Linux Cheatsheet

Table Of Contents

LPIC

101.1 Determine and configure hardware settings

Link to source

What is sysfs in Linux? a pseudo file system provided by the Linux kernel that exports information about various kernel subsystems, hardware devices, and associated device drivers from the kernel's device model to user space through virtual files.
What is in `/dev`? There are a lot of devices in `/dev/` and if you plug in any device, it will be assigned a file in `/dev` (say `/dev/sdb2`)
What details are in `/proc`? - IRQs (interrupt requests) - I/O ports (locations in memory where CPU can talk with devices) - DMA (direct memory access, faster than I/O ports) - Processes - Network Settings
Name two files under `/proc` - `cpuinfo` - `meminfo`
What are four commands to see hardware information? - `lsusb` - `lspci` - `lshw` - `lsblk`

101.2 Boot the System

Link to source

You can check /sys/firmware/efi or /boot/efi to see if you are using a UEFI system or not.

journalctl -k # to check Kernel logs
journalctl -b # check for boot logs
cat /var/log/dmesg  # show only the data during the boot.
cat /var/log/messages # Include init logs

Which init is using

$ which init
/sbin/init
$ readlink -f /sbin/init
/usr/lib/systemd/systemd
$ ps -p 1
PID TTY TIME     CMD
1   ?   00:00:06 systemd
$ pstree  # the hierarchy of processes

$ systemctl list-units
$ systemctl list-units --type=target
$ systemctl cat graphical.target
$ systemctl stop sshd
$ systemctl start sshd
$ systemctl status sshd
$ systemctl enable sshd
Which command shows the units hierarchy? `pstree`

101.3 Change runlevels / boot targets and shutdown or reboot the system

Link to source

  1. rescue: Local file systems are mounted, there is no networking, and only root user (maintenance mode)
  2. emergency: Only the root file system and in read-only mode, No networking and only root (maintenance mode)
  3. reboot
  4. halt: Stops all processes and halts CPU activities
  5. poweroff: Like halt but also sends an ACPI shutdown signal (No lights!) 
    $ systemctl isolate emergency
$ systemctl is-system-running
maintenance

shutdown command:

Notifying users:

102.1 Design hard disk layout

Link to source

Directory Description
bin Essential command binaries
boot Static files of the boot loader
dev Device files
etc Host-specific system configuration
home Home directory of the users
lib Essential shared libraries and kernel modules
media Mount point for removable media
mnt Mount point for mounting a filesystem temporarily
opt Add-on application software packages
root Home directory of the root user
sbin Essential system binaries
srv Data for services provided by this system
tmp Temporary files, sometimes purged on each boot
usr Secondary hierarchy
var Variable data (logs, …)

Commands:

$ fdisk /dev/sda
Command (m for help): p

$ sudo parted /dev/sda p

$ gparted # A graphical tool for managing disks and partitions.

3 different swap design:

102.2 Install a boot manager

Link to source

GRUB2:

On BIOS systems it is installed on /boot/grub/ or /boot/grub2/ and under UEFI it goes in /boot/efi/EFI/distro-name/ (say /boot/efi/EFI/fedora/). GRUB2’s configuration file is called grub.cfg.

Commands:

grub-install /dev/sda # Install

update-grub
# or
grub2-mkconfig > /boot/grub2/grub.cfg

update-grub: Reads the configuration files from /etc/grub.d/ and /etc/default/grub/ and create the grub.cfg

102.3 Manage shared libraries

Link to source

Linux dynamic libraries have names like libLIBNAME.so.VERSION and are located at places like /lib*/ and /usr/lib*/

The ldd command helps you find:

$ ldd /sbin/ldconfig
not a dynamic executable

Configs:

/etc/ld.so.conf

Run ldconfig to update the cache.

102.4 Use Debian package management

Link to source

apt-get update  # Updating sources information
apt-get install tmux
apt-get install -s tmux # simulate
apt-get install --download-only tmux
apt-get remove tmux
apt-get autoremove # remove automatically installed dependencies
apt-get upgrade
apt-get dist-upgrade  # going to a new distribution

102.6 Linux as a virtualization guest

Link to source

Check CPU supports hypervisor vmx (for Intel CPUs) or svm (for AMD CPUs) in /proc/cpuinfo in flags. Based on the CPU you should have kvm or kvm-amd kernel modules loaded.

lsmod | grep -i kvm
sudo modprobe kvm

Guest-specific configs

After cloning we need to change these on each machine before booting them:

Linux Commands

Command Description
rev Reverse lines of a file or files
seq Print sequences of numbers
yes Print a string until interrupted
date Print or set the system date and time
netstat -tulpn List all listening ports

Bash

Read line by line from a file:

cat peptides.txt | while read line 
do
  # do something with $line here
done

Swap

How to configure a swapfile in ubuntu.see this link

  1. How to create swapfile
  2. config swappiness percentage

Systemd

  1. good link
  2. good link2
  3. Full guide on service
  4. Full guide on units

Put services for user here: /etc/systemd/system/<name>.service. After that call systemctl daemon-reload

# something.service
[Unit]
Description=Apache web server
After=network.target
Before=nextcloud-web.service
StartLimitIntervalSec=180

[Service]
ExecStart=/usr/local/apache2/bin/httpd -D FOREGROUND -k start
ExecReload=/usr/local/apache2/bin/httpd -k graceful
Type=notify
Restart=always

[Install]
WantedBy=default.target
RequiredBy=network.target

Known parameters

  1. WorkingDirectory=

    Takes a directory path relative to the service’s root directory specified by RootDirectory=, or the special value “~”. Sets the working directory for executed processes. If set to “~”, the home directory of the user specified in User= is used. If not set, defaults to the root directory when systemd is running as a system instance and the respective user’s home directory if run as user. If the setting is prefixed with the “-“ character, a missing working directory is not considered fatal. If RootDirectory=/RootImage= is not set, then WorkingDirectory= is relative to the root of the system running the service manager. Note that setting this parameter might result in additional dependencies to be added to the unit (see above).

  2. User=, Group= Set the UNIX user or group that the processes are executed as, respectively. Takes a single user or group name, or a numeric ID as argument. For system services (services run by the system service manager, i.e. managed by PID 1) and for user services of the root user (services managed by root’s instance of systemd –user), the default is “root”, but User= may be used to specify a different user. For user services of any other user, switching user identity is not permitted, hence the only valid setting is the same user the user’s service manager is running as. If no group is set, the default group of the user is used. This setting does not affect commands whose command line is prefixed with “+”.

  3. Nice= Sets the default nice level (scheduling priority) for executed processes. Takes an integer between -20 (highest priority) and 19 (lowest priority). In case of resource contention, smaller values mean more resources will be made available to the unit’s processes, larger values mean less resources will be made available. See setpriority(2) for details.

  4. ExecStart= Commands that are executed when this service is started. The value is split into zero or more command lines according to the rules described in the section “Command Lines” below.

  5. ExecReload= Commands to execute to trigger a configuration reload in the service.

  6. ExecStop= Commands to execute to stop the service started via ExecStart=.

  7. RestartSec= Configures the time to sleep before restarting a service (as configured with Restart=). Takes a unit-less value in seconds, or a time span value such as “5min 20s”. Defaults to 100ms.

  8. Restart= Takes one of no, on-success, on-failure, on-abnormal, on-watchdog, on-abort, or always. If set to no (the default), the service will not be restarted.

  9. StartLimitIntervalSec= Configure unit start rate limiting. Units which are started more than burst times within an interval time span are not permitted to start any more. Use StartLimitIntervalSec= to configure the checking interval and StartLimitBurst= to configure how many starts per interval are allowed.

  10. BindsTo= Configures requirement dependencies, very similar in style to Requires=. However, this dependency type is stronger: in addition to the effect of Requires= it declares that if the unit bound to is stopped, this unit will be stopped too. This means a unit bound to another unit that suddenly enters inactive state will be stopped too. Units can suddenly, unexpectedly enter inactive state for different reasons: the main process of a service unit might terminate on its own choice, the backing device of a device unit might be unplugged or the mount point of a mount unit might be unmounted without involvement of the system and service manager.

  11. PropagatesReloadTo=, ReloadPropagatedFrom= A space-separated list of one or more units to which reload requests from this unit shall be propagated to, or units from which reload requests shall be propagated to this unit, respectively. Issuing a reload request on a unit will automatically also enqueue reload requests on all units that are linked to it using these two settings.

A Big Question

Hope this clarifies things! 😊 Thanks CoPilot!

tmux

Key Usage
% Split current window vertically
" Split current window horizontally
D Detach from the current window
& Kill current window
x Kill current pane
, Rename current window
c Create a new window
Command Usage
tmux ls Show all tmux daemons
tmux att -t session_name To attach to a specific one

vim

Key Usage
i insert
a append
dd Delete current line
ctrl+f Forward window
ctrl+b Backward window
G Go to last line
o Open line below of current line
v Select
V Select a line
d Delete selected characters
y Copy selected characters
p Paste selected characters

Handwrite Notes

  1. After changing user groups, The user should login again 
     su - ${USER}