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1. What is Linux alias Command?

Explanation

alias COMMAND:
     alias command allows you to create a shortcut to a command. As the name indicates, you can set alias/shortcut name for the commands/paths which is too longer to remember.

SYNTAX:
  The Syntax is
     alias [options] [ AliasName [ =String ] ]

OPTIONS:

-a	Removes all alias definitions from the current shell execution environment.
-p	Prints the list of aliases in the form alias name=value on standard output.

EXAMPLE:

1. To create a shortcut temporarily:

alias lhost='cd /var/www/html'

This command will set lhost to cd /var/www/html/.
Now if you type lhost it will take you to the specified folder/directory.

2. To create a shortcut Permanently:
You can put your aliases into the /home/user/.bashrc file. It is good to add them at the end of the file.

alias home='cd /var/www/html/hscripts/linux-commands'

Now if you type home it will take you to the specified folder/directory.

3. To create a shortcut for a command:

alias c='clear'

This command will set c to clear.
Now if you type c it will clear the screen.

2. What is Linux awk Command?

Explanation

awk COMMAND:
     awk command is used to manipulate the text.This command checks each line of a file, looking for patterns that match those given on the command line.

SYNTAX:
  The Syntax is
     awk '{pattern + action}' {filenames}

OPTIONS:

-W version	Display version information and exit.
-F	Print help message and exit.

EXAMPLE:
Lets create a file file1.txt and let it have the following data:

Data in file1.txt

14 15 16

15 15 11

5 56 6

5 25 1

1. To print the second column data in file1.txt

awk '{print $2}' file1.txt

This command will manipulate and print second column of text file (file1.txt). The output will look like

15
15
56
25

2. To multiply the column-1 and column-2 and redirect the output to file2.txt:

awk '{print $1,$2,$1*$2}' file1.txt > file2.txt

Command Explanation:
$1	: Prints 1^st column
$2	: Prints 2^ndcolumn
$1*$2	: Prints Result of $1 x $2
file1.txt	: input file
>	: redirection symbol
file2.txt	: output file

The above command will redirect the output to file2.txt and it will look like,
14    15    210
15    15    225
5      56    280
5      25    125

3. What is Linux autoreconf Command?

Explanation

autoreconf COMMAND:
     autoreconf - Update generated configuration files

Run 'autoreconf' repeatedly to remake the GNU Build System files in the DIRECTORIES or the directory trees driven by CONFIG-URE-AC.

     By default, it only remakes those files that are older than their predecessors. If you install new versions of the GNU Build System, running 'autoreconf' remakes all of the files by giving it the '--force' option.

SYNTAX:
  The Syntax is
      autoreconf [OPTION] ... [CONFIGURE-AC or DIRECTORY] ...

OPTIONS:

Operation modes:

-h,--help	print this help, then exit.
-V, --version	print version number, then exit.
-v,--verbose	verbosely report processing.
-d,--debug	don't remove temporary files.
-f, --force	consider all files obsolete.
-i, --install	copy missing auxiliary files.
-s,--symlink	with -i, install symbolic links instead of copies
-m, --make	when applicable, re-run ./configure && make
-W,--warnings=CATEGORY	report the warnings falling in CATEGORY [syntax]

Warning categories include:
'cross'	cross compilation issues
'gnu'	GNU coding standards (default in gnu and gnits modes).
'obsolete'	obsolete features or constructions
'override'	user redefinitions of Automake rules or variables
'portability'	portability issues
'syntax'	dubious syntactic constructs (default).

EXAMPLE:

autoreconf --force --install -I config -I m4

4. What is Linux a2p Command?

Explanation

a2p COMMAND:
     a2p - Awk to Perl translator
A2p takes an awk script specified on the command line (or from standard input) and produces a comparable perl script on the standard output.

SYNTAX:
  The Syntax is
      a2p [options] [filename]

OPTIONS:

-D<number>	sets debugging flags.
-F<character>	tells a2p that this awk script is always invoked with this -F switch.
-n<fieldlist>	specifies the names of the input fields if input does not have to be split into an array.
-<number>	causes a2p to assume that input will always have that many fields.

EXAMPLE:

a2p myfile - would convert the file myfile.

Awk to perl translator scripts are often embedded in a shell script that pipes stuff into and out of awk. Often the shell script wrapper can be incorporated into the perl script, since perl can start up pipes into and out of itself, and can do other things that awk can't do by itself.

5. What is Linux bc Command?

Explanation

bc COMMAND:
     bc command is used for command line calculator. It is similar to basic calculator. By using which we can do basic mathematical calculations.

SYNTAX:
  The Syntax is
     bc [options]

OPTIONS:

-c	Compile only. The output is dc commands that are sent to the standard output.
-l	Define the math functions and initialize scale to 20, instead of the default zero.
filename	Name of the file that contains the basic calculator commands to be calculated this is not a necessary command.

EXAMPLE:

1. bc

Output:

bc 1.06

This is free software with ABSOLUTELY NO WARRANTY.

For details type `warranty'.

9*2

The above command used is for mathematical calculations.

2. bc -l

Output:

bc 1.06

This is free software with ABSOLUTELY NO WARRANTY.

For details type `warranty'.

1+2

The above command displays the sum of '1+2'.

3. bc calc.txt

Output:

bc 1.06

This is free software with ABSOLUTELY NO WARRANTY.

For details type `warranty'.

'calc.txt' file have the following code:1+2. Get the input from file and displays the output.

6. What is Linux bg Command

Explanation

bg COMMAND:
     bg command is used to place a job in background. User can run a job in the background by adding a "&" symbol at the end of the command.

SYNTAX:
  The Syntax is
     bg [options] [job]

OPTIONS:

-l	Report the process group ID and working directory of the jobs.
-p	Report only the process group ID of the jobs.
-x	Replace any job_id found in command or arguments with the corresponding process group ID, then execute command passing it arguments.
job	Specifies the job that want to run in the background.

EXAMPLE:

1. To Run a process in background

2. kmail- start the email client application.

Press ctrl+z to stop the current job.

Now just type bg to move last stopped job to background.

3. To move the specified job to back ground:

Lets start some three jobs and suspend those process in background.

kmail- start the email client application.

Press ctrl+z to stop the current job.

xmms- music player application.

Press ctrl+z to stop the current job.

sleep 120- a dummy job.

Press ctrl+z to stop the current job.

jobs

The above command will display the jobs in the shell.

[1] Stopped kmail

[2]- Stopped xmms

[3]+ Stopped sleep 120

bg 2

The above command will start running the xmms application. In such way you can start running the specific background process.

jobs

[1]- Stopped kmail

[2] Running xmms &

[3]+ Stopped sleep 120

7. What is Linux bzip2 Command?

Explanation

bzip2 COMMAND:
     bzip2 linux command is used to compress the file. Each file is replaced by a compressed version of itself with .bz2 extension.

SYNTAX:
  The Syntax is
     bzip2 [ options ] filenames

OPTIONS:

- c	Compress or decompress to standard output.
- d	Force decompression.
- z	The complement to -d. Force compression.
- t	Performs the integrity test. It performs a trial decompression test and prints the result.
- f	Force overwrite of output file.
- k	Keep the original file. dont delete the input file during compression or decompression.
- q	Quiet, suppress non-essential warning messages.
- s	Reduce memory usage, for compression,decompression and testing.
- v	verbose mode shows the compression ratio for each file processed.
- V	Displays the version of the software.
- L	Displays the license terms and conditions.
- 1	Performs fast compression,creating a relatively large files.
- 9	Get the best possible compression.

EXAMPLE:

1. To Compress a file using bzip2:

Lets have a text file as an example.

$ ls -l

-rw-rw-r-- 1 hiox hiox 9150000 Sep 26 18:37 hiox.txt

$ bzip2 -c -1 hiox.txt > hiox.txt.bz2

$ ls -l

-rw-rw-r-- 1 hiox hiox 9150000 Sep 26 18:37 hiox.txt

-rw-rw-r-- 1 hiox hiox 17706 Sep 27 12:38 hiox.txt.bz2

From the above example it is clear that the filesize is reduced from 9150000 bytes to 17706.

When the File is reduced with option -9 the filesize still gets reduced.

$ bzip2 -c -9 hiox.txt > hscripts.txt.bz2

$ ls -l

-rw-rw-r-- 1 hiox hiox 9150000 Sep 26 18:37 hiox.txt

-rw-rw-r-- 1 hiox hiox 17706 Sep 27 12:38 hiox.txt.bz2

-rw-rw-r-- 1 hiox hiox 2394 Sep 27 13:01 hscripts.txt.bz2

When the file is compressed with -1 the size was 17706 bytes and now the filesize is 2394 bytes. The 9 makes best compression but the default is 6.

8. What is Linux cal Command?

Explanation

cal COMMAND:
     cal command is used to display the calendar.

SYNTAX:
  The Syntax is
     cal [options] [month] [year]

OPTIONS:

-1	Displays single month as output.
-3	Displays prev/current/next month output.
-s	Displays sunday as the first day of the week.
-m	Displays Monday as the first day of the week.
-j	Displays Julian dates (days one-based, numbered from January 1).
-y	Displays a calendar for the current year.

EXAMPLE:

1. cal

Output:

September 2008

Su Mo Tu We Th Fr Sa

1 2 3 4 5 6

7 8 9 10 11 12 13

14 15 16 17 18 19 20

21 22 23 24 25 26 27

28 29 30

cal command displays the current month calendar.

2. cal -3 5 2008

Output:

April 2008 May 2008 June 2008

Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa

1 2 3 4 5 1 2 3 1 2 3 4 5 6 7

6 7 8 9 10 11 12 4 5 6 7 8 9 10 8 9 10 11 12 13 14

13 14 15 16 17 18 19 11 12 13 14 15 16 17 15 16 17 18 19 20 21

20 21 22 23 24 25 26 18 19 20 21 22 23 24 22 23 24 25 26 27 28

27 28 29 30 25 26 27 28 29 30 31 29 30

Here the cal command displays the calendar of April, May and June month of year 2008.

9. What is Linux cat Command?

Explanation

cat COMMAND:
     cat linux command concatenates files and print it on the standard output.

SYNTAX:
  The Syntax is
     cat [OPTIONS] [FILE]...

OPTIONS:

-A	Show all.
-b	Omits line numbers for blank space in the output.
-e	A $ character will be printed at the end of each line prior to a new line.
-E	Displays a $ (dollar sign) at the end of each line.
-n	Line numbers for all the output lines.
-s	If the output has multiple empty lines it replaces it with one empty line.
-T	Displays the tab characters in the output.
-v	Non-printing characters (with the exception of tabs, new-lines and form-feeds) are printed visibly.

EXAMPLE:

1. To Create a new file:

cat > file1.txt

This command creates a new file file1.txt. After typing into the file press control+d (^d) simultaneously to end the file.

2. To Append data into the file:

cat >> file1.txt

To append data into the same file use append operator >> to write into the file, else the file will be overwritten (i.e., all of its contents will be erased).

3. To display a file:

cat file1.txt

This command displays the data in the file.

4. To concatenate several files and display:

cat file1.txt file2.txt

The above cat command will concatenate the two files (file1.txt and file2.txt) and it will display the output in the screen. Some times the output may not fit the monitor screen. In such situation you can print those files in a new file or display the file using less command.

cat file1.txt file2.txt | less

5. To concatenate several files and to transfer the output to another file.

cat file1.txt file2.txt > file3.txt

In the above example the output is redirected to new file file3.txt. The cat command will create new file file3.txt and store the concatenated output into file3.txt.

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

More tar examples:

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 "ramesh" *

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

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

< John Doe --- > John M Doe

> Jason Bourne

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

More ls examples: Unix LS Command: 15 Practical Examples

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:

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

More ftp examples:

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

More crontab examples:

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 steatus 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]$

$ 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

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

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Tuesday, 4 March 2014

Some Linux Command