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Linux Cheat Sheet

Linux Cheat Sheet

Linux basic information unified from personal notes and bookmarks

Filesystem Hierarchy Standard

Directory Description
/ root directory of the entire file system
/bin essential command binaries that need to be available in single user mode; for all users, e.g., cat, ls, cp
/boot boot loader files, e.g., kernels, initrd
/etc host-specific system-wide configuration files
/home users' home directories, containing saved files, personal settings, etc.
/lib libraries essential for the binaries in /bin and /sbin
/media mount points for removable media such as CD-ROMs
/mnt temporarily mounted filesystems
/opt optional application software packages
/proc cirtual filesystem providing process and kernel information as files
/root home directory for the root user
/run run-time variable data: Information about the running system since last boot
/sbin essential system binaries, e.g., fsck, init, route
/srv server data such as data and scripts for web servers, data offered by FTP servers
/sys information about devices, drivers, and some kernel features
/tmp temporary files (see also /var/tmp). Often not preserved between system reboots
/usr secondary hierarchy for read-only user data; contains the majority of (multi-)user applications
    /usr/bin non-essential command binaries (not needed in single user mode); for all users.
    /usr/include standard include files
    /usr/lib libraries for the binaries in /usr/bin and /usr/sbin
    /usr/local tertiary hierarchy for local data, specific to this host
    /usr/sbin non-essential system binaries
    /usr/share architecture-independent (shared) data
    /usr/src source code, e.g., the kernel source code with its header files
/var variable files
    /var/cache application cache data
    /var/lib persistent data modified by programs as they run
    /var/lock lock files
    /var/log log files
    /var/mail mailbox files
    /var/opt variable data from add-on packages that are stored in /opt
    /var/run Run-time variable data
    /var/spool spool for tasks waiting to be processed
    /var/tmp temporary files to be preserved between reboots

System Inspection Commands

Command Description
uname -a current system summary
lshw system hardware
lscpu cpu and processing units
lspci -v pci buses and details about the devices connected to them
lsblk - a all block devices
lsmod loaded kernel modules
lsusb -v usb devices
df -H partitions, their mount points and the used and available space on each
fdisk -l partition information
mount | column -t mounted file systems
free -m check the amount of used, free and total amount of RAM
dmidecode SMBOIS data structures
ifconfig -a network interfaces
cat /proc/cpuinfo
cat /proc/meminfo
cat /proc/version
cat /proc/scsi/scsi
cat /proc/partitions
cat /etc/*-release linux distro
dmesg | less boot information (/var/log/dmesg)
history | tac | less bash execution history (~/.bash_history)
uptime show current uptime
vmstat 5 10 virtual memory statistics 10 times at 5 second intervals
less /etc/passwd list users
find . -name "device" find all files containing word device

Process Management

A process refers to a program in execution; it’s a running instance of a program. Daemons are special types of background processes that start at system startup and keep running forever as a service (typically in the /etc/init.d directory)

Command Description
ps -ef display every processes on the system
top system monitoring tool
pstree display a tree diagram of processes
w list of all the users currently logged
who same as above
whoami who you are logged in as
kill pid kill process id pid
killall proc kill all processes named proc *
bg stopped or background jobs
fg brings the most recent job to foreground
fg n brings job n to the foreground

systemd Service Manager

systemctl

Parameters Description
start application.service start a systemd service
stop application.service stop a currently running service
restart application.service
reload application.service reload configuration files without restarting
reload-or-restart application.service
enable application.service
disable application.service
status application.service
is-active application.service active or inactive
is-enabled application.service enabled or disabled
is-failed application.service
list-units --all list loaded or attempted to load
list-unit-files
cat atd.service display unit file
list-dependencies sshd.service display dependencies
show sshd.service show low-level properties of a unit
mask nginx.service prevent the service from being started, automatically or manually
edit nginx.service
rescue put the system into rescue (single-user) mode = isolate rescue.target
halt halt the system
poweroff initiate a full shutdown
reboot system restart

Targets are special unit files that describe a system state or synchronization point. Targets do not do much themselves, but are instead used to group other units together.

Parameters Description
get-default get default target
set-default graphical.target
list-unit-files --type=target
list-units --type=target
isolate multi-user.target

journalctl

Parameters Description
--utc display the timestamps in UTC
-b logs from the current boot
--list-boots see the boots that journald knows about
-b -1 logs for the boot n relative to the current
--since yesterday
--since "2015-01-10" --until "2015-01-11 03:00"
--since 09:00 --until "1 hour ago"
-u nginx.service for a specific unit
_PID=8088 by process id
-k or --dmesg kernel messages
-p err priority: emerg, alert, crit, err, warning, notice, info, debug
-n 20 most recent 20 logs
-o json-pretty output as: cat, export, json, json-pretty, json-sse, short, short-iso, short-monotonic, short-precise, verbose
--no-pager
-f actively follow the logs as they are being written
--disk-usage find out the amount of space of the journal
--vacuum-size=1G shrink journal by the indicated size
--vacuum-time=1years remove entries beyond specified time
cat /etc/systemd/journald.conf journal config

udev

Working with files and folders

Command Description
pwd show current directory
cd change to home
cd dir change directory to dir
ls directory listing
ls -la formatted listing with hidden files
mkdir dir create a directory dir
rm file delete file
rm -r dir delete directory dir
rm -f file force remove file
rm -rf dir force remove directory dir
cp file1 file2 copy file1 to file2
cp -r dir1 dir2 copy dir1 to dir2; create dir2 if it doesn't exist
mv file1 file2 rename or move file1 to file2
ln -s file link create symbolic link link to file
touch file create or update file
more file output the contents of file
head file output the first 10 lines of file
tail file output the last 10 lines of file
tail -f file output the contents of file as it grows
cat > file standard input into file

Editors

Editor Command Description
Vi vi file opens file in Vi file editor
i text mode
ESC command mode
: ex mode
:q quit
:q! force quit
:w save
:wq or :x save and quit
:w save

File Permissions:

  • chmod octal file – change the permissions of file to octal, which can be found separately for user, group, and world by adding:
  • 4 – read (r)
  • 2 – write (w)
  • 1 – execute (x)

Examples:

  • chmod 777 – read, write, execute for all
  • chmod 755 – rwx for owner, rx for group and world

SSH:

  • ssh user@host – connect to host as user
  • ssh -p port user@host – connect to host on port port as user
  • ssh-copy-id user@host – add your key to host for user to enable a keyed or passwordless login

Searching:

  • grep pattern files – search for pattern in files
  • grep -r pattern dir – search recursively for pattern in dir
  • command | grep pattern – search for pattern in the output of command
  • locate file – find all instances of file

System Info:

  • date – show the current date and time
  • cal – show this month's calendar
  • uptime – show current uptime
  • w – display who is online
  • whoami – who you are logged in as
  • finger user – display information about user
  • uname -a – show kernel information
  • cat /proc/cpuinfo – cpu information
  • cat /proc/meminfo – memory information
  • man command – show the manual for command
  • df – show disk usage
  • du – show directory space usage
  • free – show memory and swap usage
  • whereis app – show possible locations of app
  • which app – show which app will be run by default

Compression:

  • tar cf file.tar files – create a tar named file.tar containing files
  • tar xf file.tar – extract the files from file.tar
  • tar czf file.tar.gz files – create a tar with Gzip compression
  • tar xzf file.tar.gz – extract a tar using Gzip
  • tar cjf file.tar.bz2 – create a tar with Bzip2 compression
  • tar xjf file.tar.bz2 – extract a tar using Bzip2
  • gzip file – compresses file and renames it to file.gz
  • gzip -d file.gz – decompresses file.gz back to file

Network:

  • ping host – ping host and output results
  • whois domain – get whois information for domain
  • dig domain – get DNS information for domain
  • dig -x host – reverse lookup host
  • wget file – download file
  • wget -c file – continue a stopped download

Installation:

  • dpkg -i pkg.deb – install a package (Debian)
  • rpm -Uvh pkg.rpm – install a package (RPM)

Install from source:

  • ./configure
  • make
  • make install

Shortcuts:

  • Ctrl+C – halts the current command
  • Ctrl+Z – stops the current command, resume with
  • fg in the foreground or bg in the background
  • Ctrl+D – log out of current session, similar to exit
  • Ctrl+W – erases one word in the current line
  • Ctrl+U – erases the whole line
  • Ctrl+R – type to bring up a recent command
  • !! - repeats the last command
  • exit – log out of current session
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