Essential Linux commands

UNIT="gpg-agent.service"
systemctl get-default                      # gets default system target
systemctl show-environment                 # shows common environment variables
systemctl list-unit-files                  # lists installed units
systemctl list-units --type=target         # lists target units
systemctl list-units --type=service        # lists service units
systemctl list-dependencies --all          # dependency tree for the default target
systemctl list-dependencies "$UNIT"        # dependency tree for $UNIT
systemctl status "$UNIT"                   # summary and logs for $UNIT
systemctl show -p Requires "$UNIT"         # reads a parameter from $UNIT
systemctl show -p ControlGroup --global    # reads a parameter from the global configuration
systemctl --user restart "$UNIT"           # restart user service $UNIT

UNIT="my-awesome-system-daemon.service"
sudo systemctl set-environment LANG=en_US  # set common environment variable
sudo systemctl edit --full "$UNIT" --force # creates unit $UNIT
sudo systemctl edit --full "$UNIT"         # edits unit $UNIT
sudo systemctl enable "$UNIT"              # enables unit $UNIT
sudo systemctl disable "$UNIT"             # disables unit $UNIT
sudo systemctl daemon-reload               # rebuilds entire dependency tree

# "systemctl enable" creates a set of symlinks in /etc/systemd/system according to the [Install] directives
# "systemctl disable" removes the symlinks from /etc/systemd/system and /etc/systemd/system/$UNIT.wants
# both do reload systemd configuration but do not start/stop relevant units except when using --now

FILE="/usr/lib/systemd/user/gpg-agent.service"
systemd-analyze verify "$FILE"             # checks if "$FILE" is a valid unit
systemd-analyze time                       # boot sequence duration to reach default target
systemd-analyze blame                      # lists loaded units, ordered by start time
ps --pid 1 -f                              # view the systemd process

man bootup                                 # detailed startup sequence description
man systemd.unit                           # configuration units guide
man systemd.service                        # service units design guide
man systemd.special                        # details on default system targets

UNIT="ssh.service"
sudo journalctl -u "$UNIT" --priority info # information logs for systemd unit $UNIT
journalctl --user --boot                   # current user log entries since last boot
journalctl --facility=help                 # list known syslog facilities
sudo journalctl --facility=daemon          # log entries for daemons & services
sudo journalctl --facility=cron            # log entries for scheduled tasks
sudo journalctl --facility=auth            # log entries for login events
sudo journalctl --facility=authpriv        # log entries for root commands

USER_NAME="someone"
id "$USER_NAME"                            # prints id and group details for $USER_NAME
sudo who -uH                               # view all connected users
sudo w                                     # more details about connected users
sudo wall "hiya"                           # message all connected users
sudo write "$USER_NAME" pts/9              # write to $USER_NAME on a specific tty

cat /etc/passwd | column -ts ":"           # view all users accounts
cat /etc/group | column -ts ":"            # view all groups accounts
sudo cat /etc/shadow | column -ts ":"      # view shadowed users accounts
sudo cat /etc/gshadow | column -ts ":"     # view shadowed groups accounts

USER_NAME="someone"
GROUP_NAME="access-to-network-drive"
passwd                                        # change current user password
sudo adduser "$USER_NAME" --shell /bin/bash   # create user, set default shell
sudo deluser "$USER_NAME"                     # delete user
sudo usermod -a -G "$GROUP_NAME" "$USER_NAME" # add user to group

USER_NAME="someone"
GROUP_NAME="access-to-network-drive"
newgrp "$GROUP_NAME"                                # changes Gid for current user to $GROUP_NAME
sudo groupadd -f "$GROUP_NAME" -g 1010              # create user group $GROUP_NAME, set Gid
sudo gpasswd -a "$USER_NAME" "$GROUP_NAME"          # add $USER_NAME to $GROUP_NAME
sudo gpasswd -d "$USER_NAME" "$GROUP_NAME"          # removes $USER_NAME from $GROUP_NAME
sudo gpasswd -M "$USER_NAME" "person" "$GROUP_NAME" # overwrite members list for $GROUP_NAME
sudo groupmod -n "some-group" "$GROUP_NAME"         # changes group name to some-group
sudo groupdel "some-group"                          # deletes group some-group

man login.defs                             # shadow password suite tools configuration
man shadow                                 # shadowed users accounts file description
man gshadow                                # shadowed groups accounts file description

USER_NAME="xrdp"
PID=582
CMD="sleep 60"
top -u "$USER_NAME"                        # watch processes with username $USER_NAME
sudo ps -fU "$USER_NAME"                   # processes whose real Uid is $USER_NAME's
sudo ps -fp "$PID"                         # details on process with PID $PID
sudo ps -aux                               # lists all processes on the system, by user
pstree -puTn                               # displays current processes tree
                                           # options : show PID, show RUID, hide threads, sort by PPID
time --verbose "$CMD"                      # time $CMD execution, displays resources usage
                                           # - User time: command instructions processing
                                           # - System time: execution of kernel system calls

sleep 60 &                                 # start command as a job
jobs                                       # show active jobs
fg 1                                       # bring job in the foreground
bg 1                                       # resume suspended job

CMD="sleep 60"
PID=582
ps -o pid,ni,cmd --ppid $$                 # niceness for current shell's processes
nice -n 5 "$CMD"                           # runs $CMD and adds 5 to the default niceness
renice -n 5 -p "$PID"                      # alters process $PID niceness by 5

DEV="/dev/sda"
PART="/dev/sda2"
FILE="/home/admin/.bashrc"
sudo fdisk -lo +UUID "$DEV"                # show block device information for $DEV
lsblk -fmo +TYPE "$PART"                   # show partition summary for $PART
sudo blkid -p "$DEV" | sed -r "s/\s/\n/g"  # show partition details for $PART
sudo dumpe2fs "$DEV" | grep -F Filesystem  # show file system details for $PART
df -h "$FILE"                              # show free space on $FILE's partition
findmnt -A --fstab --evaluate              # show details about fstab mounts

FILE="/home/someone/.bashrc"
file "$FILE"                               # show file type for $FILE
stat "$FILE"                               # show inode metadata for $FILE

MODE="644"
FILE="/home/someone/.bashrc"
USER_NAME="someone"
GROUP_NAME="someone"
DIR="/home/someone"
chmod "$MODE" "$FILE"                      # set $FILE permissions to $MODE
chown "$USER_NAME:" "$FILE"                # set $FILE Uid and Gid to $USER_NAME
chgrp "$GROUP_NAME" "$FILE"                # set $FILE Gid to $GROUP_NAME
chmod o+t "$DIR"                           # set sticky bit on $DIR
umask "$MODE"                              # set user file creation mode mask

USER_NAME="xrdp"
PID=582
CMD="sleep 60"
FILE="/home/someone/.somefile.swp"
sudo lsof -u "$USER_NAME"                     # files opened by processes owned by $USER_NAME
sudo lsof -u "^root"                          # files opened by processes NOT owned by root
sudo lsof -c "$CMD"                           # files opened by processes running $CMD
sudo lsof -c "/^s.*$/x" -c0                   # match command against regex + do not truncate
sudo lsof -a -u "$USER_NAME" -c "$CMD"        # combine multiple options with -a
sudo lsof -a -u "$USER_NAME" +D /dev/pts/. -R # list interactive processes for a user, display PPID
sudo lsof -i tcp@192.168.1.12:ssh -n          # list sockets matching protocol, host and port/service
sudo lsof -a -i TCP -s TCP:LISTEN -n          # list socket matching protocol and state
sudo lsof -a -p "$PID" -d ^mem                # files opened by process $PID, hide memory mapped files
sudo lsof -ap "$PID" -d ^mem                  # alternative form using grouped options
sudo lsof -t "$FILE"                          # processes currently accessing a specific file
vi /usr/share/doc/lsof/00QUICKSTART.gz        # lsof quickstart guide with many examples
tty                                           # print terminal / pseudo terminal for the current shell

man hier                                   # description of the entire file system
man inode                                  # description of the inode structure

DIR="/home/someone"
FILE="*.md"
CMD="file"

# search $DIR for $FILE and run $CMD on matches (recursive)
find "$DIR" -name "$FILE" -exec "$CMD" {} \;

# create a symlink to $FILE at $LINK
ln -s "$FILE" "$LINK"

PARAM="fs.inotify.max_user_watches"
VALUE="262144"
sudo sysctl "$PARAM"="$VALUE"              # sets kernel $PARAM to $VALUE
sudo sysctl -a                             # list all kernel parameters
sudo dmesg -kHxL=always                    # print kernel messages since last boot
sudo dmesg -k -l err --since "5 hour ago"  # print kernel error messages since 5 hrs ago

USER_NAME="someone"
SOME_NAME="my-new-host"
sudo EDITOR=/bin/vim visudo                     # edit sudoers file with vim
sudo -k                                         # revoke sudo session
sudo hostnamectl hostname "$SOME_NAME"          # updates system informations
sudo runuser -c '<CMD>' -P --login "$USER_NAME" # runs <CMD> as $USER_NAME without asking for password

# runuser starts a new login shell and runs the command with $USER_NAME's environment, but without executing their .bashrc
# <CMD> has to be single quoted to prevent variable expansion by the current shell with the current user environment

FILE="/usr/share/man/ja/man1/passwd.1.gz"
CMD="passwd"
man "$FILE"                                # format and display man file $FILE
whatis "$CMD" chpasswd                     # search man-db for pages matching arguments
man -f "$CMD" chpasswd                     # same as the above
man -k "$CMD" chpasswd                     # searches man-db for arguments in pages descriptions (regex supported)
apropos "$CMD" chpasswd                    # same as the above
man -a "$CMD"                              # prints all man pages for $CMD one after the other

ARCHIVE="/home/someone/tarball.tar"
tar --list -f "$ARCHIVE"                 # list $ARCHIVE contents
tar -cvf "$ARCHIVE" /home/someone/data   # archives directory contents into $ARCHIVE
tar -xvf "$ARCHIVE"                      # extracts $ARCHIVE contents to the current path

man polkit                                 # general concepts about the policy kit framework

man crontab                                # details on crontab files formatting
crontab -l                                 # print current user crontab file
crontab -e                                 # edits current users crontab using /usr/bin/editor
crontab -r                                 # removes current user crontab file

history -a                               # append the history buffer's content to user's .bash_history
history -c                               # clears history buffer

ALT="editor"
EXEC="/usr/bin/vim.tiny"
update-alternatives --get-selections        # view all alternatives names and values
update-alternatives --display "$ALT"        # symlink, alternatives, best and current value for $ALT
update-alternatives --config "$ALT"         # changes alternative configuration for $ALT (auto/manual, system wide)
update-alternatives --remove "$ALT" "$EXEC" # removes $EXEC as an alternative for $ALT symlink
update-alternatives --install $l $n $p $i   # installs alternative $p with priority $i for name $n and link $l