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Linux Cheat Sheet

Notable commands, variables and files for administering linux.

Paths ending with / is a directory, and those without it is a file. Example: ~/test is a file and ~/test/ is a folder.

Environment Variables​

Environment variables are variables accessible by a specific instance of a process, usually within a shell session.

Commands:​

  • VARIABLE=value - Assigns value to VARIABLE. If VARIABLE already exxists, this overwrites its previous value.
  • $VARIABLE - Accesses the value of VARIABLE. When a command is executed, $VARIABLE is expanded to its values.
  • echo $VARIABLE - Displays the value of VARIABLE.
  • set - Lists all currently set environment variables.
  • unset VARIABLE - Removes the environment variable VARIABLE.
  • export VARIABLE - Makes VARIABLE accessible to child processes spawned by the current process. export VARIABLE=value is equivalent to VARIABLE=value && export VARIABLE.

GRUB (Bootloader)​

UEFI system and GRUB 2 is assumed in this section.

Commands:​

  • grub-install - Installs GRUB to /boot/efi/. Pass the --efi-directory option to install to other directory.
  • update-grub - Shortcut to the command grub-mkconfig -o /boot/grub/grub.cfg. Updates GRUB configuration file (commonly located in /boot/grub/grub.cfg) from the definitions in /etc/default/grub and /etc/grub.d/
  • Press C when on GRUB menu screen to access GRUB shell.
  • Press E when on GRUB menu to edit kernel parameter for that particular entry for the current boot. Set systemd.unit=rescue.target to enter rescue mode for systemd base systems.

Variables:​

  • $prefix - GRUB configuration path location. (This is a variable for GRUB shell, not linux one)

Files:​

  • /boot/efi/ - Mount point of ESP (EFI System Partition) containing the actual bootloader UEFI accesses
  • /boot/grub/grub.cfg - Common location for GRUB configuration file. Located in root partition. Do not edit directly, edit /etc/default/grub and run update-grub instead.
  • /etc/grub.d/ - update-grub changes /boot/grub/grub.cfg based on this folder. Contents are executed in numerical order. Custom entries are usualy added to 40_custom file. Example entry is shown below.
    menuentry "Default OS" {
        set root=(hd0,1)
        linux /vmlinuz root=/dev/sda1 ro quiet splash
        initrd /initrd.img
    • set root=(hd0,1) - Sets device and partition of root folder of OS. hd0,hd1,... corresponds to /dev/sda,dev/sdb,... in linux. Partition numbering starts at 1. So, this configuration in particular equates to /dev/sda1
    • /vmlinuz - Location of linux kernel. Because the kernel is located directly inside root folder, we can assume that this configuration has a seperate boot partition and it is not located at /dev/sda1 (hd0,1). If there is no boot partition and the boot folder is located within root parition, something like /boot/vmlinuz will instead be shown instead.

systemd (Service Manager)​

Commands:​

General​

  • systemctl list-unit-files - Lists all available units and show if they are enabled. Pass the option --type=TYPE to only show units of TYPE type.
  • systemctl list-units - Lists all active units. Pass the option --type=TYPE to only show units of TYPE type.
  • systemctl reboot - Reboot the system. Same effect as systemctl isolate reboot.target
  • systemctl poweroff - Power off the system. Same effect as systemctl isolate shutdown.target
  • systemctl suspend - Sleeps the system. Data in memory is saved as is.
  • systemctl hibernate - Hibernates the system. Data in memory is moved to disk.
  • systemctl rescue - Puts the system in rescue mode (similar to "safe mode" on Windows). Same effect as systemctl isolate rescue.target

Services​

  • systemctl start [<service>]　- Starts unit
  • systemctl stop [<service>]　- Stops unit
  • systemctl restart [<service>]　- Restarts unit
  • systemctl status [<service>]　- Shows the state of unit
  • systemctl is-active [<service>]　- Shows active if unit is running, inactive otherwise.
  • systemctl enable [<service>]　- Makes unit start from the next system initialization. Does NOT start unit immediately.
  • systemctl disable [<service>]　- Unit will not start from the next system initialization.
  • systemctl is-enabled [<service>] - Outputs enabled or disabled.

Targets​

  • systemctl isolate [<target>] - Alternate to the target unit.
  • systemctl set-default [<target>] - Set the default initialization target for subsequent boots. Usually defaults to multi-user.target or graphical.target.
  • systemctl get-default - Get the default initialization target.

Files:​

  • /lib/systemd/system/ - Location of unit configuration file

Shared Libraries​

Commands:​

  • ldconfig - Reads /etc/ld.so.conf and /etc/ld.so.conf.d/* to configure shared library location and caches them into /etc/ld.so.cache.
    • -p - Shows currently cached shared library (do not update the cache).
    • -v - Updates the cache and show the cache while doing so.
  • ldd [program || so] - Shows shared library dependencies of a program or shared object.
  • objdump & readelf - Examines contents of object, binary, and shared library files

Variables:​

  • $LD_LIBRARY_PATH - Colon (:) separated set of directories to look for libraries. User configured. Empty by default.

Files:​

  • /etc/ld.so.conf.d/ - ld (dynamic linker) looks for shared library paths defined in files in this folder.

Last updated: July 28, 2023