Essential Linux Commands and Options: A Comprehensive Guide

1. ls [options] [directory]

-a   List all files, including hidden ones starting with '.'
-A   List all files except '.' and '..'
-l   Use a long listing format
-c   Sort by ctime
-t   Sort by modification time
--color[=WHEN]   Colorize output (WHEN can be 'never', 'always', or 'auto')

2. mv [options] source target

-b   Make a backup before overwriting
-f   Force overwrite without prompting
-i   Prompt before overwriting
-u   Overwrite only if the source is newer
-t   Move multiple sources into a target directory (target first, then sources)

3. cp [options] source target

-r -R   Copy directories recursively
-p   Preserve file attributes
-f   Force copy without prompting
-s   Create a symbolic link instead of copying
-a   Preserve all file attributes (equivalent to -dpR)

4. scp [options] source target

-v   Verbose output
-r   Recursively copy directories
Examples:
scp local_file user@remote_ip:/remote_folder
scp -r local_folder user@remote_ip:/remote_folder

5. rm [options] file

-r   Remove directories recursively
-f   Force removal without prompts
-i   Interactive prompting before removal
-v   Verbose output

6. touch [options] file

-a   Change only the access time
-m   Change only the modification time
-r   Use the timestamps from another file
-t   Specify an explicit timestamp (e.g., 201211142234.50)

7. pwd

Display the current working directory.

8. cd

-   Return to the previous directory
..  Return to the parent directory
~   Return to the home directory
/   Root directory

9. mkdir [options] directory

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

10. rmdir

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

11. rm [options] files…

-f   Ignore nonexistent files, never prompt
-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
Examples:
echo "hello"
echo -e "line1
line2"

13. cat [options] file…

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

14. tac

Display file contents in reverse order.

15. more

+n   Start displaying at line n
-n   Show n lines per screen
+/string   Search forward for string
-c   Clear screen before display
-p   Pause with a clear screen between pages

16. less

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

17. nl [options] file…

-b   Number non-blank lines (default)
-b a Number all lines, including blank ones
-b t Number only non‑blank lines
-n   Control number format (left, right, or zero‑filled)

18. head [options] file…

-v   Show file names
-c   Show the first *number* bytes (negative shows all but last *number* bytes)
-n   Show the first *number* lines (negative shows all but last *number* lines)

19. tail [options] file…

-v   Verbose output
-q   Quiet mode (no headers)
-n   Show the last *number* lines
+N   Start output at line *N*
-c   Show the last *number* bytes
-f   Follow the file as it grows

20. vi

:w filename      Save file as *filename*
:wq              Write and quit
:q!              Quit without saving
Insert mode: i, a, o
Command mode: ESC
Navigation: h j k l, Ctrl+b/f/u/d, G, $, ^
Delete: x, X, dd, #dd
Copy/Yank: yw, yy, #yy
Paste: p
Replace: r, R
Undo: u

21. which

Locate the executable file in the directories listed in $PATH.

22. whereis

-b   Locate binaries
-m   Locate manual pages
-s   Locate source files
-u   Locate other files
-B   Search specific binary path
-M   Search specific manual path
-S   Search specific source path

23. locate

-r   Use regular expressions for searching

24. find [path] [options] [actions]

-atime, -ctime, -mtime, -amin, -cmin, -mmin   Find files based on time criteria
-uid n   Find files owned by user ID n
-gid n   Find files owned by group ID n
-name pattern   Match file name pattern
-size [+|-]SIZE   Find files larger or smaller than SIZE (c, k, M, G)
-type f|d|l|b|c|p|s   Filter by file type
-perm mode   Exact permission match
-perm -mode   At least the given permissions
-perm +mode   Any of the given permissions
-exec command {} \;   Execute *command* on each matched file

25. grep ‘pattern’ file

-c   Print only a count of matching lines
-i   Case‑insensitive matching
-l   Print only the names of matching files
-v   Invert match (show non‑matching lines)
-n   Prefix each line with its line number

25. file

Determine file type.

26. gzip [-cdtv#] file

-d   Decompress
-c   Write output to stdout
-v   Verbose (show compression ratio)
-#   Set compression level (1‑fastest, 9‑best)

27. gunzip

Decompress .gz files.

28. bzip2

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

29. bzcat

Read bzip2‑compressed files without decompressing to disk.

30. tar [options] archive [files]

-c   Create archive
-t   List archive contents
-x   Extract archive
-j   Filter through bzip2
-z   Filter through gzip
-v   Verbose
-f   Specify archive file name
-C   Change to directory before operation
--exclude FILE   Exclude FILE from archive
-p   Preserve permissions
-P   Preserve absolute paths

31. exit

Exit the current shell.

32. logout

Log out of a login shell.

33. shutdown -h now

Power off the system immediately.

34. users

Show currently logged‑in users.

35. who

-H   Show column headings

36. w

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

40. lastlog

Show the most recent login of each user.

41. finger [options] [user]

-s   Show short info (login name, real name, terminal, idle time, login time)
-l   Show long info (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 ii="ls -l"

Create an alias.

44. unalias ii

Remove an alias.

45. useradd [options] username

-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 for useradd

46. passwd

-l   Lock the password
-u   Unlock the password
-S   Show password file entry
-n days   Set minimum days between password changes
-x days   Set maximum days before password must be changed
-w days   Set warning days before expiration
-i date   Set account expiration date

47. userdel

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

48. chage [options] username

-l   List password aging information
-d date   Set last password change date (YYYY‑MM‑DD)
-E date   Set account expiration date (YYYY‑MM‑DD)
-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 [options] username

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

50. id [username]

Display user and group IDs.

51. groups

Show groups the current user belongs to.

52. newgrp

Change the current group ID.

53. groupadd [-g gid] groupname

-g gid   Specify GID for the new group

54. groupmod [-g gid] [-n newname] groupname

-g gid   Change GID
-n name  Change group name

55. groupdel groupname

Delete a group.

56. gpasswd

Root actions:
  gpasswd -A user1,... group   Set group administrators
  gpasswd -M user1,... group   Add members to group
  gpasswd -r group             Remove group password
User actions:
  gpasswd -a user group        Add user to group
  gpasswd -d user group        Remove user from group

57. chfn

Change user personal information.

58. mount [-t type] [-o options] device dir

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

59. umount

Unmount a filesystem.

60. cut

-b   Select bytes
-c   Select characters
-d delim   Specify delimiter (default tab)
-f   Select fields

61. sort

-n   Sort numerically
-o file   Write output to *file*
-r   Reverse sort order
-t delim   Use *delim* as field separator
-k n,m   Sort by key from field n to m

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 shell variables and functions.

65. env

Display environment variables.

66. export

Mark a variable for export to the environment.

67. unset

Remove a variable or function.

68. read

-p prompt   Display *prompt* before reading
-t seconds   Timeout after *seconds*

69. declare / typeset

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

70. ulimit

-f   Set maximum file size (in blocks)
Example: ulimit -f 1024   # limit files to 1 MiB

71. df [options] [file]

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

72. du [options] [file]

-h   Human‑readable output
-s   Show only total size

73. ln [options] source target

-s   Create symbolic link
-v   Verbose output

74. diff [options] file1 file2

-b   Ignore changes in the amount of whitespace
-B   Ignore blank lines
-i   Ignore case differences
-q   Report only when files differ

75. date [options] [+format]

%H   Hour (00‑23)
%M   Minute (00‑59)
%P   am/pm
%D   Date (YYYY‑MM‑DD)
%U   Week number of the year
Set date: date -s "2015-10-17 01:01:01"
Show date: date +%Y%m%d

76. cal [options] [month] [year]

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

77. ps [options]

a   Show all processes
-u   Show processes for a specific user
-x   Show processes without a controlling terminal
-H   Show process hierarchy tree
-f   Full‑format listing

78. top [options]

Interactive display of running processes with CPU and memory usage.

79. kill [options] pid

Send signals to terminate processes.

80. free [options]

Display total, used, and free memory and swap.

81. vmstat

Report virtual memory, processes, and CPU activity.

82. iostat [options] [interval] [count]

-p [device]   Show statistics for the specified device(s)

83. watch [options] command

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

84. at [time] command

Schedule a one‑time command.
Examples:
  at 14:30   # run at 2:30 PM today
  at now + 1 hour   # run in one hour
Use atq to list pending jobs, atrm to remove.

85. crontab

-e   Edit current user's crontab
-l   List current user's crontab
-r   Remove current user's crontab

86. ifconfig [interface] [options]

Display or configure network interfaces.

87. route

Display or modify the IP routing table.

88. ping [options] host

-q   Quiet output, only summary

89. netstat

Show network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.

90. telnet [options] host

Remote login using plain‑text protocol (insecure).

91. rcp [options] source target

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

92. wget [options] URL

-O FILE   Write output to *FILE*
--limit-rate=300k   Limit download speed to 300 KB/s

93. awk

-F delim   Set field separator
{ script }   Execute *script* on each line
Example: awk -F ':' '{print $1,$7}' /etc/passwd

94. sed

a   Append text after a line
c   Change (replace) selected lines
d   Delete lines
i   Insert text before a line

95. paste

-d delim   Use *delim* as field delimiter (default tab)
-s   Paste one file per line

96. su [options] [user]

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

97. sudo

-l   List allowed (and forbidden) commands for the invoking user
-u username   Run command as *username*

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