Linux cheat sheet 🐧
An operating system is responsible for directly managing a system’s hardware resources like CPU, memory, and storage. Linux is one of the most widely-used open-source operating systems. It is a bundle of tools, programs, and services along with the Linux kernel which provides a fully functional operating system. Linux can be differentiated from other operating systems in many ways. Firstly, Linux is open-source software. Different Linux distributions come with different software options. Each distribution has its own approach to desktop interfaces. As such, users have the ability to choose core components such as graphics, user-interface, etc., as per their preference.
Here, we have attempted to cover the widely used Linux commands in the Linux Command Cheat Sheet. With new Linux distributions, there are chances of a few commands getting deprecated. So, make sure you keep up with the new releases, their new additions, and commands which are rendered deprecated.
Things to know:
· "ELF" file: The native executable file format used in Linux systems
· ltrace is used to trace library calls made by a program, which essentially logs user API calls.
· strace is used to trace system calls, which are the API calls directed towards the kernel.
· C is king for OS development due to its efficient hardware interaction and the ability to predict the assembly language output of C code.
How to diagnose a mysterious process that’s taking too much CPU, memory, IO, etc?
The diagram below illustrates helpful tools in a Linux system.
🔹‘vmstat’ - reports information about processes, memory, paging, block IO, traps, and CPU activity.
🔹‘iostat’ - reports CPU and input/output statistics of the system.
🔹‘netstat’ - displays statistical data related to IP, TCP, UDP, and ICMP protocols.
🔹‘lsof’ - lists open files of the current system.
🔹‘pidstat’ - monitors the utilization of system resources by all or specified processes, including CPU, memory, device IO, task switching, threads, etc.
Linux Performance Tools
| Application | Description | Category | Category |
|---|---|---|---|
| alias | Lists all your current aliases It allows one command to be substituted by another. | alias home=’cd home/hackr/public_html’ | Shell Commands |
| set -x | It is a built-in function that is used to define the values of the system environment. | set [--aefhkntuvx[argument]], where a, e, f, h, k, n, t, u, v, x are the different options that can be used with this command. | Shell Commands |
| \ | The backslash character is used to override any aliases. | Shell Commands | |
| script | It automatically creates a typescript or a capture-log of a session and writes it to a file. It includes all commands and their outputs as well. | script [options] [file] | Shell Commands |
| ~ | The tilde character is used as an alias to a user’s home directory. | Shell Commands | |
| reset | It reinitializes your current terminal. This is generally useful when a program dies leaving the terminal in an abnormal state. | reset [terminal] There are a number of options that can be used with reset to customize the output. | Shell Commands |
| exit | Closes your current terminal or logs out. Also, CTRL+D can be used. | exit | Shell Commands |
| logout | Logs out of the current terminal. Also, CTRL+D can be used. | logout | Shell Commands |
| echo | Repeats anything that is typed after it. | echo “text” echo rm -R | Shell Commands |
| history | It is used to list the log of typed commands. | history n – Lists the last n number of commands !<command_name> will list the last command typed with <command_name> | Shell Commands |
| CTRL + D | It is an end-of-file key combination and can be used to log out of any terminal. | Combination Keys | |
| CTRL + Z | This key combination is used to stop a process. Also, it can be used to run a background process. | Combination Keys | |
| CTRL + A | CTRL + E | These key combinations are used to navigate to the start and the end of a line on the command line. | Combination Keys | |
| CTRL + K | Used to delete what is in the front of the cursor. | Combination Keys | |
| CTRL + Y | Pastes the last text that was deleted. | Combination Keys | |
| CTRL + W | Cuts or deletes the entire line that has been typed. | Combination Keys | |
| Screen | This command is used to switch between multiple virtual terminals. It is mainly useful with remote login. By default, you start with one terminal. CTRL + A & C can be used to create a new virtual terminal. | Combination Keys | |
| CTRL + N | CTRL + N is used to go to the next virtual terminal | Combination Keys | |
| CTRL + P | CTRL + P is used to go to the previous virtual terminal. | Combination Keys | |
| CTRL + C | This combination key breaks/kills a process that is running on the terminal. | Combination Keys | |
| CTRL + ALT + DEL | This key combination can be used from a terminal to reboot or shutdown. Here the user does not have to be logged in. | Combination Keys | |
| man | Displays the summary of a program from an online manual. | man <program_name> man -f: Lists the details associated with the command. man -K <keyword> : searches the manual for the specified keyword. | Help |
| info | Gives a more detailed hyper-text manual on a particular command. | info <program_name> | Help |
| whatis | Describes a program in one-line. | whatis <program_name> | Help |
| apropos | It searches the database for strings. | apropos <string> | Help |
| > | Send information to a file. | cat file1 file2 > file1_and2.txt | Input/Output |
| < | Inserts information into a text file. It is generally used with other commands that are used to extract information through standard input. | Ex: tr ‘[A-Z]’ ‘[a-z]’ <filename.txt >newfilename.txt | Input/Output |
| >> | Appends information at the end of file. It creates the file if it does not exist. | Input/Output | |
| << | It is mainly used in shell scripting. It is also used with command that are used to extract information. | Input/Output | |
| 2> | Used for redirecting error output. | Input/Output | |
| | | It sends the output of one command as an input for another command. | Input/Output | |
| Tee | Sends the output of a file to the standard output. | tee -options file_name.txt | Input/Output |
| &> | Used to redirect output to a specific location | make &> /dev/null | Input/Output |
| ` ` | Used to direct the output of the second command to the first command. | 1st_command `2nd_command -options` | Command Substitution |
| $ | Used to direct the output of the second command to the first command. | 1st_command $(2nd Command) | Command Substitution |
| | | Used to direct the output of the second command to the first command. | cat command1 | command2 | Command Substitution |
| && | Runs the 2nd command only if the 1st command runs successfully. | 1stcommand && 2nd Command | Multiple Commands |
| || | Runs the 2nd command only if the 2nd command does not run successfully. | 1st Command || 2nd Command | Multiple Commands |
| ; | 2nd Command is executed after the 1st Command is executed. | 1st Command; 2nd Command | Multiple Commands |
| access | This command is used to check if a program has access to a specified file or whether the file exists or not. | access (const char *path, int mode) | File Management & Handling |
| awk | This command is a scripting language used for manipulating data and generating reports. | awk -options filename.txt | File Management & Handling |
| batch | This command is used to read commands from the standard input or a file and execute them when permitted by system load levels. | batch | File Management & Handling |
| bc | Used for command line calculator | bc -options file_name.txt | File Management & Handling |
| cd | Change Directory. Used to go to the previous directory. Can be used with the absolute path or the relative path. | cd cd pathname | File Management & Handling |
| ls | Lists files and directories. Does not list hidden files or directories | ls -options, where options can be l, a, d, F, S, R. | File Management & Handling |
| pwd | Print the current working directory | pwd | File Management & Handling |
| tree | Lists all files and directory recursively. | tree | File Management & Handling |
| find | This is used to search for files on a filesystem. The search can be customized with many options. | find / -name file | File Management & Handling |
| slocate | Lists all the files in the system that match the specified pattern | slocate string | File Management & Handling |
| whereis | It locates the source, binary and the manual page for a particular program. | whereis program_name | File Management & Handling |
| which | It only looks for the executable program | which program_name | File Management & Handling |
| mkdir | Make Directory. | mkdir Dir1 mkdir -p /home/dir1/dir2/dir3 This command creates dir1, dir2 and dir3 as it goes. | File Management & Handling |
| rm | Removes or delete a file or a directory | rm -options file_or_folder | File Management & Handling |
| rmdir | Removes an empty directory. | rmdir directory | File Management & Handling |
| mv | Used to move a file or directory to another location. It can also be used to rename a file or a directory. | mv existing_file/folder new_location | File Management & Handling |
| cp | Copy a file | cp -options file_to_be_copied new_location | File Management & Handling |
| ln | Used to create a link to a file. Links can be hard-links or symbolic links. A hard-link is a reference to a file while a symbolic-link is similar to a shortcut link. | ln target_name link_name ln -s target_name link_name | File Management & Handling |
| shred | Overwrites a file and removes it in order to make it irretrievable by the software or hardware. | shred -n 2 -z -v /dev/hda1 | File Management & Handling |
| du | It is used to display information about the file size. It can also be used on directories, which then displays the information about all the size of all the files and subdirectories. | du -options file_name/directory_name | File Management & Handling |
| file | It shows the type of a particular file. It displays the inode number, creation data/access date along with some advanced options. | file file_name | File Management & Handling |
| stat | Give detailed information about a file. | stat file | File Management & Handling |
| dd | Used to create copies of disks, perform conversions on files and very the block size while writing the file. | dd operand dd option dd inputfile_path outputfile_path | File Management & Handling |
| touch | Used to create empty files. It is also used to increase the timestamps on the file. | touch -t 09072019 filename.txt | File Management & Handling |
| split | It is used to split files into several small files. | split -options filename split -b xx – splits the file into xx bytes, split -k xx – splits the file into xx kilobytes split -m xx – splits the file into xx megabytes | File Management & Handling |
| rename | It is used to rename files as per regular expression. | rename -options file_name | File Management & Handling |
| time | It is a utility that measures the time taken by a program to execute. | time program_name options | System Information |
| /proc | The files under /proc displays system information. | cat /proc/cpuinfo – displays information about the CPU. | System Information |
| dmseg | This command is used to print the contents of the bootup messages displayed by the kernel. This is particularly useful in debugging issues. | dmseg | System Information |
| df | Displays the information about the space on the mounted file-systems. | df -options /dev/hdx | System Information |
| who | Displays information about the logged in users including their login time. | who | System Information |
| w | This command displays who are logged in into the system and the processes they are running. | w | System Information |
| users | This command prints the name of the currently logged in users. | users | System Information |
| last | This command displays the time of the logged-out users. This also displays the information when the computer was rebooted. | last | System Information |
| lastlog | Displays a list of the users and the time/day of their login. | lastlog | System Information |
| whoami | It tells the username of the logged in user. | whoami – it does not need or take any options. | System Information |
| free | Displays memory status. (Total, Used, Free, cached, Swap) | free -tm, where t displays the total statistics and m displays the space in megabytes. | System Information |
| uptime | w | It displays how long the computer has been up and running. Additionally, it displays the number of users and the processor load. | System Information | |
| uname | It is used to display system information such as OS type, kernel version, etc. | uname -options, where the options can be – a, n, m, s, r, p | System Information |
| xargs | This command is used to run a command as many times as required. | Ex: ls | xargs grep “search string” The various options that go with it are:
|
System Information |
| date | It is used to display the system. It can also be used to set the date/time. | date -s hh:mm:ss | System Information |
| cal | This command displays the calendar of the current month. | cal -y – Displays the calendar of the specific year. | System Information |
| acpi | This command is used to display the battery status and other ACPI(Advanced Configuration and Power Information) related information. | acpi -options Using different options will give different system information. | System Information |
| acpi_available | This command is used to test if the ACPI subsystem is available. | acpi-available | System Information |
| aptitude | This command opens up a highly built-in interface to interact with the package manager of the machine. | System Information | |
| proc | This command gives information about the hard- drive. | cd /proc/ide0/had | System Management |
| fdisk | It gives information on any hard drives that is connected to the system and their partitions. | fdisk | System Management |
| shutdown now | It starts the process of shut-down immediately. This command can also mean go to a single user mode. This command can also be used to shutdown a system at a particular time. | shutdown now | System Management |
| halt | This command shuts down the system immediately. | same as shutdown -h | System Management |
| reboot | This command is used to reboot the computer immediately. | same as shutdown -r | System Management |
| ps | This displays the list of processes running on the system. When used without any options, it lists the processes run by the current user in the controlling terminal. | ps -aux: lists all processes run by all users. | System Management |
| pstree | Lists all the processes in a tree structure. | pstree -p: lists all processes along with their ID’s | System Management |
| pgrep | This command is used for finding processes via keyword terms. Generally, this command returns only the PID no. It can be used to list process names or via usernames. | pgrep | System Management |
| top | This command is used to list process in the order of CPU usage. It has many additional options that go with it, which helps to get the desired result. | top | System Management |
| kill | This command is used to kill a process. It requires the pid or the id of the process. | kill pid kill %id | System Management |
| killall | This command is used to kill a process by its name. | killall -v: will make the process report if the kill was successful. killall -i – will prompt the user to confirm before attempting the kill. | System Management |
| pkill | This command is used to kill a process specified by a regular expression. | pkill process_name pkill -u: is used to kill the processes of a certain user. | System Management |
| skill | It is used to continue, kill, stop the process. The process can be specified via username, command name or process-id. | Ex: skill -stop unauthorised_username | System Management |
| jobs | It displays the list of currently running jobs in order of execution. This command is available only in bash, csh, ksh and tcsh shells. | jobs [job_name] | System Management |
| bg | It runs a process in the background. An & sign at the end of the command can also be used to run a job at the background. | bg job_number bg job_name | System Management |
| fg | This command is used to bring a process at the foreground. This will take over your current terminal. | fg job_number fg job_id | System Management |
| nice | This command is used to set the CPU priority for a process. In order to set a higher priority for a process, the user needs to be a root user. However, any user can lower the priority of a process. | nice -20 <process_name>: sets the maximum priority nice 20 <process_name>:: sets the minimum priority | System Management |
| renice | Changes the priority of an existing command. | System Management | |
| snice | Along with changing the priorities of a process, snice can also be used to stop, continue or kill a process. | snice -upctvi, where u,p,c,t,v,I are all options that make snice perform differently. | System Management |
| service | It allows to perform different tasks on services. | service -sfR, where s, f, R are all different options that can be used with it. | System Management |
| autoconf | This command is used in Linux to generate configuration scripts. | System Management | |
| su username | This command is used to switch to a different username. In order to return to the original username, you need to type exit or use CTRL+D. | su username | User Management |
| root | This is the superuser. The overall power of the system lies with this user. This includes all administrative powers as well. | User Management | |
| chsh | This command is used to change the login shell. | chsh [enter] <shell_name> to be used every time you login. | User Management |
| chfn | This command is used to change the finger information. This change is reflected in the /ect/passwd file | chfn user_name | User Management |
| passwd | This command can only be executed by the root user. It allows the user to change the password of a user. | passwd username | User Management |
| vi | It is a very powerful text editor which is known for its minimal use of resources. | Text Command | |
| emacs | It is an advanced text editor which enables a user to browse the internet, play games, chat, etc. | Text Command | |
| head | When used without any options, it shows the first 10 lines of a file. | head -n x, where x specifies the number of lines to be displayed from the top. | Text Command |
| tail | When used without any options, it is used to show the last 10 lines of a file. | tail -n x, where x specifies the number of lines to be displayed from the last. | Text Command |
| less | This command enables to view text and scroll backwards and forwards. | less <file-name> | Text Command |
| more | This command enables to view one page at a time. | Text Command | |
| cat | Used to concatenate/combine multiple files into one. | cat <file_1> <file_2> > <new_file> | Text Command |
| tac | Combines multiple files into one, however the files will be combines in reverse order. | tac <file_1> <file_2> > <new_file> | Text Command |
| z* | bz* commands | Commands can be prefixed with a z in order to allow them to work within a gzip compressed file. | Text Command | |
| wc | This returns the word count in a text file. | wc -wlc <filename>, where w returns the number of words, l returns the number of lines and c returns the number fo characters. | Text Command |
| style | This command allows to run various readability tests on a file. | style -options <file_name> | Text Command |
| cmp | This command is used to determine the differences between two files, at the binary level. | cmp -option filename1.txt filename2.txt | Text Command |
| diff | This command outputs the difference in text between two files. | diff <flie_name1> <file_name2> | Text Command |
| sdiff | This command displays the file contents in two columns. | sdiff <flie_name1> <file_name2> | Text Command |
| diff3 | This command outputs the difference in text between three files. | diff <flie_name1> <file_name2> <file_name3> | Text Command |
| comm | This command enables us to compare two files and displays the unique contents of file1, unique contents of file2 and those common to both in three separate columns. | comm <file1> <file2> Options -1, -2, -3 can be used to suppress the display of the corresponding column. | Text Command |
| look | This command enables you to display a list of words from the system dictionary that begins with a specified string. | look <string> | Text Command |
| sort | This command is used to sort text in a file. When used without any options, it sorts the text alphabetically. It is a powerful command and can be used with many options to achieve the required output. | cat result_list.txt | sort | Text Command |
| join | This command will allow joining of two lines under the consideration that both the lines have a specific value in common. | join <file_1> <file_2> | Text Command |
| cut | This command helps to remove certain sections of a line. This can be done based on tabs, commas, etc. | cut -options <file_name> | Text Command |
| ispell/aspell | This command enables a user to find detect misspelt words and replace the word or continue. | ispell <file_name.txt> aspell -c <file_name.txt > | Text Command |
| chcase/tr | This command is used to change the uppercase letters in a file to lowercase. | cat <original_filename.txt> | chcase ‘[A-Z]’ ‘[a-z]’ > newfile.txt | Text Command |
| fmt | It is a simple command for text formatting. | Text Command | |
| paste | This command can be used to put lines from two files together. It arranges the lines as per options used. | paste <file_name1> <file_name2> | Text Command |
| expand | This command will convert 2 tabs into spaces and output it. | expand <file_name.txt> | Text Command |
| unexpand | This command will convert spaces into tabs and display it, | unexpand <file_name.txt> | Text Command |
| uniq | This command is used to remove the duplicate entries from a file. | uniq <list.txt> | Text Command |
| nl | This command is used to add numbers to each line and indent the text. | nl <file_name.txt> | Text Command |
| perl | This command is used to search and replace text. | perl -pi -e “s/search_string/new-string/g;” <file_name.txt> | Text Command |
| dos2unix/ fromdos | This command is used to convert Microsoft style end-of-line characters to Unix System end-of-line characters. | dos2unix <file_name.txt> | Text Command |
| unix2dos/todos | This command is used to convert Unix System style end-of-line characters to Microsoft System end-of-line characters. | unix2dos file_name.txt | Text Command |
| antiword | This is a filter command that is used to convert Microsoft word documents into plain ASCII text documents. | antiword file_name.txt | Text Command |
| recode | This command is used to convert text files between various forms of text encodings. By default, recode overwrites the file, hence it is suggested the user to use necessary options to avoid losing the original content. | recode ..pc < file_name.txt > new_file.txt | Text Command |
| enscript | This command is used to convert text files to postscript, HTML, rich text file, etc. It goes with a number of options that helps to customize its output. | enscript - -language=HTML original_file.txt -o output_file.txt | Text Command |
| figlet | This is a unique command that can be used to convert an ASCII art. | Text Command | |
| grep | This command is used to search for a text within files. | grep ‘search_word’ file_name.txt Options like v,n,w,A x, B x and/or r can be used to customize the output. | Text Command |
| rgrep | It is recursive grep. It searches the respective subdirectories and files in the directory and displays the files names and the matching line. | grep -r option can be used for the same effect. | Text Command |
| fgrep | This command is used to find string literals only. | fgrep ‘string literal’ file_name.txt | Text Command |
| units | It helps to convert units of measurement between different scales. | units – verbose To exit – CTRL+D | Mathematical Commands |
| python | It is a programming language that also makes a good calculator. This command executes the Python interpreter in interactive mode. | python To exit: CTRL – D | Mathematical Commands |
| numgrep | It searches for different occurrences of numbers, basically ranges, factors and sequences of numbers. | numgrep /<expression>/ | Mathematical Commands |
| ifconfig | This command is used to configure network parameters. | ifconfig -options | Network Commands |
| tcpdump | It is a program that sniffs packets off network interface and interprets them. | sudo tcpdump -I any : captures all packets in any interface. | Network Commands |
| ping | It sends echo requests to the specified host and lists the responses received in their round-trip time. | ping ip_or_host_name To stop ping – CTRL + C | Network Commands |
| hostname | Displays the hostname of the | Network Commands | |
| traceroute | Shows the route of a packet. | traceroute machine_name or IP | Network Commands |
| tracepath | It si similar to traceroute, however it does not take complicated options with it. | tracepath machine_name or IP | Network Commands |
| findsmb | It lists information about machines that respond to SMB name queries | findsmb | Network Commands |
| telnet | This command helps to establish connection between two hosts. | telnet [hostname] | Network Commands |
| nmap | It is used to query machines, if they are up and which ports they are on. | nmap machine_name | Network Commands |
| ip | The ip command is used to perform network administration tasks. It can be used to set or delete ip addresses, verify changes, show routing table, change the default route, show network statistics, monitor netlink messages, activate and deactivate network interface and more. | ip a (ip address) ip (link) ip -s(ip stats) ip tunnel ip r | Network Commands |
| host | Looks up the internet address using the Domain Name Server. | host_ipaddress | host domain_name | Network Commands |
| dig | It is the domain information groper tool which outputs IP address, hostname and various other information. | dig -x <ip-address> returns the hostname dig works with various other options. | Network Commands |
| whois | Used to look up contact information | Network Commands | |
| wget | Used for downloading files from the world wide web | wget file_url: gets a file from the url wget -m: archives a single website wget -nc: to avoid overwriting and already archived file. wget -c: to continue archiving a file that was previously unfinished. | Network Commands |
| curl | It is used to download information remotely. It works without user interaction and supports many internet protocols. It can access dictionary servers, ftp, http, gopher,etc. | curl -M: this will take you to its manual page. It has a lot many options going with it for accomplishing various tasks. | Network Commands |
| ssh | It is used to securely connect to a remote server/system. It transfers data in an encrypted from between the host and the client. | ssh user_name@host(IP/domain_name) | Remote Administration |
| scp | Allows copying of one file from one computer to another. | scp machine-to-be-copied-from machine-to-be-copied-to scp -r: Used for recursive copying. | Remote Administration |
| sftp | Secure ftp. It is an encrypted version of the ftp command, which transfers securely over a network. | sftp [user@]host[:file…] sftp [user@]host[:dir[/]] sftp -b batchfile [user@]host There are many other options that goes with this command. | Remote Administration |
| md5sum | Computes an md5 checksum to for the specified file_name to verify its integrity. | md5sum file_name | Security |
| mkpassword -1 10 | Gives a random password of 10 characters | Security | |
| at | ‘at’ is used to execute a command once on a specified day, at a specified time. | at 22:00 Then type the command and press the end-of-file key. | Background Commands |
| at now+time | Runs the command at the + the time time specified. | at -f shell_script now+1hr | Background Commands |
| atq | Lists the job in the queue. It does not take any options. | Background Commands | |
| atrm | Removes a job from the at queue. | atrm job_no | Background Commands |
| cron | It is used to schedule a particular function every minute, hour, day, week or month. | anacron is a command used for machines which are switched off. It runs the command when the machine is next switched on. | Background Commands |
| crontab | It edits, reads and removes the files read by the cron daemon. | -e – edits file -l – lists the contents of the file -u username – to works with other uses crontab file. | Background Commands |
| @reboot | Runs the command at reboot | Shortcut Commands | |
| @yearly | @annually | Same as 0011* | Shortcut Commands | |
| @monthly | Same as 001** | Shortcut Commands | |
| @weekly | Same as 00**0 | Shortcut Commands | |
| @daily | @midnight | Same as 00*** | Shortcut Commands | |
| @hourly | Same as 0**** | Shortcut Commands | |
| Application | Description | Category | Category |
Conclusion
Linux is one of the most reliable operating systems. Being an open-source software it can be installed in as many computer systems for free. It works without any trouble with malware or viruses.