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Day 3 of 5
⏱ ~60 minutes
Linux in 5 Days — Day 3

Permissions and Users

Understand rwx permissions, change them with chmod, change ownership with chown, and understand sudo.

Reading Permissions

ls -la output
-rwxr-xr--  1 bo  staff  1234 Apr 10 script.sh
│└──┬──┘└──┬──┘└──┬──┘
│   │      │      └─── others: read only (r--)
│   │      └────────── group: read+execute (r-x)
│   └───────────────── owner: read+write+execute (rwx)
└───────────────────── file type: - = file, d = directory
chmod: Change Permissions
# Symbolic mode
chmod +x script.sh     # add execute for everyone
chmod -w file.txt      # remove write for everyone
chmod u+x script.sh    # add execute for owner only
chmod go-w file.txt    # remove write from group and others

# Octal mode (more common in practice)
chmod 755 script.sh    # rwxr-xr-x (owner all, others read/exec)
chmod 644 file.txt     # rw-r--r-- (owner rw, others read)
chmod 600 secret.key   # rw------- (owner only)
chmod 700 private_dir/ # rwx------ (owner only)
chown and sudo
# Change owner
chown bo file.txt
chown bo:staff file.txt  # owner:group
chown -R bo ./mydir/     # recursive

# sudo: run as root
sudo apt install nginx
sudo systemctl restart nginx
sudo nano /etc/nginx/nginx.conf

# Switch to root (avoid where possible)
sudo -i

# See who has sudo access
cat /etc/sudoers
💡
Permissions 755 for directories, 644 for files, 600 for secrets. This is the pattern you'll use 90% of the time. 755 = owner can do anything, everyone else can read and traverse. 600 = owner only, no one else.
📝 Day 3 Exercise
Fix Permission Problems
  1. C
  2. r
  3. e
  4. a
  5. t
  6. e
  8. a
  10. s
  11. c
  12. r
  13. i
  14. p
  15. t
  16. .
  17. s
  18. h
  19. .
  21. T
  22. r
  23. y
  25. r
  26. u
  27. n
  28. n
  29. i
  30. n
  31. g
  33. i
  34. t
  36. w
  37. i
  38. t
  39. h
  40. o
  41. u
  42. t
  44. e
  45. x
  46. e
  47. c
  48. u
  49. t
  50. e
  52. p
  53. e
  54. r
  55. m
  56. i
  57. s
  58. s
  59. i
  60. o
  61. n
  62. .
  64. A
  65. d
  66. d
  68. e
  69. x
  70. e
  71. c
  72. u
  73. t
  74. e
  76. p
  77. e
  78. r
  79. m
  80. i
  81. s
  82. s
  83. i
  84. o
  85. n
  86. .
  88. C
  89. r
  90. e
  91. a
  92. t
  93. e
  95. a
  97. f
  98. i
  99. l
  100. e
  102. r
  103. e
  104. a
  105. d
  106. a
  107. b
  108. l
  109. e
  111. o
  112. n
  113. l
  114. y
  116. b
  117. y
  119. o
  120. w
  121. n
  122. e
  123. r
  124. .
  126. V
  127. e
  128. r
  129. i
  130. f
  131. y
  133. o
  134. t
  135. h
  136. e
  137. r
  138. s
  140. c
  141. a
  142. n
  143. '
  144. t
  146. r
  147. e
  148. a
  149. d
  151. i
  152. t
  153. .
  155. P
  156. r
  157. a
  158. c
  159. t
  160. i
  161. c
  162. e
  164. c
  165. h
  166. o
  167. w
  168. n
  170. t
  171. o
  173. c
  174. h
  175. a
  176. n
  177. g
  178. e
  180. o
  181. w
  182. n
  183. e
  184. r
  185. s
  186. h
  187. i
  188. p
  189. .

Day 3 Summary

  • Permissions: read (4), write (2), execute (1). Three sets: owner, group, others.
  • chmod 755 = rwxr-xr-x. chmod 644 = rw-r--r--. chmod 600 = private.
  • sudo runs one command as root. sudo -i opens a root shell.
  • SSH keys should be 600. Directories in .ssh should be 700. Git will complain otherwise.
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Day 2: Files: Reading, Writing, Searching
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Day 4: Processes and System Monitoring
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