Day 3

File Viewing Commands

41. more filename

Purpose: Display file content page by page (forward only navigation). Usage:

• Space bar = scroll down one page

• Enter = scroll down one line

• q = quit

• b = scroll back (limited) Output: File displayed in chunks Example: more /var/log/syslog - view system log page by page Limitation: Cannot scroll backward efficiently; use less for better control

42. less filename

Purpose: Display file content with better navigation (backward and forward). Usage:

• Space bar / Page Down = scroll down one page

• Page Up / b = scroll up one page

• Arrow keys = line-by-line navigation

• /pattern = search forward for pattern

• ?pattern = search backward for pattern

• n = next search match

• N = previous search match

• g = go to beginning of file

• G = go to end of file

• q = quit Output: File displayed with flexible navigation Example: less large_logfile.log - navigate through large files efficiently Advantage: More powerful than more; allows backward scrolling and search

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System Information Commands

62. df -h .

Purpose: Show disk usage of current directory's filesystem in human-readable format. Breakdown:

• df = Disk free/usage

• -h = Human-readable (K, M, G, T)

• . = Current directory Output Columns: Filesystem, Size, Used, Available, Use%, Mounted on Example Output:

textFilesystem      Size  Used Avail Use%
/dev/sda1       100G   60G   40G  60%

Use Case: Check available space before downloading/installing software

63. free -g or free -m

Purpose: Display memory usage in gigabytes (-g) or megabytes (-m). Breakdown:

• free = Show free and used memory

• -g = Display in gigabytes

• -m = Display in megabytes Output Columns: Total, Used, Free, Shared, Buffer/Cache, Available Example with -g:

text              total   used   free
Mem:             16      8      8
Swap:             4      0      4

Example with -m:

text              total    used    free
Mem:           16384    8192    8192
Swap:            4096       0    4096

Use Case: Monitor available RAM before running memory-intensive applications

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File Sorting and Deduplication

64. uniq file

Purpose: Remove adjacent duplicate lines from a file. Important: File must be sorted first or adjacent duplicates won't be detected. Output: Lines with adjacent duplicates removed Example:

textInput:
apple
apple
banana
banana
cherry

Output (uniq file):
apple
banana
cherry

Limitation: Only removes consecutive duplicates Common Pattern: sort file | uniq to remove all duplicates

65. sort file

Purpose: Sort lines in a file alphabetically (default). Output: File contents sorted line by line Example:

textInput:
zebra
apple
banana

Output:
apple
banana
zebra

Flags:

• -n = Numeric sort

• -r = Reverse sort

• -k 2 = Sort by column 2

• -u = Sort and remove duplicates

66. sort file|uniq

Purpose: Sort file contents and remove all duplicate lines (including non-adjacent). Breakdown: sort file → sorts entire file → uniq → removes adjacent duplicates (which are all duplicates after sort) Output: Sorted unique lines only Example:

textInput:
banana
apple
banana
cherry
apple

Output:
apple
banana
cherry

Use Case: Get list of unique values from a file (e.g., unique IPs from logs)

67. sort -r file|uniq

Purpose: Sort file in reverse order and remove duplicate lines. Breakdown:

• sort -r file = Sort in reverse (Z to A)

• uniq = Remove adjacent duplicates Output: Sorted unique lines in reverse order Example:

textInput:
banana
apple
banana
cherry
apple

Output:
cherry
banana
apple

Use Case: List unique items in descending alphabetical order

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File Permissions (chmod)

Understanding Permission Notation

Permission Symbols:

• r (read) = 4

• w (write) = 2

• x (execute) = 1

Three Permission Sets (owner, group, others):

• 7 = rwx (4+2+1)

• 6 = rw- (4+2)

• 5 = r-x (4+1)

• 4 = r-- (4)

• 3 = -wx (2+1)

• 2 = -w- (2)

• 1 = --x (1)

• 0 = --- (0)

Examples of Permission Calculations:

textrwx rwx rwx = 7 7 7 (full permissions for all)
rw- r-- r-- = 6 4 4 (owner reads/writes, group/others read only)
rwx r-x r-- = 7 5 4 (owner full, group execute, others read)
rw- --- --- = 6 0 0 (owner only can read/write)

68. chmod 777 filename

Purpose: Set read, write, execute permissions for owner, group, and others on a file. Breakdown: 7 7 7 = rwx rwx rwx Effect: Everyone can read, write, and execute the file Security Risk: Avoid in production; too permissive Example: chmod 777 script.sh - makes script fully accessible Use Case: Temporary development permissions only

69. chmod 777 directory

Purpose: Set read, write, execute permissions for owner, group, and others on a directory. Breakdown: 7 7 7 = rwx rwx rwx Effect: Everyone can read, write, and enter the directory Warning: Dangerous for system directories Example: chmod 777 temp_folder/ Use Case: Temporary shared directories only

70. chmod 644 directory1 directory2 directory3

Purpose: Set read/write for owner, read-only for group/others on multiple directories. Breakdown: 6 4 4 = rw- r-- r-- Effect: Owner can read/write, group and others can only read Multiple targets: Applies to all three directories Example: chmod 644 config1 config2 config3 Use Case: Configuration files accessible for reading but not modification

71. chmod 755 file1 file2 file3

Purpose: Set read/write/execute for owner, read/execute for group/others on multiple files. Breakdown: 7 5 5 = rwx r-x r-x Effect: Owner full access, group/others can read and execute Multiple targets: Applies to all three files Example: chmod 755 script1.sh script2.sh script3.sh Use Case: Standard permissions for executable scripts and public programs

72. chmod -R 777 folder1

Purpose: Recursively set read, write, execute permissions for folder and ALL contents. Breakdown:

• -R = Recursive (applies to directory and all subdirectories/files)

• 777 = rwx rwx rwx Effect: All files and subdirectories get full permissions Warning: Very dangerous; can expose system security Example: chmod -R 777 /var/www/html/ (not recommended) Use Case: Development environments only, never in production

73. chmod a+w file

Purpose: Add write permission for all (owner, group, others). Breakdown:

• a = all (owner, group, others)

• + = add permission

• w = write Effect: Everyone gets write permission added (even if they had none before) Example: chmod a+w document.txt - makes file writable by everyone Before: rw- r-- r-- (644) After: rw- rw- rw- (666)

74. chmod a+rwx folder

Purpose: Add read, write, execute permissions for all users on a folder. Breakdown:

• a = all

• + = add permission

• rwx = read, write, execute Effect: Everyone gets all permissions on the folder Example: chmod a+rwx shared_folder/ Use Case: Shared development folders (not production)

75. chmod u+rw file

Purpose: Add read and write permissions for owner (user) only. Breakdown:

• u = user/owner

• + = add permission

• rw = read and write Effect: Owner gets read and write (execute not added) Example: chmod u+rw important_file.txt Use Case: Restore write permission on protected files

76. chmod g+w director

Purpose: Add write permission for group on a directory. Breakdown:

• g = group

• + = add permission

• w = write Effect: Group members can now modify directory contents Example: chmod g+w project_folder/ Use Case: Allow team members to edit shared folders

77. chmod o+r file

Purpose: Add read permission for others on a file. Breakdown:

• o = others (everyone except owner and group)

• + = add permission

• r = read Effect: Others can now read the file Example: chmod o+r public_document.txt Use Case: Make file readable to public/all users

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Umask (Default Permission Mask)

Umask determines default permissions for new files/directories. It SUBTRACTS from 777 (directories) or 666 (files).

78. umask 641

Purpose: Set default permission mask to 641. Effect: New files/dirs subtract 641 from defaults Breakdown (subtracting from 777):

• 777 - 641 = 136

• Owner: 7 - 6 = 1 (--x)

• Group: 7 - 4 = 3 (-wx)

• Others: 7 - 1 = 6 (rw-) Result for new directories: --x -wx rw- Example: umask 641

79. umask 777

Purpose: Set most restrictive umask (removes all permissions by default). Effect: 777 - 777 = 000 Result: New files/directories have NO permissions (--- --- ---) Example: umask 777 Use Case: Highly secure environment requiring explicit permission grants

80. umask 000

Purpose: Set most permissive umask (no permission restrictions). Effect: 777 - 000 = 777 Result: New files/directories have FULL permissions (rwx rwx rwx) Example: umask 000 Use Case: Development/testing environments only Security: Not recommended for production

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File Deletion

81. rm filename

Purpose: Delete a single file permanently. Warning: Deletion is immediate and irreversible Example: rm old_file.txt Safe option: Use -i flag for interactive confirmation: rm -i filename

82. rm -rf directory_name

Purpose: Recursively delete directory and ALL contents (files and subdirectories). Breakdown:

• -r = Recursive (delete directory and contents)

• -f = Force (don't prompt for confirmation) Warning: EXTREMELY DANGEROUS; no recovery possible Example: rm -rf temp_folder/ Safer alternative: rm -ri directory_name (interactive, asks for confirmation) Common mistake: sudo rm -rf / (system-destroying command)

83. rm f1 f2 f3

Purpose: Delete multiple files at once. Output: All three files deleted Example: rm file1.txt file2.txt file3.txt Safe option: rm -i f1 f2 f3 (prompts before each deletion)

84. rm -rf folder1 folder2 folder3

Purpose: Recursively delete multiple directories and all contents. Breakdown:

• -r = Recursive

• -f = Force Effect: All three folders and everything inside deleted Example: rm -rf backup1/ backup2/ backup3/ Warning: Very destructive; use with extreme caution

85. rm director/dir1/file

Purpose: Delete a file located in a subdirectory. Output: File deleted from nested path Example: rm /home/user/Documents/old_file.txt Safe option: rm -i director/dir1/file

86. rm -rf /dir1/dir2/dir3

Purpose: Recursively delete directory tree starting from dir1. Breakdown:

• -r = Recursive

• -f = Force (no prompts)

• /dir1/dir2/dir3 = Full or relative path Effect: Entire directory structure deleted Example: rm -rf /home/user/old_project/ Critical Warning: Using / (root) at start is extremely dangerous

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File Moving (Rename)

87. mv old_name New_name

Purpose: Rename a file or directory. Output: File/directory renamed in same location Example: mv report.txt report_final.txt Note: Same as "moving" within same directory

88. mv file dir1/file

Purpose: Move file to dir1 (can also rename in process). Output: File relocated to dir1 Example: mv document.txt /home/user/Documents/document.txt Syntax: mv [source] [destination]

89. mv dir2/f1 .

Purpose: Move file f1 from dir2 to current directory (.). Output: f1 relocated to current directory Example: mv backup/data.zip . (moves data.zip to current folder) Syntax: . represents current directory

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File Copying

90. cp file file2

Purpose: Copy file to create file2 (duplicate). Output: file and file2 both exist with same content Example: cp original.txt backup.txt Note: Original file unchanged

91. cp file dir1/dir2/file

Purpose: Copy file to nested directory structure (may need to create dirs first). Output: Copy placed in dir1/dir2/ Example: cp config.conf /etc/app/config.conf Note: Target path must exist

92. cp -i file file2

Purpose: Copy with interactive prompt if file2 already exists. Breakdown: -i = Interactive (prompts before overwriting) Output: Prompts user before overwriting Example: cp -i important.txt important.txt.backup Safe: Prevents accidental overwrites

93. cp -i test dir1/test

Purpose: Copy file 'test' to dir1 with interactive confirmation if target exists. Breakdown: -i = Interactive prompt Output: Copies to dir1, prompts if file exists Example: cp -i config config/config.old

94. cp -r dir2 dir3

Purpose: Recursively copy entire directory dir2 as dir3. Breakdown: -r = Recursive (copy directory and all contents) Output: New directory dir3 created with all contents from dir2 Example: cp -r /var/www/html /var/www/html_backup Use Case: Backing up entire directory structures

95. cp -r dir1 dir2/dir3/dir4

Purpose: Copy directory dir1 into dir2/dir3/dir4/. Output: dir1 becomes a subdirectory inside dir4 Example: cp -r project project_archive/2024/Q1/ Note: dir2/dir3/dir4 must exist

96. cp -i dir1/file dir2/dir3/dir4/

Purpose: Copy file from dir1 into nested dir with interactive prompt. Breakdown:

• -i = Interactive (prompt if exists)

• dir2/dir3/dir4/ = Destination directory (note the trailing /) Output: File copied with confirmation if target exists Example: cp -i /etc/config /backup/configs/old/

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File Listing with Wildcards

97. ls Devops*

Purpose: List all files/directories starting with "Devops". Output: Files matching pattern Example Output:

textDevopsClass
DevopsGuide.txt
DevopsProject

Pattern: * = wildcard matching any characters after "Devops"

98. ls test*

Purpose: List all files/directories starting with "test". Output: Files matching pattern Example Output:

texttest.txt
test_file.log
test_script.sh
testing.md

Pattern: * matches zero or more characters

99. ls *

Purpose: List all files and directories in current directory. Output: All items in current folder Example Output:

textfile1.txt
file2.txt
folder1
folder2
README.md

Note: Same as ls without arguments

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Grep with Regex Anchors

Anchors match specific positions in a line:

• ^ = Start of line

• $ = End of line

100. grep -i "^Devops" file_name

Purpose: Find lines STARTING with "Devops" (case-insensitive). Breakdown:

• -i = Case-insensitive (matches "Devops", "DEVOPS", "devops")

• ^Devops = Must start with "Devops" Output: Lines beginning with "Devops" Example Output:

textDevops is important
DEVOPS class 13
DevopsTeam meeting

Won't match: "Learn Devops here" (doesn't start with Devops)

101. grep -i "^t" file_name

Purpose: Find lines STARTING with "t" (case-insensitive). Breakdown:

• -i = Case-insensitive (T or t)

• ^t = Must start with "t" Output: Lines beginning with "t" or "T" Example Output:

texttest case
The beginning
training module

Won't match: "at the start" (doesn't start with t)

102. grep -i "dev$" file_name

Purpose: Find lines ENDING with "dev" (case-insensitive). Breakdown:

• -i = Case-insensitive (matches "dev", "DEV", "Dev")

• dev$ = Must end with "dev" Output: Lines ending with "dev" Example Output:

textI am a dev
Full stack dev
DevOps dev

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Summary Table

Command	Purpose	Key Option
more	Page through file (forward)	Space to scroll
less	Navigate file (bidirectional)	/pattern to search
df -h	Disk space usage	. for current
free	Memory usage	-g or -m
sort	Sort lines	-r for reverse
uniq	Remove duplicates	Works after sort
chmod	Change permissions	-R for recursive
umask	Default permissions	Subtracts from 777
rm	Delete files	-rf for recursive
mv	Move/rename	Works in same operation
cp	Copy files	-r for directories
grep ^pattern	Lines starting with	$ for end anchors