My Linux Notes

My learning notes while exploring Linux, Ubuntu and Xubuntu.

• System Tips
• Commands

System Tips

1. To execute a file: ./file

2. To create a Desktop or Launcher shortcut, you need a .desktop file. (Refer to /usr/share/applications)

3. Use TAB for auto-completion in terminal. (Won't forget names anymore!)

4. Install rar to unpack a .rar file.

5. System freezed? Try Ctrl-Alt-F1 to access the terminal.

6. SysRq: Alt + SysRq + "REISUB" ("busier")

   R: exit Raw mode (X window)	E: terminate all
   I: force terminate	S: Sync disk
   U: umount disk	B: reboot
   F: call oom_kill (kill one)

7. htop: A colorful yet handy system resource monitor.

8. Fast OS simulation: qemu-system-x86_64 [iso file]

9. List files in the recycle bin sorted by deletion time: cd ~/.local/share/Trash/files; stat -c "%z - %n" * | sort -t'-'.

10. Ubuntu 16+ DO NOT run startup scripts from /etc/rcX.d anymore; Use the crontab @reboot method instead. (But do remember that root's crontab cannot read encrypted user data)

11. Crontab can be dumped and loaded: crontab -l > dump; crontab dump

• The format of /etc/crontab is different than crontab -e.

12. Sticky bit: ------t. Only the owner can remove it, even if it's chmod 777 (eg. /tmp)

13. SetUID: Cannot be applied to scripts; Executables are also not supported under /tmp and /home. (which are mounted as nosuid)

14. Systemctl's is-enable command can check for "generated scripts".

15. Calling grep multiple time is faster than parsing with a single while-loop. (for big files)

16. MessageOfTheDay (the tty greetings) is under /etc/update-motd.d/.

17. Locate: A utility for reverse-mapping filenames to paths. However, it will scan the whole system everyday (updatedb.mlocate), so remember to turn it off if not needed.

18. Secure boot: Not all modules are signed, hence the error message: XX kernel module not found or loaded.

19. '-' is nothing special; it is just that programs such as cat view it as stdin.

20. Disable alias temporarily: 1) command [name], 2) '[name]', 3) \[name].

21. Analyze core dumps: 1) ulimit -c unlimited, 2) After crash, gdb [program] core.

• Can also be viewed with readelf or objdump -s, eg. note0 section:

  OFFSET  VALUES (SIZE)
  34      pid, ppid, pgrd, sid   (DWORD)
  44      usertime, systime, cum_usertime, cum_systime (0x10)
  84      r15, r14, r13, r12     (QWORD)
  a4      bp, bx, r11, r10       (QWORD)
  c4      r9, r8, ax, cx         (QWORD)
  e4      dx, si, di, orig_ax    (QWORD)
  104     ip, cs, flags, sp      (QWORD)
  124     ss, fsbase, gsbase, ds (QWORD)
  144     es, fs, gs             (QWORD)
  

• Cygwin: export CYGWIN="$CYGWIN error_start=dumper -d %1 %2"

• Beware that Apport/ABRT sometimes hide the core files.

22. Color is just CSI sequences, while programs detect the destination themselves (eg. ls --color=auto)

23. ptrace：only child relationship or setup PTRACE_TRACEME flag can a process been traced (/proc/sys/kernel/yama/ptrace_scope)

Commands

• Read the manual: man [something](.SECTION_NUMBER)

  • -f: search among page names.

  • -M [/usr/local/man]: search under a different MANPATH.

  • info [somthing] can also be helpful.

• Grep to filter: ls | grep "sys"

  • ^[a-z]: Starting / $: Ending / . Any / ? Optional

• Print contents: cat [textfile]

• Rename through moving: mv aaa abbb

• Create soft link: ln -s /home/afolder /home/f/Folder

• Change file permission:

  • chmod g+x file ---> ['u'=owner/'g'=group/'o'=other] [+-] ['r'=read/'w'=write/'x'=execute]

  • chmod 705 file ---> (eg. '0'=---, '1'=--x, ..., '7'=rwx)

• Change owner: chown [user]:[group] file

• File info (ls -l): -rw-rw-r-- 1 [user] [usergroup] [1024] [Sep 26 18:48] [file.name]

  • The first '-' indicates file type: '-'=file, 'd'=dir, 'l'=link

  • Inode: Record metadata (eg. size) and position of data block

  • Directory: Is a file, which map filenames to their inode number

• pwd: Print current directory path.

• apt: apt-get install ...

  • apt-cache search ...

  • apt-cache show [package name] (detail info)

  • apt autoremove (remove useless packages)

  • apt-get update (refresh updatable list)

  • apt-get upgrade (install update)

  • add-apt-repository ppa:webupd8team/java (add package source)

  • --reinstall

• dpkg: -i (install)

  • -l (list packages)

  • -r (remove)

  • -p (also purge config)

  • dpkg-reconfigure

• sudo passwd root (change root account's password)

• mount -t ntfs /dev/sda1 /mnt/winOS/ -o "umask=022"

  • umount /dev/sda1

• su [user] --shell /bin/bash: Login as user with a shell.

  • sudo -k: Reset credential, require password to use sudo again.

• rsync -av ~/Desktop/ ~/backup/fold1/ (A tool more powerful than cp)

  • --exclude={"a/*","auto*","a/.gitignore"}

• systemd-analyze (time / blame / critical-chain)

  • systemctl (status / disable / stop)

• System info:

  • uname -a: Show system info.

  • inxi: Show lots of system info (-S for distro/DE version)

  • ls /usr/bin/*session (Find out which DE has been installed)

• du -h /home/david: Show file's size. (can filter with grep ^[0-9]*G)

  • df -h: Show disk usage.

• ip link: List network interfaces (NIC). Check if there is "UP" inside <>.

  • ip addr add [ip] broadcast + dev [interface]: Add a static ip.

  • ip route add default via [gateway ip] dev [interface]: Add a default router.

  • ifconfig eth0:0 [addr]: Create a virtual interface.

• iotop: View disk usage of every process.

• nmcli dev wifi: List available Wifi APs.

  • nmcli dev wifi connect [iTaiwan] password [0123456789]

• lshw: List hardware info.

• lscpu: Show cpu architecture info.

• date -d @[seconds since epoch]: convert "seconds since epoch" to readable date.

• head -n x ./myfile: Print the first x line of file/stdin (if file not provided).

• User:

  • who: Show current user.

  • pkill -u [username]: Logout a user.

  • adduser [name] --home /home/name --shell /bin/bash

• Groups:

  • groups [user]: Show a user's groups

  • usermod -a -G [group] [user]: Add user to a group.

  • gpasswd -d [user] [group]: Remove user from a group.

• Mount eCryptfs: sudo mount -t ecryptfs [.Private] [/mnt/point] (su are needed sometimes)

• wmctrl -k on: Minimize all windows. (off=switch back)

• time [command]: Record command's execution time.

  • Get output: bash -c "time [command]" 2>&1

• xdotool: Can emulate key presses and mouse clicks.

• stat: Show file modification time, size, links, type and permission.

  • Link count: Number of hard references to this inode.

    • eg. mkdir -p A/B generates 3 count for A: A/B/.. A/. and A

  • file: Show file type.

• xxd: Hex dump (-b for binary dump)

• Message box:

  • zenity --question --window-icon="question" --title="title" --text="some text"

  • notify-send -t 4000 "some message"

• sed:

  • Substitution: 's/Regex/ReplaceStr/Options'

    • Deliminators can be other than /, eg. | : #
    • Use \(foo\) to remember and \1 \2 \3 to recall. (note that backreference runs slower)
    • Use & in ReplaceStr to get the matching part.
    • Substitute only within line 1~4: 1,4s/.../.../
      • only within matched lines: /Pattern/s/.../.../

  • Options or commands:

    • g = Replace all.
    • 2 = Perform on the 2nd occurence.
    • i = Case isensitive
    • p = Print matched line.
    • d = Delete matched line.
    • q = Quit.
    • i\ = Insert text before.

  • Find and print a pattern: sed -n '/Regex/p' (-n = disable echoing)

    • 2,$p: Print 2nd line ~ end
    • /xxx/,$p: Print after including xxx
      • 1,/xxx/d: excluding
    • /xxx/q: Print before including xxx
      • /xxx/q;p: excluding (if not match then print)

  • Regex: a* a\+ a\? \(a\|b\) a\{N,M\} (<- N~M matches)

    • Extended (-E): a+ a? (a|b) a{N,M}
    • Charset: [:space:], [:digit:], [:alpha:], [:lower:], [:punct:]
    • A * always tries to match the longest first occurence.

  • Run multiple sed sequentially: sed -e 'script1' -e 'script2'

  • x: Swap pattern space with hold space. (pattern space: the matching line)

  • Escaping the ReplaceStr (for delim=/): 's/[\/&]/\\&/g'

  • Escaping the Pattern (for BRE and delim=/): -E 's/([]\/^$.*]|\[)/\\&/g'

  • Caution:

    • Beware of shell expansion within double quotes "": $, ``` and \\.
    • Matching non-ASCII characters might go wrong.

• $!: Holds the last background process' pid.

• find . -name 'basename' (wildcards supported)

  • Options: -type f -size 100c -mtime -3 -maxdepth 5

  • find . -regex '.*full/path/regex'

  • find . -regex '\./ignore/.*' -prune -or -name 'name' (skip './ignore/' subdir)

• lsof: find out which process opened a file.

• Vim editor:

  • / (search), /\%xa9 (search hex)

  • :d30 (delete 30 lines)

  • ga (show current char)

• cut -d " " -f2: Print second column, seperated by a single space.

• awk '{print $2}': Print second column, seperated by whitespaces.

  • ls -l | awk -v perm="-rwxr-xr-x" '$1 == perm { $1="EXE: "; printf "%s\n", $0; system("sleep 1") }'

  • awk -F . '{printf("%d %d %d"), $1, $2, $3}': version string to integer

  • awk '/pty1/ {if ($1=="david") print $2 }': when contain "pty1", if $1 is "david" print $2

• history -c; history -r: Clear current bash history in memory.

• strace: Trace all syscall of a program.

• sudo rm -rf --no-preserve-root / &>/dev/null (don't try it :D)

• 	Set	Unset	Show
  Shell variables	var=[value]	unset var	set
  Environ variables	export var=[value]	export -n var	printenv

  • If a variable is already in Environ, the first method would also change it.

• trap "echo "I have been interrupted"; exit 0" SIGINT

  • Ignore signal: "" ; Restore to default: -

  • Ctrl-c will send SIGINT to all foreground processes (inner one handle first)

  • Avoid calling exit 0 directly; Write trap - SIGINT; kill -2 $$ instead.

    • For Bash uses Wait-and-Cooperative-Exit, which will depend on how its child has terminated.

• kill -SIGINT [process_id]: Send signals to processes.

• timeout [duration] [command]: Run a command with a time limit.

• mktemp --tmpdir prefix_XXXXXX: Create unique temp file with race-condition safety.

• lslogins -u: View each user's login time, number of process etc.

• swapoff -a: Move data from swap back to RAM and disable swap.

• journalctl (-u apache2): View systemd logs.

• xargs: Place input at arguments, eg. ls *.txt | xargs stat