Linux Administration¶
Overview¶
This page is dedicated to helpful commands, techniques, and packages to system administrators.
Links¶
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
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¶
Command | Description |
---|---|
shutdown –P +4 | Powers off your system in four minute |
shutdown –H +4 | Halts the operating system from executing in four minutes, but does not invoke the ACPI function in your BIOS to turn off power to your computer |
shutdown –r +4 | Reboots your system in four minutes |
shutdown –P now | Powers off your system immediately |
shutdown –r now | Reboots your system immediately |
shutdown –c | Cancels a scheduled shutdown |
halt | Halts your system immediately, but does not power it off |
poweroff | Powers off your system immediately |
reboot | Reboots your system immediately |
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¶
- UEFI / BIOS
- POST
- MBR/GPT
- UEFI System Partition
- Boot Loader
- /boot/linux-5.4.2 (kernel)
- Init or Systemd, systemd is more common now
- Daemons
Target¶
Target maps are the newer terminilogy and implementation of runlevels.
- Poweroff.target - runlevel 0
- Rescure.target - runlevel 1
- Multi-user.target - runlevels 2, 3, and 4
- Graphical.target - runlevel 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