Linux Daily Uses & Advance Commands – Part 1

 

1. How to remove files older than 7 days by creating a cron job to run every night?

find /var/log/security -type f -mtime +7 -exec rm -rf {} \;

Cron – /etc/crontab (show the cron format)

To add the command in crontab file

crontab -e

0 0 * * * /bin/find /var/log/security -type f -mtime +7 -exec rm -rf {} \;

2. Run a command that shows all the line except any lines starting with a character # in the file?

cat filename | grep -v ^#  (character name you want to omit)

3. Which 2 files contain default values when creating a user with useradd command? 


First File: /etc/default/useradd

Second File: /etc/login.defs

cat /etc/default/useradd

grep -v ^# /etc/login.defs | grep -v ^$ [first # symbol for omit and second $ for omit space)

Password max, min, warn days | password length | UID and GID min, max | umask | encryption method



4. What is the command to create user with a pre defined uid, shell and home directory?

useradd -m -d /home/username -s /bin/bash -u 9000 username

grep username /etc/passwd

5. How to delete a user with his home directory?

userdel -r username

6. How to create a user specifying a primary/secondary group?

useradd -g test -G wheel username [small g for primary group and capital G for secondary group)

id username [to check user information]

7. How to change primary group of any user?

usermod -g primarygroupname username

8. How can you give a normal user all the root level privileges? 

The file path is /etc/sudoers (But we never change the sudoer file directly, we edit the file with visudo command)

visudo

Change the file line # # Allow root to run any command anywhere

root  ALL=(ALL) ALL
gautam  ALL=(ALL) ALL (Add this line below the root line)

9. How can you give sudo access to any user without asking him to provide password every time he runs a command?

visudo

Add the line below – # # Same thing without a password [we need to add user in wheel group)

# %wheel ALL(ALL) NOPASSWD: ALL

linux ALL(ALL) NOPASSWD: ALL

10. How to view the user’s login and logout details?

last 

last username (particular user detail)