Master Essential Linux Commands: A Complete Cheat‑Sheet for System Operations

1. ls

-a  List all files including hidden files that start with a dot.
-A  List all files except "." and "..".
-l  Use long format to display detailed file information.
-c  Sort by ctime.
-t  Sort by modification time.
--color[=WHEN] Colorize output; WHEN can be 'never', 'always', or 'auto'.
Color legend: white – regular file, blue – directory, green – executable, red – compressed, light blue – symbolic link, blinking red – broken link, yellow – device file, gray – other.

2. mv

-b  Make a backup before overwriting.
-f  Force overwrite without prompting.
-i  Prompt before overwriting.
-u  Overwrite only if the source file is newer.
-t  Move multiple source files to a target directory (target first, then sources).
Example: mv a /tmp/   # Move file a to /tmp directory.
Example: mv a b       # Rename file a to b.
Example: mv /home/zenghao test1.txt test2.txt test3.txt

3. cp

-r -R  Recursively copy directories and their contents.
-p  Preserve file attributes.
-f  Force copy without prompting.
-s  Create a symbolic link.
-a  Preserve all file attributes (archive mode).

4. scp

-v  Verbose output.
-r  Recursively copy directories.
Usage examples:
scp local_file user@remote_ip:remote_folder
scp local_file user@remote_ip:remote_file
scp local_file remote_ip:remote_folder
scp local_file remote_ip:remote_file
When username is specified, password is required.
When only remote path is given, password and username are prompted.

5. rm

-r  Remove directories recursively.
-f  Force removal without prompts.
-i  Interactive removal (prompt before each delete).
-v  Verbose output showing each step.

6. touch

-a  Change only the access time.
-m  Change only the modification time.
-r  Example: touch -r a b   # Make b's timestamps match a's.
-t  Specify a custom timestamp (e.g., touch -t 201211142234.50 log.log).

7. pwd

Display the current working directory.

8. cd

-  Return to the previous directory.
.. Return to the parent directory.
<Enter> Return to the home directory.
/  Root directory.

9. mkdir

-p  Create parent directories as needed.
-m  Set custom permissions (e.g., mkdir -m 777 hehe).
-v  Verbose output showing directory creation.

10. rmdir

-v  Verbose output.
-p  Remove parent directories if they become empty.

11. rm (multiple files)

-f  Ignore nonexistent files, no warnings.
-i  Interactive deletion.
-r  Recursively delete directories.
-v  Verbose output.

12. echo

-n  Do not output the trailing newline.
-e  Enable interpretation of backslash escapes.
Example: echo "he
he"   # Prints he
he.
Example: echo -e "he
he"   # Prints he (newline) he.

13. cat

-n  Number all output lines.
-E  Display $ at the end of each line.

14. tac

Display file contents in reverse order.

15. more

+n  Start display at line n.
-n  Show n lines per screen.
+/String  Search for a string and start two lines before the match.
-c  Clear screen before displaying.
-p  Clear screen when paging.

16. less

-m  Show a percentage similar to more.
-N  Show line numbers.
/STRING  Search forward for STRING.
?STRING  Search backward for STRING.
n  Repeat previous search forward.
N  Repeat previous search backward.
b  Scroll back one page.
d  Scroll back half a page.

17. nl

-b  Number all lines, including empty ones (like cat -n).
-b a  Number all lines, even empty ones.
-b t  Do not number empty lines (default).
-n  Various formatting options for line numbers.

18. head

-v  Show file names.
-c number  Show the first number characters; negative number shows all but the last number characters.
-number/n  Show the first number lines; negative number shows all but the last number lines.

19. tail

-v  Verbose output.
-q  Quiet mode (no headers).
-num / -n num  Show the last num lines.
-n +num  Start displaying from line num.
-c  Show the last num characters.
-f  Follow the file (like `tail -f`).

20. vi

:w filename          Save file as filename.
:wq                  Write and quit.
:q!                  Quit without saving.
Insert mode: i, a, o.
Esc returns to command mode.
Cursor movement: h (left), j (down), k (up), l (right).
Page navigation: Ctrl+b (back), Ctrl+f (forward), Ctrl+u (half back), Ctrl+d (half forward).
0  Go to start of file.
G  Go to end of file.
$  End of current line.
^  Start of current line.
w  Jump to next word.
e  Jump to end of word.
b  Jump to previous word.
#l  Move to column # on the current line.
Delete: x (delete char after cursor), X (delete char before cursor), dd (delete line), #dd (delete # lines).
Copy: yw (yank to end of word), yy (yank line), #yy (yank # lines).
Paste: p.
Replace: r (replace single char), R (replace until Esc).
Undo: u.
Change: cw (change word), c#w (change # words).
Go to line: #G (e.g., 15G).

21. which

Locate the executable file of a command in the directories listed in $PATH.

22. whereis

-b  Locate binary.
-m  Locate manual.
-s  Locate source.
-u  Locate other files.
-B  Specify search path for binaries.
-M  Specify search path for manuals.
-S  Specify search path for sources.

23. locate

-r  Use regular expression for searching.

24. find

Time options: -atime, -ctime, -mtime, -amin, -cmin, -mmin.
User/group options: -uid n, -gid n, -user name, -group name, -nouser, -nogroup.
Name/size/type options: -name pattern, -size [+|-]SIZE, -type f/d/l/etc.
Permission options: -perm mode, -perm -mode, -perm +mode.
Actions: -exec command {}, -print (default), -print0, xargs.

25. grep

-c  Print only the count of matching lines.
-I  Case‑insensitive matching (for single‑byte characters).
-l  Print only the names of files with matches.
-v  Invert match (show non‑matching lines).
-n  Show line numbers with matches.

26. file

Determine file type.

27. gzip

-d  Decompress.
-c  Write output to standard output.
-v  Show compression ratio and other info.
-#  Set compression level (1 fastest, 9 best).

28. gunzip

Decompress gzip files.

29. bzip2

-d  Decompress.
-z  Compress.
-k  Keep original file.
-c  Write output to standard output.
-v  Verbose.
-#  Compression level (1 fastest, 9 best).

30. tar

Main options:
-c  Create archive.
-t  List archive contents.
-x  Extract archive.
Auxiliary options:
-j  Filter through bzip2.
-z  Filter through gzip.
-v  Verbose.
-f filename  Specify archive file name.
-C directory  Change to directory before extraction.
--exclude FILE  Exclude FILE from archive.
-p  Preserve permissions.
-P  Preserve absolute paths.
Examples:
 tar -cvf archive.tar files      # Create.
 tar -tvf archive.tar            # List.
 tar -xvf archive.tar            # Extract.
 tar -jcvf archive.tar.bz2 files  # Bzip2 compression.
 tar -zcvf archive.tar.gz files  # Gzip compression.

31. exit

Exit the current shell.

32. logout

Logout from the current login shell.

33. shutdown -h now

Power off the system immediately.

34. users

Display currently logged‑in users.

35. who

-H or --heading  Show column headings.

36. w

-s  Short format (no login time, terminal, CPU time).
-h  Hide column headings.

37. write

Send a message to another logged‑in user.

38. wall

Broadcast a message to all logged‑in users.

39. last

Show login history of users.

40. lastlog

Show the last login time of each user.

41. finger

-s  Show short info (login name, real name, terminal, idle time, login time).
-l  Show long info (including home directory, shell, mail status, .plan, .project, .forward).
-p  Same as -l but omit .plan and .project.

42. hostname

Display or set the system's host name.

43. alias

Define command aliases, e.g., alias ii="ls -l".

44. unalias

Remove a command alias.

45. useradd

-M  Do not create a home directory.
-m  Create a home directory.
-r  Create a system account (UID limited).
-e  Set account expiration date (YYYY‑MM‑DD).
-D  Show default values.
Other options: -u UID, -g initial‑group, -G secondary‑groups, -c comment, -d home‑dir, -s shell.

46. passwd

-l  Lock password.
-u  Unlock password.
-S  Show password file parameters.
-n days  Set minimum days between password changes.
-x days  Set maximum days before password must be changed.
-w days  Set warning period before expiration.
-i date  Set account expiration date.
Use pipe to set password: echo "newpass" | passwd --stdin username.

47. userdel

-r  Remove user's home directory and mail spool.

48. chage

-l  List password aging information.
-d YYYY‑MM‑DD  Set last password change date.
-E YYYY‑MM‑DD  Set account expiration date.
-I days  Set password inactivity period.
-m days  Set minimum password age.
-M days  Set maximum password age.
-W days  Set warning period before expiration.

49. usermod

-c comment  Change GECOS field.
-d dir      Change home directory.
-e YYYY‑MM‑DD  Set account expiration date.
-f days     Set password inactivity period.
-g gid      Change primary group.
-G groups   Set secondary groups.
-l newname  Change login name.
-s shell    Change login shell.
-u UID      Change user ID.
-L          Lock account.
-U          Unlock account.

50. id

Display user and group IDs for a given username.

51. groups

List groups the current user belongs to; the first group is the primary group.

52. newgrp

Log in to a new group.

53. groupadd

-g gid  Specify GID for the new group.

54. groupmod

-g gid  Change GID.
-n name  Change group name.

55. groupdel

Delete a group.

56. gpasswd

Root actions:
 -gpasswd groupname          Set group password.
 -gpasswd -A user1,... -M user2,... groupname  Assign administrators and members.
 -gpasswd -r groupname       Remove group password.
Group member actions:
 -gpasswd -a user groupname   Add user to group.
 -gpasswd -d user groupname   Remove user from group.

57. chfn

Change personal information (GECOS fields) for a user.

58. mount

-ro  Mount read‑only.
-rw  Mount read‑write.
Example: mount /home/mydisk.iso /tmp/mnt

59. umount

Unmount a filesystem.

60. cut

-b  Cut by byte positions.
-c  Cut by character positions.
-d delim  Use DELIM as field delimiter (default TAB).
-f  Select only these fields (used with -d).

61. sort

-n  Sort numerically.
-o file  Write output to FILE.
-r  Reverse sort order.
-t delim  Use DELIM as field separator.
-k  Sort by key (field range).

62. wc

-l  Count lines.
-c  Count bytes.
-m  Count characters.
-w  Count words.

63. uniq

-c  Prefix lines by the number of occurrences.
-d  Only print duplicate lines.
-f N  Skip N fields when comparing.
-s N  Skip N characters when comparing.
-u  Only print unique lines.
-w N  Compare only the first N characters.

64. set

Display all shell variables and functions.

65. env

Display the environment variables.

66. export

Mark a variable for export to the environment of subsequent commands.

67. unset

Remove a variable or function definition.

68. read

-p prompt  Display PROMPT before reading.
-t seconds  Timeout after SECONDS.

69. declare / typeset

-i  Declare integer.
-a  Declare array.
-f  Declare function.
-r  Declare read‑only.

70. ulimit

-f  Set maximum file size (in blocks, usually kilobytes).
Example: ulimit -f 1024  # Limit files to 1 MiB.

71. df

-a  Show all filesystems.
-h  Human‑readable sizes.
-l  Show local filesystems only.
-i  Show inode information.
-T  Show filesystem type.

72. du

-h  Human‑readable output.
-s  Show only a total for each argument.

73. ln

-s  Create a symbolic link.
-v  Verbose output.

74. diff

-b  Ignore changes in the amount of whitespace.
-B  Ignore changes whose lines are all blank.
-i  Ignore case differences.
-q  Report only when files differ.
Example: diff -u file1 file2 > patch.log

75. date

Format specifiers:
%H  Hour (00‑23).
%M  Minute (00‑59).
%P  am/pm.
%D  Date (MM/DD/YY).
%U  Week number of the year.
Set date: date -s "2015-10-17 01:01:01"
Display date: date +%Y%m%d
Relative dates: date -d "2 weeks"

76. cal

-1  Show current month.
-3  Show previous, current, and next month.
-m  Monday as first day of week.
-s  Sunday as first day (default).
-j  Show day of year.
-y  Show the whole year.

77. ps

a   Show all processes.
-a  Show processes for all users on the current terminal.
-e  Show environment.
-f  Show process hierarchy.
-H  Show tree view.
-r  Show processes for the current terminal.
-T  Show all processes for the current terminal.
-aux  Show all processes with detailed info.
-u user  Show processes for a specific user.

78. top

Interactive real‑time view of running processes.

79. kill

Terminate a process by PID.

80. free

Display memory usage (total, used, free, buffers, swap).

81. vmstat

Report virtual memory, processes, CPU activity.

82. iostat

-p [device]  Show statistics for the specified device or partition.

83. watch

-n seconds  Interval between executions.
-d  Highlight differences between updates.

84. at

Schedule a one‑time command.
Time format: HH:MM[am|pm] [+ number unit] (units: minutes|hours|days|weeks).
Commands: atq (list pending jobs), atrm n (remove job n), at -c n (show job n).

85. crontab

file   Load crontab from file.
-e      Edit current user's crontab.
-l      List current user's crontab.
-r      Remove current user's crontab.

86. ifconfig

Configure network interfaces (display, set IP, netmask, etc.).

87. route

Display or modify the IP routing table.

88. ping

-q  Quiet output (summary only).

89. netstat

Show network statistics for IP, TCP, UDP, and ICMP.

90. telnet

Remote login using plain‑text transmission (insecure).

91. rcp

-r  Recursive copy.
-p  Preserve file attributes.
Usage: rcp -r remote_host:remote_dir local_dir

92. wget

-O FILE  Write output to FILE.
--limit-rate=300k  Limit download speed.

93. awk

-F delim  Set input field separator.
{ script }  Action to perform on each line.
Example: cat /etc/passwd | awk -F ':' '{print $1"\t"$7}'

94. sed

a  Append after the current line.
 c  Change lines.
 d  Delete lines.
 i  Insert before the current line.
Example: sed '1,2c Hi' file

95. paste

-d delim  Use DELIM as the output delimiter.
-s  Paste sequentially, one file per line.

96. su

-l  Simulate a full login (change environment, directory).
-c command  Execute COMMAND as the target user and return.

97. sudo

-l  List allowed commands for the invoking user.
-u username  Run command as USERNAME.