Wednesday, May 29, 2013

Linux Commands(Continues)

Linux Commands


SSH


ssh user@host – connect to host as user
ssh -p port user@host – connect to host on port
port as user
ssh-copy-id user@host – add your key to host for
user to enable a keyed or passwordless login


Searching


grep pattern files – search for pattern in files
grep -r pattern dir – search recursively for
pattern in dir
command | grep pattern – search for pattern in the
output of command
locate file – find all instances of file

System Info


date – show the current date and time
cal – show this month's calendar
uptime – show current uptime
w – display who is online
whoami – who you are logged in as
finger user – display information about user
uname -a – show kernel information
cat /proc/cpuinfo – cpu information
cat /proc/meminfo – memory information
man command – show the manual for command
df – show disk usage
du – show directory space usage
free – show memory and swap usage
whereis app show possible locations of app
which app – show which app will be run by default

Compression


tar cf file.tar files – create a tar named
file.tar containing files
tar xf file.tar – extract the files from file.tar
tar czf file.tar.gz files – create a tar with
Gzip compression
tar xzf file.tar.gz – extract a tar using Gzip
tar cjf file.tar.bz2 – create a tar with Bzip2
compression
tar xjf file.tar.bz2 – extract a tar using Bzip2
gzip file – compresses file and renames it to
file.gz
gzip -d file.gz – decompresses file.gz back to
file

Network


ping host – ping host and output results
whois domain – get whois information for domain
dig domain – get DNS information for domain
dig -x host – reverse lookup host
wget file – download file
wget -c file – continue a stopped download\

Installation


Install from source:
./configure
make
make install
dpkg -i pkg.deb – install a package (Debian)
rpm -Uvh pkg.rpm – install a package (RPM)

Shortcuts in Linux


Ctrl+C – halts the current command
Ctrl+Z – stops the current command, resume with
fg in the foreground or bg in the background
Ctrl+D – log out of current session, similar to exit
Ctrl+W – erases one word in the current line
Ctrl+U – erases the whole line
Ctrl+R – type to bring up a recent command
!! - repeats the last command
exit – log out of current session

* use with extreme caution.

Linux Commands

Linux Commands


File Commands


ls – directory listing

ls -al – formatted listing with hidden files

cd dir - change directory to dir

cd – change to home

pwd – show current directory

mkdir dir – create a directory dir

rm file – delete file

rm -r dir – delete directory dir

rm -f file – force remove file

rm -rf dir – force remove directory dir *

cp file1 file2 – copy file1 to file2

cp -r dir1 dir2 – copy dir1 to dir2; create dir2 if it

doesn't exist

mv file1 file2 – rename or move file1 to file2

if file2 is an existing directory, moves file1 into

directory file2

ln -s file link – create symbolic link link to file

touch file – create or update file

cat > file – places standard input into file

more file – output the contents of file

head file – output the first 10 lines of file

tail file – output the last 10 lines of file

tail -f file – output the contents of file as it

grows, starting with the last 10 lines 



Process Management


ps – display your currently active processes

top – display all running processes

kill pid – kill process id pid

killall proc – kill all processes named proc *

bg – lists stopped or background jobs; resume a

stopped job in the background

fg – brings the most recent job to foreground

fg n – brings job n to the foreground 



File Permissions


chmod octal file – change the permissions of file

to octal, which can be found separately for user,

group, and world by adding:

● 4 – read (r)

● 2 – write (w)

● 1 – execute (x)

Examples:

chmod 777 – read, write, execute for all

chmod 755 – rwx for owner, rx for group and world

For more options, see man chmod.

Sunday, February 17, 2013

Linux ls Command

Linux "ls" Command


1. List Files using ls with no option

ls with no option list files and directories in bare format where we won’t be able to view details like file types, size, modified date and time, permission and links etc.
 
# ls
 
0001.pcap        Desktop    Downloads         index.html   install.log.syslog  Pictures  Templates
anaconda-ks.cfg  Documents  fbcmd_update.php  install.log  Music               Public    Videos

 

2. List Files With option –l

Here, ls -l (-l is character not one) shows file or directory, size, modified date and time, file or folder name and owner of file and it’s permission.
 
# ls -l
 
total 176
-rw-r--r--. 1 root root   683 Aug 19 09:59 0001.pcap
-rw-------. 1 root root  1586 Jul 31 02:17 anaconda-ks.cfg
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Desktop
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Documents
drwxr-xr-x. 4 root root  4096 Aug 16 02:55 Downloads
-rw-r--r--. 1 root root 21262 Aug 12 12:42 fbcmd_update.php
-rw-r--r--. 1 root root 46701 Jul 31 09:58 index.html
-rw-r--r--. 1 root root 48867 Jul 31 02:17 install.log
-rw-r--r--. 1 root root 11439 Jul 31 02:13 install.log.syslog
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Music
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Pictures
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Public
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Templates
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Videos

 

3. View Hidden Files

 
List all files including hidden file starting with ‘.‘.
 
# ls -a
 
.                .bashrc  Documents         .gconfd          install.log         .nautilus     .pulse-cookie
..               .cache   Downloads         .gnome2          install.log.syslog  .netstat.swp  .recently-used.xbel
0001.pcap        .config  .elinks           .gnome2_private  .kde                .opera        .spice-vdagent
anaconda-ks.cfg  .cshrc   .esd_auth         .gtk-bookmarks   .libreoffice        Pictures      .tcshrc
.bash_history    .dbus    .fbcmd            .gvfs            .local              .pki          Templates
.bash_logout     Desktop  fbcmd_update.php  .ICEauthority    .mozilla            Public        Videos
.bash_profile    .digrc   .gconf            index.html       Music               .pulse        .wireshark

 

4. List Files with Human Readable Format with option -lh



With combination of -lh option, shows sizes in human readable format.
 
# ls -lh
 
total 176K
-rw-r--r--. 1 root root  683 Aug 19 09:59 0001.pcap
-rw-------. 1 root root 1.6K Jul 31 02:17 anaconda-ks.cfg
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Desktop
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Documents
drwxr-xr-x. 4 root root 4.0K Aug 16 02:55 Downloads
-rw-r--r--. 1 root root  21K Aug 12 12:42 fbcmd_update.php
-rw-r--r--. 1 root root  46K Jul 31 09:58 index.html
-rw-r--r--. 1 root root  48K Jul 31 02:17 install.log
-rw-r--r--. 1 root root  12K Jul 31 02:13 install.log.syslog
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Music
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Pictures
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Public
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Templates
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Videos

 

5. List Files and Directories with ‘/’ Character at the end


Using -F option with ls command, will add the ‘/’ Character at the end each directory.
 
# ls -F
 
0001.pcap        Desktop/    Downloads/        index.html   install.log.syslog  Pictures/  Templates/
anaconda-ks.cfg  Documents/  fbcmd_update.php  install.log  Music/              Public/    Videos/

 

6. List Files in Reverse Order


The following command with ls -r option display files and directories in reverse order.
 
# ls -r
 
Videos     Public    Music               install.log  fbcmd_update.php  Documents  anaconda-ks.cfg
Templates  Pictures  install.log.syslog  index.html   Downloads         Desktop    0001.pcap

 

7. Recursively list Sub-Directories


ls -R option will list very long listing directory trees. See an example of output of the command.
 
# ls -R
 
total 1384
-rw-------. 1 root     root      33408 Aug  8 17:25 anaconda.log
-rw-------. 1 root     root      30508 Aug  8 17:25 anaconda.program.log
 
./httpd:
total 132
-rw-r--r--  1 root root     0 Aug 19 03:14 access_log
-rw-r--r--. 1 root root 61916 Aug 10 17:55 access_log-20120812
 
./lighttpd:
total 68
-rw-r--r--  1 lighttpd lighttpd  7858 Aug 21 15:26 access.log
-rw-r--r--. 1 lighttpd lighttpd 37531 Aug 17 18:21 access.log-20120819
 
./nginx:
total 12
-rw-r--r--. 1 root root    0 Aug 12 03:17 access.log
-rw-r--r--. 1 root root  390 Aug 12 03:17 access.log-20120812.gz

 

8. Reverse Output Order


With combination of -ltr will shows latest modification file or directory date as last.
 
# ls -ltr
 
total 176
-rw-r--r--. 1 root root 11439 Jul 31 02:13 install.log.syslog
-rw-r--r--. 1 root root 48867 Jul 31 02:17 install.log
-rw-------. 1 root root  1586 Jul 31 02:17 anaconda-ks.cfg
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Desktop
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Videos
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Templates
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Public
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Pictures
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Music
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Documents
-rw-r--r--. 1 root root 46701 Jul 31 09:58 index.html
-rw-r--r--. 1 root root 21262 Aug 12 12:42 fbcmd_update.php
drwxr-xr-x. 4 root root  4096 Aug 16 02:55 Downloads
-rw-r--r--. 1 root root   683 Aug 19 09:59 0001.pcap

 

9. Sort Files by File Size


With combination of -lS displays file size in order, will display big in size first.
 
# ls -lS
 
total 176
-rw-r--r--. 1 root root 48867 Jul 31 02:17 install.log
-rw-r--r--. 1 root root 46701 Jul 31 09:58 index.html
-rw-r--r--. 1 root root 21262 Aug 12 12:42 fbcmd_update.php
-rw-r--r--. 1 root root 11439 Jul 31 02:13 install.log.syslog
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Desktop
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Documents
drwxr-xr-x. 4 root root  4096 Aug 16 02:55 Downloads
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Music
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Pictures
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Public
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Templates
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Videos
-rw-------. 1 root root  1586 Jul 31 02:17 anaconda-ks.cfg
-rw-r--r--. 1 root root   683 Aug 19 09:59 0001.pcap

 

10. Display Inode number of File or Directory


We can see some number printed before file / directory name. With -i options list file / directory with inode number.
 
# ls -i
 
20112 0001.pcap        23610 Documents         23793 index.html          23611 Music     23597 Templates
23564 anaconda-ks.cfg  23595 Downloads            22 install.log         23612 Pictures  23613 Videos
23594 Desktop          23585 fbcmd_update.php     35 install.log.syslog  23601 Public

 

11. Shows version of ls command


Check version of ls command.
 
# ls --version
 
ls (GNU coreutils) 8.4
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Written by Richard M. Stallman and David MacKenzie.

 

12. Show Help Page


List help page of ls command with their option.
 
# ls --help
 
Usage: ls [OPTION]... [FILE]...

 

13. List Directory Information


With ls -l command list files under directory /tmp. Wherein with -ld parameters displays information of /tmp directory.
 
# ls -l /tmp
 
total 408
drwx------. 2 narad narad   4096 Aug  2 02:00 CRX_75DAF8CB7768
-r--------. 1 root  root  384683 Aug  4 12:28 htop-1.0.1.tar.gz
drwx------. 2 root  root    4096 Aug  4 11:20 keyring-6Mfjnk
drwx------. 2 root  root    4096 Aug 16 01:33 keyring-pioZJr
drwx------. 2 gdm   gdm     4096 Aug 21 11:26 orbit-gdm
drwx------. 2 root  root    4096 Aug 19 08:41 pulse-gl6o4ZdxQVrX
drwx------. 2 narad narad   4096 Aug  4 08:16 pulse-UDH76ExwUVoU
drwx------. 2 gdm   gdm     4096 Aug 21 11:26 pulse-wJtcweUCtvhn
-rw-------. 1 root  root     300 Aug 16 03:34 yum_save_tx-2012-08-16-03-34LJTAa1.yumtx
 
 
# ls -ld /tmp/
 
drwxrwxrwt. 13 root root 4096 Aug 21 12:48 /tmp/

 

14. Display UID and GID of Files


To display UID and GID of files and directories. use option -n with ls command.
 
# ls -n
 
total 36
drwxr-xr-x. 2 500 500 4096 Aug  2 01:52 Downloads
drwxr-xr-x. 2 500 500 4096 Aug  2 01:52 Music
drwxr-xr-x. 2 500 500 4096 Aug  2 01:52 Pictures
-rw-rw-r--. 1 500 500   12 Aug 21 13:06 tmp.txt
drwxr-xr-x. 2 500 500 4096 Aug  2 01:52 Videos

 

15. ls command and it’s Aliases


We have made alias for ls command, when we execute ls command it’ll take -l option by default and display long listing as mentioned earlier.
 
# alias ls="ls -l"

Note: We can see number of alias available in your system with below alias command and same can be unalias as shown below example.
 
# alias
 
alias cp='cp -i'
alias l.='ls -d .* --color=auto'
alias ll='ls -l --color=auto'
alias ls='ls --color=auto'
alias mv='mv -i'
alias rm='rm -i'
alias which='alias | /usr/bin/which --tty-only --read-alias --show-dot --show-tilde'

To remove an alias previously defined, just use the unalias command.
 
# unalias ls


The 7 Deadly Linux Commands

The 7 Deadly Linux Commands



1. Code:

rm -rf /

This command will recursively and forcefully delete all the files inside the root directory.

50 most frequently used commands in Linux / UNIX

50 most frequently used commands in Linux / UNIX


This article provides practical examples for 50 most frequently used commands in Linux / UNIX.
This is not a comprehensive list by any means, but this should give you a jump start on some of the common Linux commands. Bookmark this article for your future reference.

Did I miss any frequently used Linux commands? Leave a comment  and let me know.

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 "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

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
< 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

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

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

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 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]$
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

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