Shell Programmimg

Overview:

A shell program combines Linux commands in such a way as to perform a specific task. The Linux shell provides you with many programming tools with which to create shell programs. You can define variables and assign values to them. You can also define variables in a script file and have a user interactively enter values for them when the script is executed.

What is Shell:

The shell provides you with an interface to the UNIX system. It gathers input from you and executes programs based on that input. When a program finishes executing, it displays that program's output.

A shell is an environment in which we can run our commands, programs, and shell scripts. There are different flavors of shells, just as there are different flavors of operating systems. Each flavor of shell has its own set of recognized commands and functions.

Shell Prompt:

The prompt, $, which is called command prompt, is issued by the shell. While the prompt is displayed, you can type a command.

Shell Types:

In UNIX there are two major types of shells: 

1. The Bourne shell. If you are using a Bourne-type shell, the default prompt is the $ character. 
2. The C shell. If you are using a C-type shell, the default prompt is the % character. 

There are again various subcategories for Bourne Shell which are listed as follows: 

• Bourne shell ( sh) 
• Korn shell ( ksh) 
• Bourne Again shell ( bash)
• POSIX shell ( sh)

The different C-type shells follow: 

• C shell ( csh) 
• TENEX/TOPS C shell ( tcsh)

1. The original UNIX shell was written in the mid-1970s by Stephen R. Bourne while he was at AT&T Bell Labs in New Jersey. 
2. The Bourne shell was the first shell to appear on UNIX systems, thus it is referred to as "the shell". 
3. The Bourne shell is usually installed as /bin/sh on most versions of UNIX. For this reason, it is the shell of choice for writing scripts to use on several different versions of UNIX.

Shell Scripts:

The basic concept of a shell script is a list of commands, which are listed in the order of execution. A good shell script will have comments, preceded by a pound sign, #, describing the steps.

We are going to write a many scripts in the next several tutorials. This would be a simple text file in which we would put our all the commands and several other required constructs that tell the shell environment what to do and when to do it. 

Example Script:

Assume we create a test.sh script. Note all the scripts would have .sh extension. Before you add anything else to your script, you need to alert the system that a shell script is being started. This is done using the shebang construct.

For example:-

#!/bin/sh

This tells the system that the commands that follow are to be executed by the Bourne shell. It's called a shebang because the # symbol is called a hash, and the ! symbol is called a bang. To create a script containing these commands, you put the shebang line first and then add the commands:

#!/bin/bash pwd ls

Shell Comments: 

You can put your comments in your script as follows:

#!/bin/bash # Author : Zara Ali # Copyright (c) Tutorialspoint.com # Script follows here: pwd ls

Now you save the above content and make this script executable as follows:

$chmod +x test.sh

Now you have your shell script ready to be executed as follows:

$./test.sh

This would produce following result:

/home/amrood index.htm unix-basic_utilities.htm unix-directories.htm test.sh unix-communication.htm unix-environment.htm

Note: To execute your any program available in current directory you would execute using./program_name

Following script use the read command which takes the input from the keyboard and assigns it as the value of the variable PERSON and finally prints it on STDOUT.

#!/bin/sh # Author : Zara Ali # Copyright (c) Thehandsonbook.blogspot.in # Script follows here: echo "What is your name?" read PERSON echo "Hello, $PERSON"

Here is sample run of the script:

$./test.sh What is your name? Zara Ali Hello, Zara Ali $

Shell Script - Using Variables:

Avariable is a character string to which we assign a value. The value assigned could be a number, text, filename, device, or any other type of data.

A variable is nothing more than a pointer to the actual data. The shell enables you to create, assign, and delete variables.

The name of a variable can contain only letters ( a to z or A to Z), numbers ( 0 to 9) or the underscore character ( _).

By convention, Unix Shell variables would have their names in UPPERCASE. The following examples are valid variable names:

_ALI TOKEN_A VAR_1 VAR_2

Following are the examples of invalid variable names:

2_VAR -VARIABLE VAR1-VAR2 VAR_A!

The reason you cannot use other characters such as !,*, or - is that these characters have a special meaning for the shell.

Defining Variables:

variable_name=variable_value

For example:

NAME="James Bond"

Above example defines the variable NAME and assigns it the value "James Bond". Variables of this type are called scalar variables. A scalar variable can hold only one value at a time. The shell enables you to store any value you want in a variable. For example:

VAR1="James Bond" VAR2=100

Accessing Values:

To access the value stored in a variable, prefix its name with the dollar sign ( $): For example, following script would access the value of defined variable NAME and would print it on STDOUT:

#!/bin/sh NAME="James Bond" echo $NAME

This would produce following value:

James Bond

Variable Types:

When a shell is running, three main types of variables are present: 

1. Local Variables: A local variable is a variable that is present within the current instance of the shell. It is not available to programs that are started by the shell. They are set at command prompt. 
2. Environment Variables: An environment variable is a variable that is available to any child process of the shell. Some programs need environment variables in order to function correctly. Usually a shell script defines only those environment variables that are needed by the programs that it runs. 
3. Shell Variables: A shell variable is a special variable that is set by the shell and is required by the shell in order to function correctly. Some of these variables are environment variables whereas others are local variables.

Special Variables:

$$: Displays PID of the current shell.

$0: Displays Filename of the current script

$#: The number of arguments supplied to a script.

#?: The exit status of the last command executed.

$!: The process number of the last background command.

$*: All the arguments are double quoted. If a script receives two arguments, $* is equivalent to $1 $2.

$$: The process number of the current shell. For shell scripts, this is the process ID under which they are executing

$@: All the arguments are individually double quoted. If a script receives two arguments, $@ is equivalent to $1 $2.

$n: These variables correspond to the arguments with which a script was invoked. Here n is a positive decimal number corresponding to the position of an argument (the first argument is $1, the second argument is $2, and so on).

Command-Line Arguments:

The command-line arguments $1, $2, $3,...$9 are positional parameters, with $0 pointing to the actual command, program, shell script, or function and $1, $2, $3, ...$9 as the arguments to the command.

Following script uses various special variables related to command line:

#!/bin/sh echo "File Name: $0" echo "First Parameter : $1" echo "First Parameter : $2" echo "Quoted Values: $@" echo "Quoted Values: $*" echo "Total Number of Parameters : $#"

Here is a sample run for the above script:

$./test.sh James Bond File Name : ./test.sh First Parameter : James First Parameter : Bond Quoted Values: James Bond Quoted Values: James Bond Total Number of Parameters : 2

Special Parameters $* and $@:

There are special parameters that allow accessing all of the command-line arguments at once. $* and $@ both will act the same unless they are enclosed in double quotes, "".

We can write the shell script shown below to process an unknown number of command-line arguments with either the $* or $@ special parameters:

#!/bin/sh for TOKEN in $* do echo $TOKEN done

There is one sample run for the above script:

$./test.sh James Bond 10 Years Old Zara Ali 10 Years Old

Exit Status: 

The $? variable represents the exit status of the previous command. Exit status is a numerical value returned by every command upon its completion. As a rule, most commands return an exit status of 0 if they were successful, and 1 if they were unsuccessful.

Following is the example of successful command:

$./test.sh James Bond File Name : ./test.sh First Parameter : James First Parameter : Bond Quoted Values: James Bond Quoted Values: James Bond Total Number of Parameters : 2 $echo $? 0 $

Unix Basic Operators:-

There are various operators supported by each shell.

• Arithmetic Operators. 
• Relational Operators. 
• Boolean Operators. 
• String Operators. 
• File Test Operators.

The Bourne shell didn't originally have any mechanism to perform simple arithmetic but it uses external programs, either awk or the must simpler program expr.

Here is simple example to add two numbers:

#!/bin/sh val=`expr 2 + 2` echo "Total value : $val"

Arithmetic Operators:

Operator	Description	Example
+	Addition - Adds values on either side of the operator	`expr $a + $b` will give 30
-	Subtraction - Subtracts right hand operand from left hand operand	`expr $a - $b` will give -10
*	Multiplication - Multiplies values on either side of the operator	`expr $a * $b` will give 200
/	Division - Divides left hand operand by right hand operand	`expr $b / $a` will give 2
%	Modulus - Divides left hand operand by right hand operand and returns remainder	`expr $b % $a` will give 0
=	Assignment - Assign right operand in left operand	a=$b would assign value of b into a
==	Equality - Compares two numbers, if both are same then returns true.	[ $a == $b ] would return false.
!=	Not Equality - Compares two numbers, if both are different then returns true.	[ $a != $b ] would return true.

Example:

Here is an example which uses all the arithmetic operators

#!/bin/sh a=10 b=20 val=`expr $a + $b` echo "a + b : $val" val=`expr $a - $b` echo "a - b : $val" val=`expr $a \* $b` echo "a * b : $val" val=`expr $b / $a` echo "b / a : $val" val=`expr $b % $a` echo "b % a : $val" if [ $a == $b ] then echo "a is equal to b" fi if [ $a != $b ] then echo "a is not equal to b" fi

Relational Operators:

Operator	Description	Example
-eq	Checks if the value of two operands are equal or not, if yes then condition becomes true.	[ $a -eq $b ] is not true.
-ne	Checks if the value of two operands are equal or not, if values are not equal then condition becomes true.	[ $a -ne $b ] is true.
-gt	Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true.	[ $a -gt $b ] is not true.
-lt	Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true.	[ $a -lt $b ] is true.
-ge	Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true.	[ $a -ge $b ] is not true.
-le	Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true.	[ $a -le $b ] is true.

Example:

Here is an example which uses all the relational operators

#!/bin/sh a=10 b=20 if [ $a -eq $b ] then echo "$a -eq $b : a is equal to b" else echo "$a -eq $b: a is not equal to b" fi if [ $a -ne $b ] then echo "$a -ne $b: a is not equal to b" else echo "$a -ne $b : a is equal to b" fi if [ $a -gt $b ] then echo "$a -gt $b: a is greater than b" else echo "$a -gt $b: a is not greater than b" fi if [ $a -lt $b ] then echo "$a -lt $b: a is less than b" else echo "$a -lt $b: a is not less than b" fi if [ $a -ge $b ] then echo "$a -ge $b: a is greater or equal to b" else echo "$a -ge $b: a is not greater or equal to b" fi if [ $a -le $b ] then echo "$a -le $b: a is less or equal to b" else echo "$a -le $b: a is not less or equal to b" fi

Linux Imp Commands

1. tar command examples

Create a new tar archive.

$ tar cvf archive_name.tar dirname/

Extract from an existing tar archive.

$ tar xvf archive_name.tar

View an existing tar archive.

$ tar tvf archive_name.tar

2. grep command examples

Search for a given string in a file (case in-sensitive search).

$ grep -i "the" demo_file

Print the matched line, along with the 3 lines after it.

$ grep -A 3 -i "example" demo_text

Search for a given string in all files recursively

$ grep -r "shiva" *

3. find command examples

Find files using file-name ( case in-sensitve find)

# find -iname "MyCProgram.c"

Execute commands on files found by the find command

$ find -iname "MyCProgram.c" -exec md5sum {} \;

Find all empty files in home directory

# find ~ -empty

4. ssh command examples

Login to remote host

ssh -l jsmith remotehost.example.com

Debug ssh client

ssh -v -l jsmith remotehost.example.com

Display ssh client version

$ ssh -V
OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2003

5. sed command examples

When you copy a DOS file to Unix, you could find \r\n in the end of each line. This example converts the DOS file format to Unix file format using sed command.

$sed 's/.$//' filename

Print file content in reverse order

$ sed -n '1!G;h;$p' thegeekstuff.txt

Add line number for all non-empty-lines in a file

$ sed '/./=' thegeekstuff.txt | sed 'N; s/\n/ /'

6. awk command examples

Remove duplicate lines using awk

$ awk '!($0 in array) { array[$0]; print }' temp

Print all lines from /etc/passwd that has the same uid and gid

$awk -F ':' '$3==$4' passwd.txt

Print only specific field from a file.

$ awk '{print $2,$5;}' employee.txt

7. vim command examples

Go to the 143rd line of file

$ vim +143 filename.txt

Go to the first match of the specified

$ vim +/search-term filename.txt

Open the file in read only mode.

$ vim -R /etc/passwd

8. diff command examples

Ignore white space while comparing.

# diff -w name_list.txt name_list_new.txt

2c2,3
< Shiva Salugu --- > Shiva P Salugu
> Shiva Prakash

9. sort command examples

Sort a file in ascending order

$ sort names.txt

Sort a file in descending order

$ sort -r names.txt

Sort passwd file by 3rd field.

$ sort -t: -k 3n /etc/passwd | more

10. export command examples

To view oracle related environment variables.

$ export | grep ORACLE
declare -x ORACLE_BASE="/u01/app/oracle"
declare -x ORACLE_HOME="/u01/app/oracle/product/10.2.0"
declare -x ORACLE_SID="med"
declare -x ORACLE_TERM="xterm"

To export an environment variable:

$ export ORACLE_HOME=/u01/app/oracle/product/10.2.0

11. xargs command examples

Copy all images to external hard-drive

# ls *.jpg | xargs -n1 -i cp {} /external-hard-drive/directory

Search all jpg images in the system and archive it.

# find / -name *.jpg -type f -print | xargs tar -cvzf images.tar.gz

Download all the URLs mentioned in the url-list.txt file

# cat url-list.txt | xargs wget –c

12. ls command examples

Display filesize in human readable format (e.g. KB, MB etc.,)

$ ls -lh
-rw-r----- 1 ramesh team-dev 8.9M Jun 12 15:27 arch-linux.txt.gz

Order Files Based on Last Modified Time (In Reverse Order) Using ls -ltr

$ ls -ltr

Visual Classification of Files With Special Characters Using ls -F

$ ls -F

13. pwd command

pwd is Print working directory. What else can be said about the good old pwd who has been printing the current directory name for ages.

14. cd command examples

Use “cd -” to toggle between the last two directories

Use “shopt -s cdspell” to automatically correct mistyped directory names on cd

More cd examples: 6 Awesome Linux cd command Hacks

15. gzip command examples

To create a *.gz compressed file:

$ gzip test.txt

To uncompress a *.gz file:

$ gzip -d test.txt.gz

Display compression ratio of the compressed file using gzip -l

$ gzip -l *.gz
         compressed        uncompressed  ratio uncompressed_name
              23709               97975  75.8% asp-patch-rpms.txt

16. bzip2 command examples

To create a *.bz2 compressed file:

$ bzip2 test.txt

To uncompress a *.bz2 file:

bzip2 -d test.txt.bz2

More bzip2 examples: BZ is Eazy! bzip2, bzgrep, bzcmp, bzdiff, bzcat, bzless, bzmore examples

17. unzip command examples

To extract a *.zip compressed file:

$ unzip test.zip

View the contents of *.zip file (Without unzipping it):

$ unzip -l jasper.zip
Archive:  jasper.zip
  Length     Date   Time    Name
 --------    ----   ----    ----
    40995  11-30-98 23:50   META-INF/MANIFEST.MF
    32169  08-25-98 21:07   classes_
    15964  08-25-98 21:07   classes_names
    10542  08-25-98 21:07   classes_ncomp

18. shutdown command examples

Shutdown the system and turn the power off immediately.

# shutdown -h now

Shutdown the system after 10 minutes.

# shutdown -h +10

Reboot the system using shutdown command.

# shutdown -r now

Force the filesystem check during reboot.

# shutdown -Fr now

19. ftp command examples

Both ftp and secure ftp (sftp) has similar commands. To connect to a remote server and download multiple files, do the following.

$ ftp IP/hostname
ftp> mget *.html

To view the file names located on the remote server before downloading, mls ftp command as shown below.

ftp> mls *.html -
/ftptest/features.html
/ftptest/index.html
/ftptest/othertools.html
/ftptest/samplereport.html
/ftptest/usage.html

20. crontab command examples

View crontab entry for a specific user

# crontab -u john -l

Schedule a cron job every 10 minutes.

*/10 * * * * /home/ramesh/check-disk-space

21. service command examples

Service command is used to run the system V init scripts. i.e Instead of calling the scripts located in the /etc/init.d/ directory with their full path, you can use the service command.

Check the status of a service:

# service ssh status

Check the status of all the services.

service --status-all

Restart a service.

# service ssh restart

22. ps command examples

ps command is used to display information about the processes that are running in the system.

While there are lot of arguments that could be passed to a ps command, following are some of the common ones.

To view current running processes.

$ ps -ef | more

To view current running processes in a tree structure. H option stands for process hierarchy.

$ ps -efH | more

23. free command examples

This command is used to display the free, used, swap memory available in the system.

Typical free command output. The output is displayed in bytes.

$ free
             total       used       free     shared    buffers     cached
Mem:       3566408    1580220    1986188          0     203988     902960
-/+ buffers/cache:     473272    3093136
Swap:      4000176          0    4000176

If you want to quickly check how many GB of RAM your system has use the -g option. -b option displays in bytes, -k in kilo bytes, -m in mega bytes.

$ free -g
             total       used       free     shared    buffers     cached
Mem:             3          1          1          0          0          0
-/+ buffers/cache:          0          2
Swap:            3          0          3

If you want to see a total memory ( including the swap), use the -t switch, which will display a total line as shown below.

ramesh@ramesh-laptop:~$ free -t
             total       used       free     shared    buffers     cached
Mem:       3566408    1592148    1974260          0     204260     912556
-/+ buffers/cache:     475332    3091076
Swap:      4000176          0    4000176
Total:     7566584    1592148    5974436

24. top command examples

top command displays the top processes in the system ( by default sorted by cpu usage ). To sort top output by any column, Press O (upper-case O) , which will display all the possible columns that you can sort by as shown below.

Current Sort Field:  P  for window 1:Def
Select sort field via field letter, type any other key to return

  a: PID        = Process Id              v: nDRT       = Dirty Pages count
  d: UID        = User Id                 y: WCHAN      = Sleeping in Function
  e: USER       = User Name               z: Flags      = Task Flags
  ........

To displays only the processes that belong to a particular user use -u option. The following will show only the top processes that belongs to oracle user.

$ top -u oracle

25. df command examples

Displays the file system disk space usage. By default df -k displays output in bytes.

$ df -k
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda1             29530400   3233104  24797232  12% /
/dev/sda2            120367992  50171596  64082060  44% /home

df -h displays output in human readable form. i.e size will be displayed in GB’s.

ramesh@ramesh-laptop:~$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1              29G  3.1G   24G  12% /
/dev/sda2             115G   48G   62G  44% /home

Use -T option to display what type of file system.

ramesh@ramesh-laptop:~$ df -T
Filesystem    Type   1K-blocks      Used Available Use% Mounted on
/dev/sda1     ext4    29530400   3233120  24797216  12% /
/dev/sda2     ext4   120367992  50171596  64082060  44% /home

26. kill command examples

Use kill command to terminate a process. First get the process id using ps -ef command, then use kill -9 to kill the running Linux process as shown below. You can also use killall, pkill, xkill to terminate a unix process.

$ ps -ef | grep vim
ramesh    7243  7222  9 22:43 pts/2    00:00:00 vim

$ kill -9 7243

27. rm command examples

Get confirmation before removing the file.

$ rm -i filename.txt

It is very useful while giving shell metacharacters in the file name argument.

Print the filename and get confirmation before removing the file.

$ rm -i file*

Following example recursively removes all files and directories under the example directory. This also removes the example directory itself.

$ rm -r example

28. cp command examples

Copy file1 to file2 preserving the mode, ownership and timestamp.

$ cp -p file1 file2

Copy file1 to file2. if file2 exists prompt for confirmation before overwritting it.

$ cp -i file1 file2

29. mv command examples

Rename file1 to file2. if file2 exists prompt for confirmation before overwritting it.

$ mv -i file1 file2

Note: mv -f is just the opposite, which will overwrite file2 without prompting.

mv -v will print what is happening during file rename, which is useful while specifying shell metacharacters in the file name argument.

$ mv -v file1 file2

30. cat command examples

You can view multiple files at the same time. Following example prints the content of file1 followed by file2 to stdout.

$ cat file1 file2

While displaying the file, following cat -n command will prepend the line number to each line of the output.

$ cat -n /etc/logrotate.conf
    1	/var/log/btmp {
    2	    missingok
    3	    monthly
    4	    create 0660 root utmp
    5	    rotate 1
    6	}

31. mount command examples

To mount a file system, you should first create a directory and mount it as shown below.

# mkdir /u01

# mount /dev/sdb1 /u01

You can also add this to the fstab for automatic mounting. i.e Anytime system is restarted, the filesystem will be mounted.

/dev/sdb1 /u01 ext2 defaults 0 2

32. chmod command examples

chmod command is used to change the permissions for a file or directory.

Give full access to user and group (i.e read, write and execute ) on a specific file.

$ chmod ug+rwx file.txt

Revoke all access for the group (i.e read, write and execute ) on a specific file.

$ chmod g-rwx file.txt

Apply the file permissions recursively to all the files in the sub-directories.

$ chmod -R ug+rwx file.txt

33. chown command examples

chown command is used to change the owner and group of a file. \

To change owner to oracle and group to db on a file. i.e Change both owner and group at the same time.

$ chown oracle:dba dbora.sh

Use -R to change the ownership recursively.

$ chown -R oracle:dba /home/oracle

34. passwd command examples

Change your password from command line using passwd. This will prompt for the old password followed by the new password.

$ passwd

Super user can use passwd command to reset others password. This will not prompt for current password of the user.

# passwd USERNAME

Remove password for a specific user. Root user can disable password for a specific user. Once the password is disabled, the user can login without entering the password.

# passwd -d USERNAME

35. mkdir command examples

Following example creates a directory called temp under your home directory.

$ mkdir ~/temp

Create nested directories using one mkdir command. If any of these directories exist already, it will not display any error. If any of these directories doesn’t exist, it will create them.

$ mkdir -p dir1/dir2/dir3/dir4/

36. ifconfig command examples

Use ifconfig command to view or configure a network interface on the Linux system.

View all the interfaces along with status.

$ ifconfig -a

Start or stop a specific interface using up and down command as shown below.

$ ifconfig eth0 up

$ ifconfig eth0 down

37. uname command examples

Uname command displays important information about the system such as — Kernel name, Host name, Kernel release number,
Processor type, etc.,

Sample uname output from a Ubuntu laptop is shown below.

$ uname -a
Linux john-laptop 2.6.32-24-generic #41-Ubuntu SMP Thu Aug 19 01:12:52 UTC 2010 i686 GNU/Linux

38. whereis command examples

When you want to find out where a specific Unix command exists (for example, where does ls command exists?), you can execute the following command.

$ whereis ls
ls: /bin/ls /usr/share/man/man1/ls.1.gz /usr/share/man/man1p/ls.1p.gz

When you want to search an executable from a path other than the whereis default path, you can use -B option and give path as argument to it. This searches for the executable lsmk in the /tmp directory, and displays it, if it is available.

$ whereis -u -B /tmp -f lsmk
lsmk: /tmp/lsmk

39. whatis command examples

Whatis command displays a single line description about a command.

$ whatis ls
ls		(1)  - list directory contents

$ whatis ifconfig
ifconfig (8)         - configure a network interface

40. locate command examples

Using locate command you can quickly search for the location of a specific file (or group of files). Locate command uses the database created by updatedb.

The example below shows all files in the system that contains the word crontab in it.

$ locate crontab
/etc/anacrontab
/etc/crontab
/usr/bin/crontab
/usr/share/doc/cron/examples/crontab2english.pl.gz
/usr/share/man/man1/crontab.1.gz
/usr/share/man/man5/anacrontab.5.gz
/usr/share/man/man5/crontab.5.gz
/usr/share/vim/vim72/syntax/crontab.vim

41. man command examples

Display the man page of a specific command.

$ man crontab

When a man page for a command is located under more than one section, you can view the man page for that command from a specific section as shown below.

$ man SECTION-NUMBER commandname

Following 8 sections are available in the man page.

1. General commands


2. System calls


3. C library functions


4. Special files (usually devices, those found in /dev) and drivers


5. File formats and conventions


6. Games and screensavers


7. Miscellaneous


8. System administration commands and daemons

For example, when you do whatis crontab, you’ll notice that crontab has two man pages (section 1 and section 5). To view section 5 of crontab man page, do the following.

$ whatis crontab
crontab (1)          - maintain crontab files for individual users (V3)
crontab (5)          - tables for driving cron

$ man 5 crontab

42. tail command examples

Print the last 10 lines of a file by default.

$ tail filename.txt

Print N number of lines from the file named filename.txt

$ tail -n N filename.txt

View the content of the file in real time using tail -f. This is useful to view the log files, that keeps growing. The command can be terminated using CTRL-C.

$ tail -f log-file

43. less command examples

less is very efficient while viewing huge log files, as it doesn’t need to load the full file while opening.

$ less huge-log-file.log

One you open a file using less command, following two keys are very helpful.

CTRL+F – forward one window
CTRL+B – backward one window

44. su command examples

Switch to a different user account using su command. Super user can switch to any other user without entering their password.

$ su - USERNAME

Execute a single command from a different account name. In the following example, john can execute the ls command as raj username. Once the command is executed, it will come back to john’s account.

[john@dev-server]$ su - raj -c 'ls'

[john@dev-server]$

Login to a specified user account, and execute the specified shell instead of the default shell.

$ su -s 'SHELLNAME' USERNAME

45. mysql command examples

mysql is probably the most widely used open source database on Linux. Even if you don’t run a mysql database on your server, you might end-up using the mysql command ( client ) to connect to a mysql database running on the remote server.

To connect to a remote mysql database. This will prompt for a password.

$ mysql -u root -p -h 192.168.1.2

To connect to a local mysql database.

$ mysql -u root -p

If you want to specify the mysql root password in the command line itself, enter it immediately after -p (without any space).

46. yum command examples

To install apache using yum.

$ yum install httpd

To upgrade apache using yum.

$ yum update httpd

To uninstall/remove apache using yum.

$ yum remove httpd

47. rpm command examples

To install apache using rpm.

# rpm -ivh httpd-2.2.3-22.0.1.el5.i386.rpm

To upgrade apache using rpm.

# rpm -uvh httpd-2.2.3-22.0.1.el5.i386.rpm

To uninstall/remove apache using rpm.

# rpm -ev httpd

48. ping command examples

Ping a remote host by sending only 5 packets.

$ ping -c 5 gmail.com

More ping examples: Ping Tutorial: 15 Effective Ping Command Examples

49. date command examples

Set the system date:

# date -s "01/31/2010 23:59:53"

Once you’ve changed the system date, you should syncronize the hardware clock with the system date as shown below.

# hwclock –systohc

# hwclock --systohc –utc

50. wget command examples

The quick and effective method to download software, music, video from internet is using wget command.

$ wget http://prdownloads.sourceforge.net/sourceforge/nagios/nagios-3.2.1.tar.gz

Download and store it with a different name.

$ wget -O taglist.zip http://www.vim.org/scripts/download_script.php?src_id=7701