Linux Administration¶

Overview¶

This page is dedicated to helpful commands, techniques, and packages to system administrators.

Links¶

Ubuntu Cloud Images

Common Tasks¶

Mounting SMB Share¶

mount -t cifs -o username={{username}},password={{password}},domain={{domain}} //{{server}}/{{share}} //mnt/{{directory}}

Mounting NFS Share¶

mount -t nfs 10.10.0.10:/path/to/share /path/to/mount

File Permissions¶

Change ownership chown <username> </path/to/file>

Change ownership recursively with the -r flag chown -r <username> </path/to/file>

Networking¶

etc/network/interfaces will contain information related to the interfaces

DHCP release dhclient -r

DHCP renew dhclient

DHCP verbose dhclient -v

Manually set IP addresses ip addr add 192.168.1.100/24 dev eth0

Netplan Configuration¶

network:
    version: 2
    ethernets:
        eth0:
            addresses:
                - 10.100.10.2/23
            nameservers:
                addresses: [1.1.1.1]
            routes:
                - to: default
                  via: 10.100.10.1

Cron¶

Cron is a great way to schedule tasks on a linux system. Cron format looks like

# minute hour day_of_month month day_of_week
0 6 * * * /path/of/script

# you can also call commands
0 6 * * * uname -r >> /var/log/uname

It is good practice to set the time zone in the crontab file so your crons will run at the proper time

# setting timezone for cron timers
CRON_TZ=America/Detroit

Systemd Timers¶

Use systemctl edit service_name.timer to edit a service or timer with overrides. The OnCalendar directive is additive and does not override the existing value. You need to set a blank value before setting the "override" value.

[Timer]
OnCalendar=
OnCalendar=Mon *-*-* 03:00:00

Reference:

• https://unix.stackexchange.com/questions/479702/cannot-override-systemd-timer-with-specific-time
• https://www.freedesktop.org/software/systemd/man/latest/systemd.timer.html#OnActiveSec=

Command/Package Mappings and Locations¶

When looking for the files a command maps to, you can use the which command. This shows the location of a command

# ls -l $(which vi)
-rwxr-xr-x. 1 root root 1180368 Jun 14  2022 /bin/vi

You can also look for alternatives if a command might map to two packages

# ls -l /etc/alternatives/java
lrwxrwxrwx. 1 root root 62 Jul 29 08:51 /etc/alternatives/java -> /usr/lib/jvm/java-11-openjdk-11.0.23.0.9-3.el8.x86_64/bin/java

More command/file troubleshooting

# rpm -qf /usr/lib/jvm/java-11-openjdk-11.0.23.0.9-3.el8.x86_64/bin/java
java-11-openjdk-headless-11.0.23.0.9-3.el8.x86_64

# alternatives --config java

There are 2 programs which provide 'java'.
  Selection    Command
-----------------------------------------------
*+ 1           java-11-openjdk.x86_64 (/usr/lib/jvm/java-11-openjdk-11.0.23.0.9-3.el9.x86_64/bin/java)
   2           java-1.8.0-openjdk.x86_64 (/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.412.b08-2.el9.x86_64/jre/bin/java)
Enter to keep the current selection[+], or type selection number: 1
# 

Swap¶

Remove swap swapoff -a

Allocate swap fallocate -l 2G /swapfile

Set permissions for swap chmod 600 /swapfile

Make file usable as swap mkswap /swapfile

Activate swap swapon /swapfile

Make sure swap is permanent vim /etc/fstab # /swapfile swap swap defaults 0 0

Show swap swapon --show

Show swap free -h

Show swappiness cat /proc/sys/vm/swappiness

Zgrep¶

Sometimes you need to search the contents of .gz files in your system. Unfortunately, grep doesn’t work on compressed files. To overcome this, people usually advise to first decompress the file(s), and then grep your text, after that finally re-compress your file(s)…

You don’t need to decompress them in the first place. You can use zgrep on compressed or gzipped files.

To search in compressed file, execute the command :

root@ck [~]#zgrep ‘put-your-text-here’ /your-file-path-here/file.gz

Example : I want to grep ‘plugged’ in all of my exim_paniclog archived files.

root@ck [~]# zgrep ‘plugged’ /var/log/exim_paniclog.*

Kernel Info¶

The core component of Linux is the kernel. It is written almost entirely in the C programming language. Kernel version are denoted by major verision, minor version, and revision number.

Ex: typing uname -r into a a terminal will give you the following:

root@ubuntu:~# uname -r
5.15.60-1

Power States¶

Shebang¶

The Shebang specifies the pathname to the shell that is used to interpret the contents of the script.

#!usr/bin/bash #!usr/bin/python3

Boot Process¶

1. UEFI / BIOS
2. POST
3. MBR/GPT
4. UEFI System Partition
5. Boot Loader
6. /boot/linux-5.4.2 (kernel)
7. Init or Systemd, systemd is more common now
8. Daemons

Target¶

Target maps are the newer terminilogy and implementation of runlevels.

1. Poweroff.target - runlevel 0
2. Rescure.target - runlevel 1
3. Multi-user.target - runlevels 2, 3, and 4
4. Graphical.target - runlevel 5
5. Reboot.target - runlevel 6

Runlevels¶

Runlevels are the older terminology that correspond to different target maps.

The most common runlevels are 3 and 5 - terminal and GUI, respectively.

0 - Halt 1 - Single User 2 - Multi User 3 - Multi User with Networking 4 - User defined 5 - Multi User with Networking and GUI 6 - Reboot