Linux Commands

MAN Sections Numbers and Meaning

1: User Executable Programs or Shell Commands

2: System Calls (Functions provided by the Kernel)

3: Library Calls (Functions within program libraries)

4: Devices and Special Files (generally found in /dev)

5: File Formats and Conventions (i.e. /etc/passwd)

6: Games

7: Miscellaneous (includes macro packages and conventions), e.g. man(7), groff(7)

8: System Administration tools and Daemons (requires root)

9: Kernel routines [Non standard]

Tricks:

• • • $ man -s 1 printf

    • $ man -k '^printf'

Tips and Tricks

Creating One-Liners Command (i.e. stringing together commands)

• ';' runs one command right after another.

  • Ex: commandName1; commandName2

• '&' runs one command right after another.

  • Ex: commandName1 & commandName2

• '&&' Will only run the second command if the first one doesn't fail.

  • Ex: commandName1 && commandName2

You can use the following options to recall the last command or arguments from the last command.

• Reuse the previous command, type: !!

  • The 'n' represents the position of the argument you want to recall, type: !!:n

• Reuse the last argument from the previous command, type: !$ --OR-- $_

• Reuse the first argument from the previous command, type: !^

• Reuse all the arguments from the previous command, type: !&

• Reuse a specific argument in the last command, enter its position (i.e. 1 = 'a.txt', 2 = 'b.txt', etc.), type: !:1

• The most recent command matching pattern, type: !pattern

• Last command, substitute 'find' with 'replace', type: !!:s/find/replace

Utilities

• Used to monitor user activities (i.e. process accounting)

  • Ex: sudo accton [on|off]

• Displays the battery status and other ACPI information

  • Note: Information from the /proc or the /sys.

  • Ex: acpi [<options>]

• Manages physical or virtual terminals multi-user access

  • Ex: sudo agetty <options>

• Creates an alias (or short name) for a command or sequence of commands.

  • Notes:

    • To see all the aliases, type: alias  

    • To delete an alias with unalias <aliasName> command.

  • Ex [type: ll instead of ls -l]: alias ls -l=ll

• Allows you to search help system on any commands installed on the local system.

  • Ex: apropos <commandName>

• A package management utility, for installing software on Debian Linux Distros

  • Note: apt was first available in Ubuntu 16.04 to simplify the package management and to merge multiple commands (i.e. apt-get, etc.) into one single command.

  • Tip: sudo apt update && sudo apt upgrade (Updates the local system)

  • Ex: sudo apt install packageName

• A package management utility, for installing software on Debian Linux Distros

  • Tip: sudo apt-get update && sudo apt upgrade (Updates the local system)

  • Ex: sudo apt-get install packageName

• Displays the computer CPU architecture.

  • Ex: arch

• Manages (i.e. creates, modifies, or extracts) archive files.

  • Ex: ar r fileName.a *.ext

• Shows the current process that was moved to the background (user must be logged in).

  • Notes:

    • Appending an ampersand ( & ) to end of a command runs the job in the background.

    • Pressing Ctrl+Z suspends the current foreground process.

  • Ex: bg

• Bind a key sequence to a Readline function or a macro, or set a Readline variable.

  • Ex: bind -l

• Executes shell builtins with arguments without performing command lookup.

  • Ex: builtin -b

• Removes sections of text from each line of file.

  • Ex: echo 'Hello, World' | cut -b 1,3

• Displays local system information using the /proc directory.

  • Ex: cat /proc/meminfo

  • Ex: cat /proc/cpuinfo

• Manipulates the real-time attributes of a process.

  • Ex: chrt -p <processID>

• Switch between the different TTY (TeleTYpewriter) terminals

  • Ex: chvt

• Changes a processes root directory.

  • Note: Every process/command in Linux has a current working directory called 'root' directory. 

  • Ex: sudo chroot /directoryName/path command

• Displays the kernel ring buffer.

  • Ex: dmesg

• Displays environment variables.

  • Ex: env | less

• Exits the shell or console window.

  • Ex: exit

• Edits and re-execute the commands previously entered into the interactive shell

  • Ex: fc

• Sends a job to the foreground (user must be logged in).

  • Ex: fg %1

• Displays the total amount of free memory used and swap space on the local system.

  • Ex: free -h

• Display entries important text files called databases (i.e. passwd, group, etc.)

  • Troubleshooting: Useful tool checking several configuration files.

  • Ex: getent passwd ubuntu

• Displays a hash table of recently executed programs.

  • Ex: hash

• Instructs the hardware to stop all the CPU functions.

  • Ex: halt

• Displays help for internal commands [i.e. ls, exit, etc.]

  • Ex: help <commandName>

• Displays a list of the most recently used commands.

  • Note: To re-execute a command (i.e. 199) from the list, type: !199

  • Ex: history.

  • Tip: history | grep "searchString" (Searches the contents of STDOUT)

• Displays a list of running processes on local system

  • Install: sudo apt install htop

  • Ex: htop

• Manages the hardware clock (aka Real Time Clock [RTC])

  • Ex: sudo hwclock

• Displays help on any command installed on the local system.

  • Note: similar to the man command.

  • Ex: info <commandName>

• Installs program binaries.

  • Ex: install sourceFile.ext destFile.ext

• Monitors local system input/output statistics for devices and partitions

  • Install: sudo apt install sysstat

  • Ex: iostat

• Removes IPC resources and associated data structure from the local system

  • Note: IPC is Inter-Process Communication

  • Ex: ipcrm

• Displays information on the IPC facilities for the calling processes

  • Note: IPC is Inter-Process Communication

  • Ex: ipcs

• Queries the 'systemd' journal.

  • Ex: journalctl

• Displays the status of all current job (i.e. running process or group of processes).

  • Ex: jobs

• Kills a running process by PID (Process ID).

  • Ex: kill -9 <processID>

• Kills a running processes by name.

  • Ex: killall -9 <processName>

• Logs the user off the local or remote system.

  • Ex: logout

• Displays file attributes on a second extended file system.

  • Ex: lsattr -a

• Displays a list of block device.

  • Ex: lsblk

• Displays information about the CPU.

  • Ex: lscpu

• Displays information about the file locks.

  • Ex: lslocks

• Displays information about the PCI.

  • Ex: lspci

• Displays information about the users accounts on the local system.

  • Ex: lslogins

• Displays information on the IPC facilities for the calling processes

  • Note: IPC is Inter-Process Communication

  • Ex: lsipc

• Displays information about the memory on the local system.

  • Ex: lsmem

• Displays detailed information of the local system’s hardware configuration (/proc directory)

  • Ex: lshw

• Displays the status of Linux kernel modules.

  • Ex: sudo lsmod

  • Tip: ls -l /lib/modules/$(uname -r)/kernel (directory of installed modules)

• Displays a list of open files.

  • Ex: lsof

• Displays information about USB bus and the connected devices

  • Ex: lsusb

• Displays information about the OS. 

  • Ex: lsb_release -a

• Displays help on any command installed on the local system.

  • Ex: man <commandName>

  • Ex: <commandName> --help 

• Displays information about a Linux Kernel module

  • Ex: sudo modinfo -V

  • Tip: ls -l /lib/modules/$(uname -r)/kernel (directory of installed modules)

• Runs a command in the background, even when the user is not logged in.

  • Ex: nohup pi 200000 (calculates pi to 20,000 place in the background)

• Reboots the local system.

  • Ex: reboot

• Displays a memory map of a process identifier.

  • Ex: pmap <processID>

• Shuts down the local system.

  • Ex: poweroff

• Displays environment variables.

  • Ex: printenv | less

• Displays a list of running processes.

  • Ex: ps -aux

• Shuts down the local system.

  • Ex: shutdown now 

• Sorts text in a file or stream.

  • Ex: cat filename.ext | sort

• Manages (changes communication settings) the console line settings

  • Ex: stty

• Executes commands as a root administrator privileges temporarily.

  • Ex: sudo programName

• Manages (i.e. start, stop, and restart) the state of the 'systemd' system and service manager.

  • Ex: systemctl

• Displays how long it takes a command to execute.

  • Ex: time echo "Hello, World"

• Displays a list of running processes on the local system

  • Ex: top

Reference: Process States

D: Uninterruptible sleep, R: Running, S: Sleeping, T: Traced (stopped), Z: Zombie 

• Displays the terminal connection you are connected on.

  • Ex: tty [<options>]

• Displays information about command type.

  • Ex: type <commandName>

• Executes a program periodically, showing a fullscreen output.

  • Ex: watch <commandName>

• A system performance monitoring command, displays hardware information

  • Ex: vmstat

• Displays information about the OS.

  • Ex: uname -a

• Removes duplicate text in a file or stream.

  • Ex: cat filename.ext | uniq

• Displays how long the local system has been up (i.e. running)

  • Ex: uptime

• Displays a brief description of a console utility.

  • Ex: whatis <commandName>

• Displays the path of executable file.

  • Ex: whereis <commandName>

File System

• Pattern scanning and processing language.

  • Ex: tail -f access_log | awk '/myhome.html/ { system("nmap " $1 ">> logdir/myhome.html") }' (Runs an external command for particular lines of data)

• Removes directory and suffix from filenames

  • Ex: basename /usr/bin/sort

• BZ compression utilities.

  • Ex: bzcmp, bzdiff, bzgrep, bzip2, bzless, bzmore

• Displays the contents of a text file.

  • Ex: cat fileName.ext

  • Ex: cat fileName.ext | less (Displays a text file one screen full at a time.)

• Changes the path from the current directory, to a different one.

  • Ex: cd directoryName

• Manages (i.e. add, delete, or change) disk partition table [uses TUI]

  • Ex: sudo cfdisk

• Changes attributes on a file in a directory.

  • Ex: chattr +i fileName.ext

• Changes permissions on a file.

  • Ex: chmod +rwx +ugw fileName.ext 

Reference: Permission Codes 

• • • r: read, w: write, x: execute

    • u: user, g: group, w: world

    • 0: None, 1: Execute, 2: Write, 3: Write & Execute, 4: Read, 5: Read & Execute, 6: Read & Write, 7: Read, Write & Execute

• Displays a CRC(Cyclic Redundancy Check) value, the byte size of the file in STDOUT

  • Ex: cksum nameFile.ext

• Changes the owner:group of a file

  • Ex: chown userName:groupName fileName.ext

• Compares two files byte by byte.

  • Ex: cmp fileName1.ext fileName2.ext

• Makes a copy of a file(s). 

  • Ex: cp fileName1.ext fileName2.ext

• Creates an empty disk image.

  • Ex: dd if=/dev/zero of=imageName bs=1024 count=10240

• Displays disk usages for directories.

  • Ex: du -h 

• Starts VIM editor in 'extended' mode (e.g. line editor mode).

  • Note: To displays the first line of file type '1p'

  • Ex: ex fileName.ext

• Displays the size, space used, and space available on the mounted filesystems

  • Note: Filesystems listed in /etc/fstab are mounted during the booting process. 

  • Ex: df -h

• Compares two files, and shows the differences.

  • Ex: diff fileName1.ext fileName2.ext

• Converts file or STDIN tabs to spaces

  • Ex: expand fileName.ext

• Formats a disk using a text based UI for managing disk partition tables

  • Ex: sudo fdisk -l

• Finds files or directories in the local filesystem that matches designated criteria.

  • Ex: find ./ -type f -name fileName.ext

  • Ex: find ./ -type d -name directoryName

• Determine the MIME file type (i.e. ‘text/plain; charset=us-ascii’)

  • Ex: file fileName.ext

• A simple text formatter that reads STDIN or a file sends to STDOUT.

  • Ex: fmt -w 15 fileName.ext

• Wraps each line of an input file to fit a specified width and displays to STDOUT.

  • Ex: fold fileName.ext

• Searches the contents of a file.

  • Ex: grep "searchString" fileName.ext

• Creates auto-decompressing executables

  • Ex: gzexe directoryName/

• Utility that compresses multiple files into a single file.

  • Ex: gzip fileName.ext

• Used to get statistics about the hard disk, alter writing intervals, acoustic management, and DMA settings

  • Ex: sudo hdparm -I /dev/sda

• Displays the top lines of a text file.

  • Ex: head fileName.ext

  • Tip: head /proc/meminfo 

• Displays a file or STDIN in ASCII, decimal, hexadecimal, octal format to STDOUT

  • Ex: hexdump fileName.ext

  • Tip: echo "Hello, World!" > helloworld && hexdump helloworld

• Converts text from one encoding into another encoding.

  • Ex: iconv -f UTF-8 -t ASCII//TRANSLIT -o outputFileName.txt inputFileName.txt

• Displays one screen of STDOUT at a time.

  • Note: The less command is better then more command.

  • Ex: less fileName.ext

  • Tip: cat /proc/meminfo | less

• Searches for files or directories in the local filesystem that match designated criteria.

  • Ex: locate fileName.ext

• Creates symbolic links to a specific file.

  • Ex: ln fileName.ext link2File.ext

  • Ex: ln -s fileName.ext link2File.ext

• Displays a list of files.

  • Ex: ls -l

  • Tip: ls -al ~ (Lists all the hidden files in the home directory)

• Verify file integrity using MD5 (Message Digest Algorithm 5)

  • Ex: md5sum nameFile.ext

  • Tip: md5sum /proc/meminfo

• Creates a new directory.

  • Ex: mkdir directoryName

• Displays one screen full of text to STDOUT at a time.

  • Ex: more fileName.ext

  • Ex: cat nameFile.ext | more

• Attaches a separate filesystem (such as an flash USB drive or external hard drive ) to the main filesystem on the local system.

  • Note: Filesystems listed in /etc/fstab are mounted during the booting process. 

  • Ex: mount -t iso9660 -o ro /dev/cdrom /mnt

• Moves or renames a file.

  • Ex: mv fileName1.ext fileName2.ext

• ncdu (NCurses Disk Usage) provides a navigable overview of file space usage.

  • Install: sudo apt install ncdu

  • Ex: ncdu

• Displays line numbers of files.

  • Ex: nl fileName.ext

  • Tip: nl /proc/meminfo

• Dumps a file in octal (and other) formats

  • Ex: od fileName.ext

  • Tip: od /proc/meminfo

• Combines two files by merging data, separated by a delimiter to STDOUT.

  • Ex: paste -d "|," number fileName1.ext fileName2.ext

• Changes current directory to the last one stored in the stack.

  • Note: This stack is LIFO (Last In First Out) based, (i.e. like a stack of paper, where you take the sheet off the top).

  • Related: pushd

  • Ex: popd

• Pushes the current directory into the stack.

  • Note: This allows you to return to the directory at a later time.

  • Related: popd

  • Ex: pushd

• Displays the current directory path.

  • Ex: pwd

• Displays resolved symbolic links or canonical file names

  • Ex: touch fileName1.ext; ln -s fileName1.ext fileName2.ext; readlink fileName2.ext

• Changes the name of one file or set of files.

  • Ex: rename fileName1.ext fileName2.ext

  • Tip: rename -n 's/jpeg/png/' *.jpeg (Renames files according to a regular expression.)

• Reverse the characters in STDIN or a file

  • Ex: rev fileName.ext

  • Tip: rev /proc/meminfo

• Deletes a file (or multiple files when using a wildcard).

  • Ex: rm fileName.ext

  • Tip: rm -rf directoryName (Deletes a directory, including all files and subdirectories)

• Deletes the current directory, unless you specify one. 

  • Note: The target directory must be completely empty.

  • Ex: rmdir directoryName

• Removes a Linux kernel module. 

  • Ex: sudo rmmod moduleName

  • Tip: ls -l /lib/modules/$(uname -r)/kernel (directory of installed modules)

• Monitors local system resources utilization, such as: CPU, Memory, I/O devices, etc.

  • Ex: sar -u 2 5

• Compares two files and then displays the results to STDOUT in a side-by-side format

  • Ex: sdiff fileName1.ext fileName2.ext

• Manipulate lines of a file or stream by replacing specified parts.

  • Ex: sed 's/keyword1/keyword2/g' fileName.ext

• Displays the last time file was accessed, modified, and changed.

  • Ex: stat fileName.ext

  • Tip: stat /proc/meminfo 

• Utility for joining lines in two files.

  • Ex: join fileName1.txt fileName2.txt

• Displays the checksum and block count of a file

  • Ex: sum fileName.ext

  • Tip: sum /proc/meminfo 

• Displays files in reverse (i.e. bottom line first, first line last).

  • Ex: tac fileName.ext 

  • Tip: tac /proc/meminfo 

• Displays the bottom lines of a text file.

  • Ex: tail fileName.ext

  • Tip: tail /proc/meminfo 

• Combines and compresses files in to an archive (aka TAR)

  • Ex: tar -cvzf fileName.tar.gz directoryName/

• Decompresses and extracts files from an archive (aka TAR)

  • Ex: tar -xvzf fileName.tar.gz directoryName/

• Reads from STDIN and write to STDOUT and files

  • Ex: wc -l fileName1.txt| tee fileName2.txt

• Creates an empty file.

  • Ex: touch fileName.ext

• Converts characters for strings (i.e. from lower to uppercase).

  • Ex: tr "[:lower:]" "[:upper:]" < fileName.ext

  • Tip: tr "[:lower:]" "[:upper:]" < /proc/meminfo 

• A recursive directory listing of depth-indented listing of files

  • Ex: tree /directoryName/path

  • Tip: tree /proc/

• Removes a separate filesystem from the main filesystem on the local system.

  • Ex: umount -f /mnt

• Converts file or STDIN spaces to tabs

  • Ex: unexpand fileName.ext

• The default text editor that comes with the OS (aka VIsual editor).

  • Tip: vim fileName.ext

  • Ex: vi fileName.ext (generally a shortcut to vim)

• Displays line, word, byte count.

  • Ex: cat fileName | wc -l

• Searches for executables files in the $PATH system environment variable.

  • Ex: which <commandName>

  • Tip: which touch

• Executes commands from STDIN.

  • Ex: find /directoryName -type f -name "*.ext" | xargs grep -i 'searchText'

• Displays a hexdump (or the reverse).

  • Ex: xxd fileName

  • Tip: echo "Hello, World!" > helloworld && xxd helloworld

Runs commands with arguments suppressing shell function lookup.

Ex: command -v grep

Ex: command -V grep

Displays possible completions depending on the options.

Ex: compgen -v

Specify how arguments are to be completed by Readline.

Ex: complete -p

Sets variable values and attributes.

Ex: declare -p 

Display the list of currently remembered directories.

Note: Works with popd and pushd

Ex: dirs -v

Removes a job from the table of active jobs.

Note: Works with bg command.

Ex: disown <jobID>

Enables and disables builtin shell commands.

Ex: enable -a

Executes arguments as a shell command. 

Note: This is useful when you build dynamic commands via scripts

Ex: cmd="echo"; txt1="Hello, "; txt2="World!"; eval $cmd $txt1 $txt2

Replace the shell with the given command.

Ex: exec > fileName.ext (redirection the output to a files)

Creates an environmental variable 

Ex: export country=USA

Mark a variable or function as read-only, so that it can not be changed

Ex: readonly [-af] [name[=value] ...]

Creates a function, multiple command working together to complete a specific task.

Syntax: functionName() { commandName ; commandName } 

Ex: 

Shift the positional parameters left by N number of times and renames 

Ex: shift [n]

 getopts optstring name [arg]

 jobs [-lnprs] [jobspec ...] or job

 let arg [arg ...]

 local name[=value] ...

 select NAME [in WORDS ... ;] do CO

 set [--abefhkmnptuvxBCHP] [-o opti

 shopt [-pqsu] [-o long-option] opt

 suspend [-f]

 test [expr]

 times

 trap [-lp] [arg signal_spec ...]

 typeset [-afFirtx] [-p] name[=valu

 ulimit [-SHacdfilmnpqstuvx] [limit

 umask [-p] [-S] [mode]

Networking

• Manages the local system's ARP cache.

  • Ex: arp -a

• Displays the output from HTTP/HTTPS request.

  • Ex: curl https://www.example.com 

• Displays DNS information for a domain name.

  • Ex: dig domainName

• Displays remote IP address of a domain name.

  • Ex: host domainName

• Displays the host ID in hexadecimal format.

  • Note: A unique number that is is based on the local system's IP address, in hexadecimal format.

  • Ex: hostid

• Displays the IP address of a local host system.

  • Ex: hostname -i

• Manages Linux system hostname and related settings.

  • Ex: hostnamectl

• Displays IP address, network interfaces, etc. of the local system.

  • Ex: ip a

• Maintain iptables for the Netfilter firewall for IPv4, included in the Linux kernel

  • Ex: sudo iptables

• Save the current iptables rules in a user specified file.

  • Ex: sudo iptables-save

• Controls the NetworkManager. manages (status, create, edit, etc.) network connections

  • Ex: nmcli device show

• Performs a port scans a hosts on a local network, or a single hosts

  • Install: sudo apt  install nmap

  • Ex: nmap -sP 10.0.0.0/24

• Pings a remote system continuously to see if it responds.

  • Ex: ping example.com

• Syncs files and directories between two hosts.

  • Ex: rsync -av /source/directoryName /destination/directoryName

• Securely copies a file from remote device to local system.

  • Ex: scp userName@remotehost:filename.ext /local/directoryName/

• Securely copies a file from local system to a remote device.

  • Ex: scp fileName.ext userName@remotehost:/remote/directoryName/

• Create a secure console connection to a remote system.

  • Ex: ssh userName@example.com

• Displays network path to the destination, show MTU along the path

  • Ex: tracepath example.com

• Look up the owner information of the domain name.

  • Ex: whois domainName

• Download a file via HTTP/HTTPS.

  • Ex: wget https://www.example.com/filename.ext 

User Management

• Manage user password expiry information.

  • Ex: chage --list

• Changes the user’s login shell.

  • Ex: chsh <userName>

• Changes user’s name and other details (finger information).

  • Note: Stored in: /etc/passwd

  • Ex: chfn

• Changes the group ownership of a file or directory.

  • Ex: chgrp <groupName> fileName.ext

• Used to change password for multiple users at a time

  • Ex: sudo chpasswd

• Displays groups that the current user is a member.

  • Ex: id

• Creates a new local group.

  • Ex: groupadd <groupName>

• Creates a gshadow file from the group

  • Ex: grpconv

• Deletes an existing group.

  • Ex: groupdel

• Manage existing groups on the local system.

  • Ex: groupmod

• Displays a list the groups installed on the local system.

  • Ex: groups

  • Tip: groups userName (Displays the groups that a user is currently a member)

• Manage the /etc/group and /etc/gshadow.

  • Ex: gpasswd -a userName groupName

• Show a listing of last logged in users

  • Ex: last

  • Ex: lastb (shows the bad login attempts (from: /var/log/btmp file).

  • Ex: last reboot ('reboot ' is pseudo user used to log each time the local system is rebooted.)

• Changes the password of a user.

  • Ex [current user]: passwd

  • Ex [another user]: sudo passwd userName

• Creates a new local user.

  • Ex: useradd <userName>

• Deletes a user account and related files on the local system

  • Ex: userdel <userName>

• Modifies a local user account.

  • Ex: usermod -a -G <groupName> <userName>

• Displays the user names of users currently logged in on the local host.

  • Ex: users

• Shows who is logged into the local system.

  • Ex: who

• Lists logged in users on the local system

  • Ex: w

• Displays a text message on the consoles of all currently logged in users.

  • Ex: wall "Hello, World"

• Displays the user name who you're logged in as.

  • Ex: whoami

• Send a message to another user on the local system.

  • Ex: write <userName>

• Displays information about currently logged in users on the local system

  • Ex: who

=====

Scripting

• Creates a variable.

  • Ex: TestVariable="Hello, World!" && echo $TestVariable

  • Tip: unset TestVariable (deletes a variable)

• Terminates a for, while and until loop.

  • Ex: break

• Case command, selectively executes commands when a condition is meet.

  • Syntax: case WORD in [PATTERN [| PATTERN]...) COMMANDS ;;]... esac

  • Ex: a=1; case "$a" in "1") echo "a is 1";; "2") echo "a is 2";; esac

• Displays a string of text in the console.

  • Ex: echo "Hello, World"

• If command, executes commands when a condition is meet.

  • Syntax: if COMMANDS; then COMMANDS; fi

  • Ex: if true; then echo 'Hello'; fi

• Exits a shell function.

  • Ex: return

• FOR command, repeats commands a specific number of times.

  • Syntax: for NAME [in WORDS ... ;] do COMMANDS; done

• Reads the specified file descriptor into the buffer

  • Ex: echo "what's your age?";read age; echo "Age: $age"

• Displays a given string, number or any other format specifier on the console

  • Ex: printf "%f\n" "5.62" "2.679"

  • Ex: printf "%s\n" "Hello, World!"

• Suspend the execution (in seconds) of a script for an interval of time.

  • Ex: sleep 10

• Reads and executes the contents (i.e. commands) in a file.

  • Ex: source fileName.ext

  • Tip: source ~/.bashrc (reloads the .bashrc file)

• While command, repeats commands until a TRUE condition is meet.

  • Syntax: while COMMANDS; do COMMANDS; done

  • Ex: while true; do echo 'Hello'; done

• Until command, repeats commands until a FALSE condition is meet.

  • Syntax: until COMMANDS; do COMMANDS; done

  • Ex: until false; do echo 'Hello'; done

Variables Constants

• Used for testing TRUE

  • Ex: true

• Used for testing FALSE

  • Ex: false

Miscellaneous

• Checks a file for spelling errors

  • Ex: aspell -c fileName.txt

• Displays lines that start with the specified string

  • Note: Default: /usr/share/dict/words

  • Ex: look ubuntu /etc/passwd

• Performs calculation functions.

  • Note: In interactive mode, type 'quit' to exit.

  • Ex: echo "10+10" | bc

• Displays a monthly or yearly calendar.

  • Ex: cal

  • Ex: cal 2020

• Clears the console screen.

  • Tip: Press Ctrl+l

  • Ex: clear

• Displays current date and time.

  • Ex: date

• Evaluates arithmetic expressions in the form of a postfix expression (i.e. entering a number pushed into the stack and followed by operator used to evaluate the expression).

  • Note: In interactive mode, type 'quit' to exit.

  • Ex: dc --expression="10 10 + p"

• Performs calculation functions (non-interactive).

  • Ex: expr 10 + 10

• Displays factors of a decimal number.

  • Ex: factor 42

• Scans the font directories and builds font cache.

  • Install: sudo apt install fontconfig

  • Ex: fc-cache

• Lists the available fonts and font styles.

  • Install: sudo apt install fontconfig

  • Ex: fc-list

• Creates text banners.

  • Install: sudo apt install figlet

  • Ex: figlet "Hello, World"

• Displays out of the blue poignant, inspirational, or silly phrases.

  • Ex: fortune 

• GNU C and C++ Compilers

  • Install: sudo apt-get install gcc

  • Ex: gcc new.c -o new && ./new

  • Install: sudo apt-get install g++

  • Ex: g++ new.cpp -o new && ./new

• Invokes Ghostscript, which is postscript interpreter.

  • Install: sudo apt-get install ghostscript

  • Ex: gs -dSAFER fileName.pdf

• File Management: Midnight Commander (aka mc) [uses TUI]

  • Install: sudo apt install mc

  • Ex: mc

• A simple text file editor (much easier to use then VI or VIM).

  • Ex: nano fileName.ext

• A powerful multi-purpose calculator (interactive).

  • Note: Type qcal -e to update currency, in interactive mode, type: 1 USD to CAD.

  • Install: sudo apt install qalc

  • Ex: qalc

• Re-initializes the terminal, use if the console is in an abnormal state.

  • Ex: reset

• Records a typescript file, that records all the terminal activities

  • Ex: script typescriptFile

• Replays a typescript/terminal_activity stored in the log file that was recorded by the script command

  • Ex: scriptreplay timingFile typescriptFile

• Allows the launching of multiple shell sessions from a single ssh session

  • Ex: screen

• Displays 'scan', 'key' or 'ascii' code for each key pressed

  • Note: Press Ctrl-D to quit.

  • Ex: showkey -a

• Generates numbers using, passed parameters FIRST, INCREMENT, LAST

  • Ex: seq 0 10 100

  • Ex: seq 1 2 10

• Terminal multiplexer (create multiple text consoles)

  • Ex: tmux

• Creates text banners.

  • Install: sudo apt install toilet

  • Ex: toilet "Hello, World"

• Opens a file or URL in the default application for the resource.

  • Note: Requires X-Windows.

  • Ex: xdg-open <fileNameOrURL>

• Activates the current screensaver.

  • Note: Requires X-Windows.

  • Ex: xdg-screensaver activate

Controlled Failures

• Running any of follow commands will result in a 'kernel panic'. The only way fix this is to reboot the local system.

  • Ex: dd if=/dev/random of=/dev/port

  • Ex: echo 1 > /proc/sys/kernel/panic

  • Ex: cat /dev/port

  • Ex: cat /dev/zero > /dev/mem

• Creates a 'fork bomb', that causes a 'kernel panic' by recursively calls itself multiple times by spawning new child processes in an infinite loop. The only way fix this is to reboot the local system.

  • Ex: :(){:|:&};:

Login shells 

When you log in to a local system, you loading an interactiveconsole shell (i.e. Bash, Zsh, etc.).  These are shells that accept commands, and have the OS execute them. There are two type of shells interactive which can be logged into, and non-interactive (or non-login) which can't be logged into.

Bash supports the following list of script files (i.e. ~/.bashrc) that are automatically executed when the shell starts (or when you log out):

Login Script Files

~/.bashrc

/etc/profile

~/.bash_profile

~/.bash_login

~/.profile

Logout Script Files

~/.bash_logout

/etc/bash.bash_logout

Input and Output Redirection

By default, shell commands read their input from the STDIN (aka Standard Input stream [0]) and write to the STDOUT (aka Standard Output stream [&1>]), unless there is an error which is written to STDERR (Standard Error stream [&2>]).

The '|' pipe operator, redirects the output of the first command as the input of the second command.

Ex: ls -l | more

The '<' redirects input from another source, other then STDIN.

Ex: sort <(printf "1\n3\n2")

The '>' redirects STDOUT into a file.

Ex: ls -l > fileName.ext

The '&1>' redirects STDOUT into a file.

Ex: ls -l &1> fileName.ext

The '&2>' redirects STDERR into a file.

Ex: ls -l &2> fileName.ext

The '&>' redirects all STDOUT into a file.

Ex: ls -l &> fileName.ext