Run a process detached to the current terminal and monitor its output:

1 $ nohup my_command &
2 $ tail -F ./nohup.out

Same as above but with multiple commands, the last cloning a disk to another:

1 $ nohup zsh -c "my_command && cat /dev/da1 > /dev/da0" &
2 $ tail -F ./nohup.out

Run a process with a shell for a system user which has none (i.e. its default shell is set to /bin/false in /etc/passwd):

1 $ su sys_user -s /bin/bash -c "my_command"

Get the exit code of the latest ran command:

1 $ echo $?

Run the last command as root (source):

1 $ sudo !!

Show the user under which I’m currently logged in:

1 $ whoami

List of most used commands:

1 $ history | awk '{a[$2]++}END{for(i in a){print a[i] " " i}}' | sort -rn | head

List cron jobs of the current user:

1 $ crontab -l

If you have the following error:

1 $ -bash: ./myscript.sh: /bin/bash^M: bad interpreter: No such file or directory

Then the fix consist of removing the bad characters:

1 $ sed -i 's/\r//' ./myscript.sh

Extract strings from a binary file:

1 $ strings ./firmware.bin | less

Free up some memory by clearing RAM caches (source):

1 $ sync ; echo 3 > /proc/sys/vm/drop_caches

Display which distro is running the system (source):

1 $ lsb_release -a

or

1 $ cat /etc/lsb-release

Disable a service on Debian/Ubuntu, then re-enable it:

1 $ update-rc.d my-service-name remove
2 $ update-rc.d my-service-name defaults

Same thing as above but on a RedHat-like system:

1 $ chkconfig sshd --del
2 $ chkconfig sshd --add

Speed-up Grub boot, but always show the boot menu:

1 $ sudo sed -i 's/GRUB_TIMEOUT=10/GRUB_TIMEOUT=1/g' /etc/default/grub
2 $ sudo sed -i 's/GRUB_HIDDEN_TIMEOUT/#GRUB_HIDDEN_TIMEOUT/g' /etc/default/grub
3 $ sudo update-grub

List fonts available on the system:

1 $ fc-list | cut -d ':' -f 2 | sort | uniq