Linux

General tips

• When you get a "file system is read only" error on a USB drive (mounted at sdb1) try unmounting and remounting as read/write with the following command: sudo mount -o remount,rw /dev/sdb1.
• Linux interprets #! (called a shebang), when they are the first two characters in a text file, as indicating that the file is a script and the rest of the line gives the absolute path of the interpreter. E.g. Bash #!/bin/sh; Python 3.1 #!/usr/bin/python3.1.

Bash/zsh Command Line

• Make the command-line prompt reflect Version Control using vcprompt.
• Terminal Multiplexer Byobu (Man Page; Add new terminal: F2; Scroll through terminals: F3 and F4).
• Change your password: passwd
• Repeatedly run command: watch -d -n 0.5 [command]
• Count number of files in current directory: ls -l | wc -l
• Search for string in files (include sub-directories): grep -rl "string" *.
• Delete all empty .xml files: find . -name "*.xml" -size 0 -exec rm {} \;.
• Recursively remove folders and their contents: rm -rf.
• List files with / added to directory names and * to executables: ls -F.
• Mass rename: zmv -n (*).txt $1.tmp (-n option shows result but does not make the change)
• Checksum: md5sum ubuntu-10.04-desktop-i386.iso.
• Find which executable will be used: which python.
• Change to the previous working directory: cd -.
• Substitution: !$ Repeats last argument of previous command; !! Repeat previous line command; ^foo^bar^ Repeat the previous shell command but replace string "foo" with "bar".
• echo $?: Display exit code.
• su userid: Switch User.
• traceroute and ping combined: mtr google.com.
• Run graphical application as root: gksudo nautilus.
• Create link: ln [-s] file [link].
• Reconnect to network connection by restarting network manager service: sudo service network-manager restart.
• Monitor several logs: tail -f [logfile1] [logfile2]....
• df: Show disk usage.
• Job Control: jobs List background jobs; bg [jobspec] Run job in background; fg [jobspec] Bring specific or most recent job to foreground; kill %1 Kill first listed job.

Ultimate Linux Shutdown Process

If all is stuck, and even Ctrl+Alt+backspace does not work, remember this mnemonic: Raising Skinny Elephants Is Utterly Boring:

• ALT+SysRq+r (put keyboard in raw mode)
• ALT+SysRq+s (sync the disk)
• ALT+SysRq+e (terminate all processes)
• ALT+SysRq+i (kill all processes)
• ALT+SysRq+u (remount all file systems read only)
• ALT+SysRq+b (reboot the system)

(SysRq is the same button as print screen on most keyboards)

Less drastically, you can use ALT+SysRq+k to kill all processes on the current virtual console (has the effect of restarting the X server).

SysRq is disabled by default so you will have to edit /etc/sysctl.conf and change the kernel.sysrq value to 1 (and then reboot) to enable these shortcut keys.

Ubuntu

Settings

• Bulk change file associations: sudo vi /usr/share/applications/defaults.list
  • :%s/gedit/geany/g Change default text editor from gedit to geany.
• Prevent keyobard resetting to US layout after reboot:
  1. In dconf-editor, navigate to desktop > ibus > general.
  2. Enable use-system-keyboard-layout.
• Disable auto-open of Nautilus on drive mount: gsettings set org.gnome.desktop.media-handling automount-open false
• Stop Wifi being activated after boot (from):
  1. sudo vi /etc/rc.local
  2. Add line: rfkill block wifi
  3. Also, to keep Bluetooth deactivated): rfkill block bluetooth
• Enable Ctrl+Alt+Backspace to restart the X server, to recover after the Desktop Environment freezes:
  1. sudo dpkg-reconfigure keyboard-configuration
  2. Accept first 5 keyboard options.
  3. Choose Yes to use Control+Alt+Backspace to terminate the X server.

OpenDNS

Add OpenDNS servers by editing /etc/dhcp3/dhclient.conf and appending the following line to the document:

prepend domain-name-servers 208.67.222.222,208.67.220.220;

Logout Keyboard Shortcut

To re-enable Control+Alt+Backspace shortcut to kill the X server:

1. Open Keyboard preferences.
2. Choose Layouts Tab.
3. Press Options Button.
4. Find "Key sequence to kill the x server' option on list.
5. Tick to enable.

Monitor Network Usage

1. Install vnstat: sudo apt-get install vnstat.
2. Create database: sudo vnstat -u -i eth0.
3. Show monthly usage: vnstat -m.

Lost Password

Steps to recover lost password in Ubuntu:

1. Choose your Recovery Mode from Grub menu.
2. On the Recovery Menu choose "Drop to root shell prompt".
3. At the command prompt enter ls /home to list Userids. (optional)
4. Enter passwd <userid> to change the password of that Userid.
5. Enter exit.
6. On the Recovery Menu choose "Resume Normal Boot".