Linux interview questions

From time to time you might need to interview somebody for a Linux role. It's hard to think of good questions; you don't want to scare somebody with your questions, but you do want to know if the person is knowledgable.

Questions for a junior.

"People that use your Linux server complain it's slow. What tools would you use to check resource usage?"
top, sar, netstat -an, iostat, free, df.

"You discover a disk is full on your Linux server. What do you use to discover where the bigest files/directories are?"

  • To create a short report on the largest directories: du -sk * | sort -nr | head -n10
  • You might use "find" to find large files: find / -size +100M

"A system needs more disk space. You want to add a partition /var/log. You add a drive to the machine, it becomes /dev/sdb. What actions do you take to use this disk?"
Depending if you use LVM or not. If you don't use LVM:

  1. "fdisk /dev/sdb" to add one partition so /dev/sdb1 becomes available.
  2. "mkfs /dev/sdb1" to create a filesystem on it.
  3. "mount /dev/sdb1 /var/log". Actually the data needs to be copied into the new /var/log.

If you do use LVM:

  1. "fdisk /dev/sdb" to add one partition so /dev/sdb1 becomes available.
  2. "pvcreate /dev/sdb1" to make it an LVM device.
  3. "vgextend SomeVolumeGroup /dev/sdb1" to extend an existing Volume Group.
  4. "lvcreate -L 1G -n SomeLogicalVolumeName SomeVolumeGroup" to create a new Logical Volume.
  5. "mkfs /dev/SomeVolumeGroup/SomeLogicalVolumeName" to create a filesystem on it.
  6. "mount /dev/SomeVolumeGroup/SomeLogicalVolumeName /var/log". Actually the data needs to be copied into the new /var/log.

Questions for a medior.

"People that use your Linux server complain it's slow. You've seen the disk usage is high. What can you do to improve this?"

  • Move away services, devide them over different server.
  • Add more memory so disk caching can be used better.
  • Look into the application, why is it reading/writing so much.
  • User faster disks.

"What happens in relation to DNS, SMTP and IMAP when I send send an email to [email protected]?"
My computer is likely to be configure to send the email to a mail server on port 25, SMTP protocol. That mailserver will query the DNS for the MX records of example.com. The mailserver that show up with the lowest priority will be contacted to deliver the email. That mailserver at example.com can accept the email and put in into the imap folder for the user or alias of [email protected]

"You need to setup 100 Red Hat Enterprise Linux systems. If you don't want to walk around and eject and insert a boot CD 100 times, what options would you have?"
Kickstarting would help out. Install one machine as you like it, save /root/anaconda-ks.cfg to a webserver. Setup a PXE (DHCP, TFTP, HTTP, DNS) environment and use that kickstart file to install the rest.

Questions for a senior.

"You have destroyed /etc/pam.d/system-auth and can't login anymore. Another machine has a propper version of /etc/pam.d/system-auth. How would you fix that broken machine?"
The machine needs to be booted in single user mode so you don't get a login prompt. After that here are some option:

  • Start network, use "nc" to get and replace that file.
  • Mount a CD that has the package to fix the broken file.
  • Manually repair it.

"You have installed apache, php and mysql and a webapplication such as Drupal. The webapplication tries to send emails to an external mailserver but fails. What could be the cause when these items have been verified:

  • The web application is correctly configured to use the external mailserver.
  • It's possible to connect to the mailserver on the specified port from the command line.
  • The logs of the mailserver don't record anything when the web application tries to send email."
  • IPtables allows connections out to port 25.

SELinux could be blocking apache from using port 25 on an external system. The logfile /var/log/messages might inform you about it. To fix it issue "setsebool -P httpd_can_network_connect=1".

"What determines the load (w, uptime) of a system?"
The number of processes that are waiting for execution. These processes are in the run queue. Processes could be waiting for io, network or memory allocation.

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References Red Hat Certified Architect By Robert de Bock Robert de Bock
Curriculum Vitae By Fred Clausen +31 6 14 39 58 72
By Nelson Manning [email protected]