Linux administrator commands: Practical sessions

Swap Space Allocation using swapfile:

Swap is a portion of hard drive storage that has been set aside for the operating system to temporarily store data that it can no longer hold in RAM.

1. Check swap space is allocated or not, if not proceed below.

free -h

2. Check the / disk whether space is available

df -h

3. Allocate swap space:

fallocate -l 1G /swapfile

4. Change read write permissions to root user:

chmod 600 /swapfile

5. Mark the file as swapfile by below command:

mkswap /swapfile

6. Enable swap file for system to start using on it:

swapon /swapfile

7. check swap space by below command:

free -h

swapon -s

8. Add below line to Permanently add swap file entry fstab file.

/swapfile none swap sw 0 0

9. Swap space performance has been based on swappiness value 1 to 100. Less value would be better.

To check current system swappiness value:

cat /proc/sys/vm/swappiness

10. To set swappiness temporarily:

sysctl vm.swappiness=10

11. To permanently set swappiness value:

vi /etc/sysctl.conf

Add below line at the end of the file.

vm.swappiness=10

12. To release the swap space.

rm /swapfile

13. To clear swap space:

Before clearing check whether you have enough RAM.

swapoff -a

wait 30s, then

swapon -a

Services check:

1. To check all configured services in linux system:

systemctl --type=service

2. To check only running services in linux:

systemctl --type=service --state=running

3. To check all failed services in linux system:

systemctl --type=service --state=failed

4. To configure a new user service follow below steps:

Place the script in /usr/local/sbin as test.sh

Check execute permissions for test.sh

Navigate to below path:

cd /lib/systemd/system

Append below lines in test.service file.

vi /lib/systemd/system/test.service

[Unit]

Description=Test

[Service]

ExecStart=/usr/local/sbin/test.sh

[Install]

WantedBy=multi-user.target

chmod 755 test.service

Reload the system daemon for the service to take effect.

systemctl daemon-reload

Enable the new service:

systemctl enable test.service

start the new service:

systemctl start test.service

To remove service:

systemctl stop test.service

systemctl disable test.service

rm /lib/systemd/system/test.service

rm /usr/local/sbin/test.sh

systemctl daemon-reload

To increase CPU performance:

Change the priority of the process using nice command. The amount of time process spends on cpu is called virtual runtime.

renice +10 pid

This will increase virtual runtime of a process, so CPU will allocate less time in the next cycle.

renice -10 pid

This will decrease the virtual runtime of a process, so CPU will allocate more time in the next cycle.

Above will be valid until the system reboot.

Clearing cache in Linux:

Note: Dentries are the memory representation of directory in linux.

1. To clear page cache in linux:

sync; echo 1 > /proc/sys/vm/drop_caches

2. To clear dentries and inode usage:

sync; echo 2 > /proc/sys/vm/drop_caches

3. To clear all above three.

sync; echo 3 > /proc/sys/vm/drop_caches

Kernel upgrade and Patching in Linux

1. To check available updates:

yum check-updates

2. To search for a package:

yum search sysstat

yum list sysstat

3. To install particular package:

yum install sysstat

4.To update all packages:

yum update -y

5. To exclude a package while updating:

yum -x "sysstat" update

To exclude multiple packages while updating:

yum -x "sysstat" -x "lib" update

6. To install only security patches:

yum update -y --security

7. To remove a package:

yum remove sysstat

8. To list installed package:

yum list installed sysstat

9. To get info about package:

yum info sysstat

10. To clear yum cache:

yum clean all

11. To install a package from local system:

yum localinstall sysstat.rpm

12. To downgrade a package:

yum downgrade sysstat

13. To view history of yum commands:

yum history

14. To rollback a update:

yum history rollback 10

Kernel upgrade:

1. To check kernel version:

uanme -a

2. To list available kernel:

yum list kernel

3. To upgrade kernel:

yum update -y

Mail command:

1. To send a mail using mailx command:

mailx -s "subject" -c cc@gmail.com to@gmail.com << EOM

Hi,

Test Mail.

Regards,

Team.

EOM

2. To send mail using sendmail command:

/usr/sbin/sendmail -f "from@gmail.com" "to@gmail.com" << EOM

From: from@gmail.com

To: to@gmail.com

MIME-Version: 1.0

Content-Type: text/html;

Subject: Test Mail

Hi,<br /><br />

Test Mail.<br /><br />

Regards,<br />

Team.<br />

EOM

Java Installation:

1. To check list of available java packages:

yum list *jdk*

2. To install java:

yum install java-latest-openjdk or yum install jdk

3. To check java version:

java -version

4. Command to check Java installed or not

which java

Mount and Unmount a Filesystem:

1. To unmount a FS:

umount /fs_name

To lazy unmount a FS:

umount -l /FS_name

To force unmount a FS when its in use:

umount -f /FS_name

2. To mount a FS:

mount -l /fs_name

To mount all FS available in /etc/fstab:

mount -a

3. To list all mount points available:

mount -l or mount

And list of mounted FS are available in /etc/mtab and /proc/mounts.

4. To list FS based on type xfs:

mount -t xfs

FS types are ext1,ext2,ext3,ext4,xfs,etc

Crontab in Linux:(Job Scheduling Tool)

Min Hour DOM Month DOW command > log

DOM --> Day of month

DOW --> Day of week

1. To list all cronjobs:

crontab -l

2. To add new cronjob:

crontab -e

00 * * * * /home/script.sh > /home/script.log

3. To view crontab of specific user:

crontab -u test -l

4. Execute command at system reboot:

@reboot /home/script.sh > /home/script.log

5. To enable crontab for users:

vi /etc/cron.d/cron.allow

root

test