Work and Note

Op Notes

Follow me on GitHub

common

  • strings: print the printable characters in file

  • mv: seems to only remove after copy is done
    • be aware of shell expansion which might cause mv to be run on individual files instead
      • which cause remove to happen after each file
  • rm: fail if file not writable
    • rm -f: no warning if file does not exist
    • unlink: rm
  • tree: directory stucture
  • dd bs=4M if=.img of=/dev/sdX status=progress oflag=sync

  • scripts: record current session

  • free: view memory usage

  • vmtouch: pin file in cache & etc

  • findmnt | grep "\[": look for bind mount

  • git submodule sync --recursive: sync .gitmodules with submodules

apt

  • alien: convert package between distro
    • --scripts: also try to convert script
  • tasksel: package group
    • --list-task
    • --task-packages
  • apt-cache
    • show: dump of various info
    • depends, rdepends
    • policy: to see which source a package is installed from
  • apt-mark
    • hold, unhold: stop automatic update
    • auto, manual: mark dependency
  • /etc/apt/preferences
    • Pin: origin ""
    • Pin: release *
    • Pin-Priority < 0 : prevent installation
  • use mirror list: deb mirror://mirrors.ubuntu.com/mirrors.txt
  • non interactive: https://stackoverflow.com/questions/33370297/apt-get-update-non-interactive

backport is not enabled even after apt source update -t bionic-backports

install-recommends https://askubuntu.com/questions/351085/how-to-remove-recommended-and-suggested-dependencies-of-uninstalled-packages

show obsolete package

aptitude search '~o' apt-show-versions | grep 'No available version'

clean residual config

Reference dpkg -l | grep '^rc' | awk '{print $2}' | xargs sudo apt-get purge

list installed packages by size

Reference dpkg-query -W --showformat='${Installed-Size;10}\t${Package}\n' | sort -k1,1n

bash

  • login vs non-login: $0 contains - if login
  • interactive vs non-interactive. $- contains i if interactive
  • reference: shell
  • reference: scp fail if shell prints
  • a shell running a script is always a non-interactive shell
    • used to change it via set -i. Not anymore because interactive only affect startup process.
  • echo $0: display current shell name reliably
  • cat /etc/shells: list valid login shells
  • grep "^$USER" /etc/passwd: print default shell name for user
  • chsh -s /bin/bash: change login shell

awesome cheatsheet

  • cron
    • cron guru
    • crontab: command
      • -e: edit
      • -l: list
    • format:
      • MAILTO=email@example.com for following cron job
      • @reboot
      • minute hour dayofmonth month dayofweek command
      • * * * * *: means that this command will run at all times
        • system cron also contains user name
    • cron file
      • /var/spool/cron/crontab: users’ cron setting
      • /etc/crontab, /etc/cron.*: system cron
  • timeout: run a command with time limit
  • at: run a command at specified time

git

  • to print git log, set GIT_TRACE=1 GIT_CURL_VERBOSE=1 before git command
  • git object

rsync

  • A trailing slash on a source path means “copy the contents of this directory”.
  • Without a trailing slash it means “copy the directory”.

  • later arguments will override previous one?

  • -a: copy recursively, with permission
  • -X: include extended permision
    • --perms: copy over original permission
    • --no-perms: original permission + umask
    • --no-perms --chmod=ugo=rwX: override original permission. i.e. umask only
      • acl is not kept
  • owner, group: keep original
    • usermap, groupmap
  • -u: means update only. skip file if the reciveing end is newer

  • --delele: delete file if source does not exist

  • -S: sparse file optimization
  • -z: compress on transfer
  • -e 'ssh -p <port-number>': use sepecified remote shell
  • -P: print progress
  • –exclude: exclude file pattern
  • –backup –backup-dir: do extra backup

  • -n: dry run

  • check:
    • size is always checked, you cannot skip on that
    • -I to not check mtime
    • -c to also checksum
rsync -aXSP
# for synology
rsync -aSP --no-perms --no-owner --no-group --no-links --exclude '#recycle' --exclude '@eaDir' --delete
# copy directory only
rsync --include '*/' --exclude '*'
# same command but with filter syntax
rsync -f'+ */' -f '- *'

user management

  • /etc/passwd: user and group

  • /etc/group: supplementary groups

  • useradd
    • -m: create home dir
  • usermod -aG
  • mkhomedir_helper
  • passwd -e <user_name>
  • chage -l <user_name>
usermod -l new old
usermod -d /newdir -m new
groupmod -n new old

login msg

make executable to enable the message.

/etc/update-motd.d

name service switch

  • specify how different services are resolved
    • /etc/nsswitch.conf
  • getent: read database through nss

xxd

  • every option start with its own -
  • -i: dump c header
  • -p: plain dump style
  • -r: revert from hex

nc

  • -v: verbose
  • -w 2: timeout
  • -z: scanning mode
  • -p: specify a port
  • -l: listen

file path

/home/$user might not exist due to network mounted system /tmp is writable by all users /run/user/$uid created by pam_systemd store files used by running process for that user wiped when no active session remain

$XDG_RUNTIME_DIR
https://doc.qt.io/qt-5/qstandardpaths.html
https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

code page

encoding set https://www.iana.org/assignments/character-sets/character-sets.xhtml

gnome

  • alt f2: run command
    • r : restrart gnome
    • lg: gnome diagnostic terminal?

ssh

  • ssh-keygen -b 4096 -t rsa 4096 bit rsa key
  • ssh-keygen -a 100 -t ed25519
  • ssh -A agent forwarding:
    • remote machine can use local ssh-agent to authenticate
    • no need to setup key everywhere
  • port-forward enable access to another service which is otherwise inaccessible
    • -L 123:remote:456 bind local port 123 to remote:456
    • -R 123:local:456 bind remote port 123 to local:456
    • -f -N -T
      • -f: make ssh a background process after authentication
      • -N: make a connection without running any commmand
      • -T: disable pseudo-tty allocation
  • ssh -J Jumphost jump host
    • for older version,
      • -o ProxyCommand="ssh -W %h:%p Jumphost"
      • -tt allocate pseudo-tty
Host betajump
  HostName jumphost1.example.org
  User test
  Port 233
  IdentityFile ~/.ssh/rsa
  # same as -A
  ForwardAgent yes

Host behindbeta
  HostName behindbeta.example.org
  # same as -J
  ProxyJump betajump

udev

  • respond to hotplug event, and assign dev name based on dev property
    • udevadm monitor: print all hotplut event
    • udevadm info --attribute-walk --name <dev>: print device property
    • udevadm control --reload-rules

grub

# whether to boot from saved entry
GRUB_DEFAULT=saved
# whether to update saved entry
# GRUB_SAVEDEFAULT=true

# then run following command
grub-set-default
update-grub

vscode

remote docker cmake config

note on china network

  • go_url: https://golang.google.cn/dl
  • GOPROXY: https://goproxy.cn/
  • docker: https://yeasy.gitbook.io/docker_practice/install/mirror
  • ngc: ngc registry image pull

https://toroid.org/sudoers-syntax