Tuesday, 25 May 2021

Top DevOps Interview Questions with Answers


  1. A Pod is crashed, how you will troubleshoot it?
  2. What is difference between $*, $@,$_ and $0?
  3. What are hosted zones in AWS?
  4. How to setup Weighted routing in route 53?
  5. How to create a network load balancer?
  6. What is provisioner and provider in Terraform?
  7. What are the modules you have used in Ansible?
  8. How to setup modules in Ansible?
  9. What are hostVars and groupVars in Ansible?
  10. How to enable gather_facts in Anisble?
  11. What are j2 templates in Ansible?
  12. What are tasks and handlers in Ansible?


Adding more with answers........stay tuned !!!!

Build and Release Engineer Interview Questions

Build and Release Engineer Interview Questions
Hi All,,

Here I would like to give some Interview Questions that I have faced in Telephonic and Face to Face Interview

The Basic Questions you need to be Prepared:-
===========================================
1. Tell about yourself?
2. What are your current Roles and Responsibilities?
3. How Much experience do you have?
4. What was the flow of Software development?
5. What tools are you aware of?
6. Have you worked on LINUX?
7. Make an overview of tools that will be used in Build and Release?
8. List out Version control commands?
9. Will update more......


Technical Questions:-
===================
1. What are the version controls you have used?
2. Have you worked on any Continuous Integration Tools?
3. Do you have any experience with scripting languages?
4. Do you know shell scripting?
5. What software do you use for Installers?
6. Have you written any build files using ANT/NANT?
7. Linux Commands-->cp,df,du,ps,umount,awk,time,ls,grep,top,head
8. How to create new user in LINUX?
9. How to Configure SVN on Linux?
10. Questions on SQL?
11. Windows platforms?
12. Questions on IIS?
13. How to create a branch in SVN?
14. When you will create a branch?
15. What is the structure of branches in the repo?
16. How to create backups of the repository?
17. Commands to create New User, New branch, and code Merging?
18. How you will release products to the client?
19. Do you know about Batch files?
20. Do you know about NSIS/Installshield/WIX Installer?
21. What are the tasks you will do in daily routine?
22. What will do for repeated tasks?
23. What do you know about SVN/CVS/TFS/GIT?
24. How to migrate from SVN to GIT?
25. Questions on JOINS, HAVING, GROUP BY, Alias in SQL?
26. Difference between DROP, DELETE, Truncate?
27. Difference between branch, tag, trunk?
28. Write a shell script to find the largest of 3 numbers, while given as input from the shell command line?
29. What is the difference between export and update in SVN?
30. What is the difference between update and commit in SVN?
31. What are the basic svn commands frequently used?
32. How to revert to a specific revision in svn without loosing the local changes?
33. What happens if a directory is deleted from server suddenly? How will you get the directory back?
34. What is the difference between  checkout and export in SVN?
35. How to tag a branch?
36. Salve config
37. How to create backup in SVN?
38. Commands for grep, Find, dr, du?
39. How to copy error logs > Dev/Null?
40. What are Jenkins plugins?
41. What are different Jenkins jobs?
42. Mail command?
43. How to post build in Jenkins?
44. Msbuild
45. Explain P4 commands
46. What is difference between $*, $@ , $_and $0?
47.  Explain p4 client command.
48. How to find files under sub directories?
49. How to get first column values?
50. Which port is free?
51. What is process on back ground? how to get it foreground?
52. How to get last column on ls-l?
53. What you need to login onto p4 client?
54. Which is the default p4 port?
55. What are installsheild conditions?
56. What are different multilanguages in IS?
57. How to create upgrade Installer?
58. What are the registry entry values?
59. How to know which services are running?
60. How to differentiate between Service and process?
61. How to create symbolic link?
62. Explain nslookup command.
63. How to configure UAC settings?
64. What are different SVN commands?
65. How to set up cron job in Jenkins?
66. How to grep only filenames that match?
67. Explain Teamcity.
68. What are file properties in SVN?
69. How to get started with SVN externals?
70. Explain relocate and switch in SVN.
71. What are the different backup types in SVN?
72. What is the maximum revision SVN will support?
73. What are the 4 ways of executing a shell script in Unix/Linux?
74. How to create jobs in Jenkins?
75. What are the plugins used?
76. What is the difference between target and task?
77. How you setup slave?
78. How you launch slave?
79. How to run slave as specific user?
80. How to run a specific job on slave?
81. What is the connection between master and slave?
82. What are the environment variables you set on slave?
83. Explain about view map.
84. How to create p4 client spec ?
85. How you exclude some files from using view map?
86. Explain p4 branch spec .
87.What are different types of views?Explain the difference.
88. What is default spec?
89. What is the command to edit config spec?
90. How to set view using script?
91. Explain clearcase spec.
92. Explain  extended view path in clearcase.

Will update more.....










AWS How to setup weighted routing in route53

Routing policy is one of the important AWS feature, it determines how Amazon Route53 responds to queries when you create a record.


Routing policies let you choose how Route 53 routes traffic to you resources. If you have multiple resources that perform the same operation, such as serve content for a website, choose a routing policy other than simple. Here's a brief comparison:

There are 7 types of routing policy in AWS. The top 4 routing policy are mostly used ones based on requirement.

  1. Simple: Simple records use standard DNS functionality.
  2. Weighted: Weighted records let you specify what portion of traffic to send to each resource.
  3. Geolocation: Geolocation records let you route traffic to your resources based on the geographic location of your users.
  4. Latency: Latency records let you route traffic to resources in the AWS Region that provides the lowest latency. All resources must be in AWS Regions.
  5. Failover: Failover records let you route traffic to a resource when the resource is healthy or to a different resource when the first resource is unhealthy.
  6. Multivalue answer: Multivalue answer records let you configure Route 53 to return multiple values, such as IP addresses for your web servers, in response to DNS queries.
  7. Geoproximity routing:  when you want to route traffic based on the location of your resources and, optionally, shift traffic from resources in one location to resources in another.



Weighted routing

Weighted routing lets you associate multiple resources with a single domain name (devopsvm.com) or subdomain name (dev.devopsvm.com,) and choose how much traffic is routed to each resource. This can be useful for a variety of purposes, including load balancing and testing new versions of software.

To configure weighted routing, you create records that have the same name and type for each of your resources. You assign each record a relative weight that corresponds with how much traffic you want to send to each resource. Amazon Route 53 sends traffic to a resource based on the weight that you assign to the record as a proportion of the total weight for all records in the group:


Formula for how much traffic is routed to a given resource: 
weight for a specified record / sum of the weights for all records.

For example, if you want to send a tiny portion of your traffic to one resource and the rest to another resource, you might specify weights of 1 and 255. The resource with a weight of 1 gets 1/256th of the traffic (1/(1+255)), and the other resource gets 255/256ths (255/(1+255)). You can gradually change the balance by changing the weights. If you want to stop sending traffic to a resource, you can change the weight for that record to 0.



Once you have records in hosted zones, select a record you want to add routing policy. Then you get an option to Edit Record, at the bottom you can see Routing policy select the policy you want to use based on your requirement.





Monday, 24 May 2021

AWS Hosted Zones

 

Hosted zones is one of the top feature of Route53, it tells Route 53 how to respond to DNS queries for a domain such as example.com.

It has records and those records contains information about how you want to route traffic for a specific domain, such as from example.com, and its subdomains (dev.example.com, qa.example.com). A hosted zone and the corresponding domain will have the same name. 

There are two types of hosted zones:

  1. Public hosted zones 
  2. Private hosted zones
Public hosted zones

It Contain records that specify how you want to route traffic on the internet

Private hosted zones

It contain records that specify how you want to route traffic in an Amazon VPC





Search for Route 53 in search, then click on Hosted Zones which is under DNS management.

Provide domain name like example.com, devopsvm.com, a bit of description and select which type of hosted zone is needed either public or private. Then click on create hosted zone button.
















Kubernetes - How to check logs of a running and crashed pods

 This is the most common issue that every DevOps guy faces, while working on Kubernetes. One day suddenly you will find that one of the pod you are working is in bad state. And you dont know what went wrong. So as a DevOps engineer you need to be aware of how to view logs of running pod or crashed pod.


First you need to know which pod you want see logs for, to do that

Get the pods

kubectl get pods -A      To list all pods irrespective of namespace

kubectl get pods -n <namespace>  To get list of pods from a specific namespace

kubectl get pods  To get list of pods from default namespace


[devopsvm@dev ~]$ kubectl get pods 

NAMESPACE     NAME                                                    READY   STATUS      RESTARTS   AGE

default       jenkins-master-wed66fcfc-kasdas                      1/1     Running           1               18d

default       nfs-client-provisioner-6756-75vc5q                 1/1     Running         0               86d


Get logs of running pods

If pod running in default namespace

kubectl log jenkins-master-wed66fcfc-kasdas  


if pod running in a different namespace

kubectl log jenkins-master-wed66fcfc-kasdas  -n <namespace>


To see logs in realtime (use -f option)

-f option gives you logs in real time on your screen, to exit press CTRL+C or CTRL+Z

kubectl logs -f jenkins-master-wed66fcfc-kasdas  


Get logs of crashed pods (use -p option)

-p or --previous option give option to see logs of a pod which got restarted or crashed. In above case you see jenkins is restarted 1 time..so if you want to see like why it got restarted, use either of the option.


kubectl logs jenkins-master-wed66fcfc-kasdas -p 

This will show logs of pod before it got crashed, best usage in realtime to troubleshoot why a pod is restarted.


Get logs a specific container inside a pod having multiple containers

When you have a pod having multiple containers running in it, then this way we can find logs of that specific container.

[devopsvm@dev ~]$ kubectl get pods 

NAMESPACE     NAME                                                    READY   STATUS      RESTARTS   AGE

default       jenkins-master-wed66fcfc-kasdas                      2/2     Running           1               18d


If you see 2/2 in under READY , it means that pod is having two containers inside it.

so if you try to run just kubectl logs jenkins-master-wed66fcfc-kasdas it will throw you an error saying "container name must be specified"

error: a container name must be specified for pod  jenkins-master-wed66fcfc-kasdas 
choose one of: [master slave]

From above error you got to know two containers are running master and slave.

use -c option to pass container name like

kubectl logs  jenkins-master-wed66fcfc-kasdas  -c slave

kubectl logs  jenkins-master-wed66fcfc-kasdas  -c slave -n <namespace>

kubectl logs  jenkins-master-wed66fcfc-kasdas  -c slave -p


you can pass namespace and -p options as well to get previous logs.


Above commands are mostly used in troubleshooting pod while running and crashed state.


Monday, 10 August 2020

Ubuntu - How to prevent updating of a specific package?


There are couple of ways to prevent packages getting updated by default.

1. apt

2. dpkg

3. aptitude

4. dselect


1. apt

Using hold and unhold you can stop packages getting updated

hold (To stop)

sudo apt-mark hold <package>


ex:- sudo apt-mark hold kubeadm kubelet kubectl kubelet

unhold (To Allow)

sudo apt-mark unhold <package>

ex:- sudo apt-mark unhold kubeadm kubelet kubectl kubelet

To view all packages on hold

sudo apt-mark showhold


2. dpkg

To hold the Package

echo "<package> hold" | sudo dpkg --set-selections

To remove the hold

echo "<package> install" | sudo dpkg --set-selections

To display status of a single package

dpkg --get-selections | grep "<package>"

To display status of all packages

dpkg --get-selections


3. aptitude

To hold a package:

sudo aptitude hold <package>

To remove the hold

sudo aptitude unhold <package>


Saturday, 5 November 2016

Important Linux Commands





LINUX 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 "venkatesh" *

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 venky remotehost.example.com
Debug ssh client
ssh -v -l venky 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' sample.txt
Add line number for all non-empty-lines in a file
$ sed '/./=' sample.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 venky 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/venky/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
Tricks and Tips