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Misc Linux tools cheatsheet

Less

Less without line wrapping

less -S

Colorful less

less -r

Less: leave uncleared output on exit

export LESS="-X"

Screen

screen -S new_screen_name # create named screen
# detach with Ctrl+a, d
screen -ls # list screens
screen -r 5050 # reattach
screen -d -r byname # reattach not-detached session

Nmap

Discover hosts, MACs, hostnames with ping scan

sudo nmap -sP 192.168.0.1/24

Check if TCP port is open

sudo nmap -sS -p22 192.168.0.50 # SYN scan
sudo nmap -sY -p22 192.168.0.50 # open/filtered/closed

Scan port range

nmap -p 1-65535 localhost

Scan OS and detect services

nmap -A -T4 192.168.0.49

Detect service versions

nmap -sV 192.168.0.49

CVE detection

nmap -Pn --script vuln 192.168.0.49

Shell

Write file without text editor: cat + EOF

cat << 'EOF' > task.xml
EOF

Turn on strict mode in bash

set -euxo pipefail

xargs

Run command on each line: 

ls -1 *.sh | xargs -I %s echo "mv '%s' '%s.bak'"

Disk usage of subfolders

du -sch .[!.]* * 2>/dev/null | sort -h

Linux Rescue Kit

Magic Key

Enable Magic Key: 

# Temporary
sudo sysctl -w kernel.sysrq=1
# or
sudo echo 1 > /proc/sys/kernel/sysrq

# Permanent
sudo vim /etc/sysctl.conf
# Add:
kernel.sysrq=1

To use the magic SysRq key, press the key combo ALT-SysRq-<command key>:

  • r - Turns off keyboard raw mode and sets it to XLATE
  • e - Send a SIGTERM to all processes, except for init.
  • i - Send a SIGKILL to all processes, except for init.
  • s - Will attempt to sync all mounted filesystems.
  • u - Will attempt to remount all mounted filesystems read-only.
  • b - Will immediately reboot the system without syncing or unmounting your disks.
  • o - Will shut your system off (if configured and supported).

Note

Some keyboards may not have a key labeled ‘SysRq’. The ‘SysRq’ key is also known as the ‘Print Screen’ key.

Add GPG key to trusted keys (fix NO_PUBKEY)

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <PUBKEY>

Restart touchpad driver

sudo modprobe -r psmouse
sudo modprobe psmouse

Disable jbd2 gvfsd

pkill gvfsd-metadata
rm -rf ~/.local/share/gvfs-metadata

Diagnostic messages of the kernel

sudo dmesg --reltime --ctime

Journal logs

Filter all errors: 

sudo journalctl --priority 2..3 -e
Filter by service name: 
sudo journalctl -b 0 -u NetworkManager -e
All logs from current boot: 
sudo journalctl -b 0 -e

Crashing GNOME

Errors in journal: 

sudo journalctl -b 0 -e /usr/bin/gnome-shell
# Following output:
sudo journalctl -f -o cat /usr/bin/gnome-shell

Reset Gnome configuration: 

dconf reset -f /org/gnome/
dconf reset /org/gnome/desktop/interface/cursor-theme