myshell foo bar

Shell Script name i.e. myshell
First command line argument passed to myshell i.e. foo
Second command line argument passed to myshell i.e. bar

In shell if we wish to refer this command line argument we refer above as follows

myshell it is $0
foo it is $1
bar it is $2

Here $# (built in shell variable ) will be 2 (Since foo and bar only two Arguments), Please note at a time such 9 arguments can be used from $1..$9, You can also refer all of them by using $* (which expand to `$1,$2...$9`). Note that $1..$9 i.e command line arguments to shell script is know as " positional parameters".

Exercise
Try to write following for commands
Shell Script Name ($0),
No. of Arguments (i.e. $#),
And actual argument (i.e. $1,$2 etc)
$ sum 11 20
$ math 4 - 7
$ d
$ bp -5 myf +20
$ Ls *
$ cal
$ findBS 4 8 24 BIG

Answer

 

Shell Script Name  No. Of Arguments to script Actual Argument ($1,..$9)
$0 $# $1 $2 $3 $4 $5
sum 2 11 20      
math 3 4 - 7    
d 0          
bp 3 -5 myf +20    
Ls 1 *        
cal 0          
findBS 4 4 8 24 BIG  

Following script is used to print command ling argument and will show you how to access them:

$ vi demo
#!/bin/sh
#
# Script that demos, command line args
#
echo "Total number of command line argument are $#"
echo "$0 is script name"
echo "$1 is first argument"
echo "$2 is second argument"
echo "All of them are :- $* or $@"

Run it as follows

Set execute permission as follows:
$ chmod 755 demo

Run it & test it as follows:
$ ./demo Hello World

If test successful, copy script to your own bin directory (Install script for private use)
$ cp demo ~/bin

Check whether it is working or not (?)
$ demo
$ demo Hello World

NOTE: After this, for any script you have to used above command, in sequence, I am not going to show you all of the above command(s) for rest of Tutorial.

Also note that you can't assigne the new value to command line arguments i.e positional parameters . So following all statements in shell script are invalid:
$1 = 5
$2 = "My Name"

 

Redirection of Standard output/input i.e. Input - Output redirection

Mostly all command gives output on screen or take input from keyboard, but in Linux (and in other OSs also) it's possible to send output to file or to read input from file.

For e.g.
$ ls command gives output to screen; to send output to file of ls command give command

$ ls > filename
It means put output of ls command to filename.

There are three main redirection symbols >,>>,<

(1) > Redirector Symbol
Syntax:
Linux-command > filename
To output Linux-commands result (output of command or shell script) to file. Note that if file already exist, it will be overwritten else new file is created. For e.g. To send output of ls command give
$ ls > myfiles
Now if ' myfiles' file exist in your current directory it will be overwritten without any type of warning.

(2) >> Redirector Symbol
Syntax:
Linux-command >> filename
To output Linux-commands result (output of command or shell script) to END of file. Note that if file exist , it will be opened and new information/data will be written to END of file, without losing previous information/data, And if file is not exist, then new file is created. For e.g. To send output of date command to already exist file give command
$ date >> myfiles

(3) < Redirector Symbol
Syntax:
Linux-command < filename
To take input to Linux-command from file instead of key-board. For e.g. To take input for cat command give
$ cat < myfiles

 Click here to learn more about I/O Redirection

You can also use above redirectors simultaneously as follows
Create text file sname as follows

$cat > sname
vivek
ashish
zebra
babu
Press CTRL + D to save.

Now issue following command.
$ sort < sname > sorted_names
$ cat sorted_names
ashish
babu
vivek
zebra

In above example sort ( $ sort < sname > sorted_names) command takes input from sname file and output of sort command (i.e. sorted names) is redirected to sorted_names file.

Try one more example to clear your idea:
$ tr "[a-z]" "[A-Z]" < sname > cap_names
$ cat cap_names
VIVEK
ASHISH
ZEBRA
BABU

tr command is used to translate all lower case characters to upper-case letters. It take input from sname file, and tr's output is redirected to cap_names file.

Future Point : Try following command and find out most important point:
$ sort > new_sorted_names < sname
$ cat new_sorted_names

 

Pipes

A pipe is a way to connect the output of one program to the input of another program without any temporary file.

Pipe - Redirecting output of 1st command to 2nd without creating temporary file

Pipe Defined as:
" A pipe is nothing but a temporary storage place where the output of one command is stored and then passed as the input for second command. Pipes are used to run more than two commands ( Multiple commands) from same command line."

Syntax:
command1 | command2

Examles:

 

Command using Pipes Meaning or Use of Pipes
$ ls | more Output of ls command is given as input to more command So that output is printed one screen full page at a time.
$ who | sort  Output of who command is given as input to sort command So that it will print sorted list of users
$ who | sort > user_l ist Same as above except output of sort is send to (redirected) user_list file
$ who | wc -l  Output of who command is given as input to wc command So that it will number of user who logon to system
$ ls -l | wc  -l    Output of ls command is given as input to wc command So that it will print number of files in current directory.
$ who | grep raju Output of who command is given as input to grep command So that it will print if particular user name if he is logon or nothing is printed (To see particular user is logon or not)

 

Filter

If a Linux command accepts its input from the standard input and produces its output on standard output is know as a filter. A filter performs some kind of process on the input and gives output. For e.g.. Suppose you have file called 'hotel.txt' with 100 lines data, And from 'hotel.txt' you would like to print contains from line number 20 to line number 30 and store this result to file called 'hlist' then give command:
$ tail +20 < hotel.txt | head -n30 >hlist

Here head command is filter which takes its input from tail command (tail command start selecting from line number 20 of given file i.e. hotel.txt) and passes this lines as input to head, whose output is redirected to 'hlist' file.

Consider one more following example
$ sort < sname | uniq > u_sname

Here uniq is filter which takes its input from sort command and passes this lines as input to uniq; Then uniqs output is redirected to "u_sname" file.

 

What is Processes

Process is kind of program or task carried out by your PC. For e.g.
$ ls -lR
ls command or a request to list files in a directory and all subdirectory in your current directory - It is a process.

Process defined as:
" A process is program (command given by user) to perform specific Job. In Linux when you start process, it gives a number to process (called PID or process-id), PID starts from 0 to 65535."

 

Why Process required

As You know Linux is multi-user, multitasking Os. It means you can run more than two process simultaneously if you wish. For e.g. To find how many files do you have on your system you may give command like:

$ ls / -R | wc -l

This command will take lot of time to search all files on your system. So you can run such command in Background or simultaneously by giving command like

$ ls / -R | wc -l &

The ampersand ( &) at the end of command tells shells start process ( ls / -R | wc -l) and run it in background takes next command immediately.

Process & PID defined as:
" An instance of running command is called process and the number printed by shell is called process-id (PID), this PID can be use to refer specific running process."

 

Linux Command Related with Process

Following tables most commonly used command(s) with process:

For this purpose Use this Command Examples*
To see currently running process  ps $ ps
To stop any process by PID i.e. to kill process kill    [PID] $ kill  1012
To stop processes by name i.e. to kill process killall   [Process-name] $ killall httpd
To get information about all running process ps -ag $ ps -ag
To stop all process except your shell kill 0 $ kill 0
For background processing (With &, use to put particular command and program in background) linux-command  & $ ls / -R | wc -l &
To display the owner of the processes along with the processes   ps aux $ ps aux
To see if a particular process is running or not. For this purpose you have to use ps command in combination with the grep command  

ps ax | grep  process-U-want-to see

 

For e.g. you want to see whether Apache web server process is running or not then give command

$ ps ax | grep httpd

To see currently running processes and other information like memory and CPU usage with real time updates. top
See the output of top command.

$ top


Note
that to exit from top command press q.
To display a tree of processes pstree $ pstree

* To run some of this command you need to be root or equivalnt user.

NOTE that you can only kill process which are created by yourself. A Administrator can almost kill 95-98% process. But some process can not be killed, such as VDU Process.

Exercise:
You are working on your Linux workstation (might be learning LSST or some other work like sending mails, typing letter), while doing this work you have started to play MP3 files on your workstation. Regarding this situation, answer the following question:

1) Is it example of Multitasking?
2) How you will you find out the both running process (MP3 Playing & Letter typing)?
3) "Currently only two Process are running in your Linux/PC environment", Is it True or False?, And how you will verify this?
4) You don't want to listen music (MP3 Files) but want to continue with other work on PC, you will take any of the following action:

  1. Turn off Speakers
  2. Turn off Computer / Shutdown Linux Os
  3. Kill the MP3 playing process
  4. None of the above

Click here for answers.