Shell — Permissions

A progressive study of Linux file permissions: user management, ownership, file modes (symbolic and octal), directory permissions, and conditional operations.

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Learning Objectives

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Task-by-Task Reference

Each task below highlights the unique challenge it posed and the new command/flag introduced — commands from earlier tasks are not repeated.

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Task 0 — Switch User (0-iam_betty)

Challenge: Change the active user identity to betty — introducing user switching.

Approach: Use su betty to switch the current session to user betty.

New commands introduced:

Key takeaway: su (switch user) lets you assume another user's identity. Without arguments, it defaults to root. Use su - for a full login shell with the target user's environment.

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Task 1 — Current User (1-who_am_i)

Challenge: Display the effective username of the current user — introducing identity introspection.

Approach: Use whoami to print the username of the user running the script.

New commands introduced:

Key takeaway: whoami tells you who you are in the current shell — useful after su to confirm the user switch worked.

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Task 2 — Group Memberships (2-groups)

Challenge: List all groups the current user belongs to — introducing group introspection.

Approach: Use groups to print all group memberships.

New commands introduced:

Key takeaway: A user can belong to multiple groups. groups shows them all. File permissions check both the user and group ownership.

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Task 3 — Change Owner (3-new_owner)

Challenge: Transfer ownership of a file to another user — introducing chown.

Approach: Use chown betty hello to change the owner of hello to betty.

New commands introduced:

Key takeaway: Only root can change file ownership. chown affects the user owner; use chown user:group to change both.

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Task 4 — Create Empty File (4-empty)

Challenge: Create a file with no content — introducing touch.

Approach: Use touch hello to create an empty file named hello.

New commands introduced:

Key takeaway: touch is the simplest way to create an empty file. If the file already exists, touch updates its access and modification timestamps without changing content.

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Task 5 — Add Execute Permission (5-execute)

Challenge: Make a file executable by its owner — introducing chmod with symbolic notation.

Approach: Use chmod u+x hello to add execute permission for the owner.

New commands introduced:

Key takeaway: Symbolic chmod uses u (user/owner), g (group), o (others), a (all). The operators are + (add), - (remove), = (set exactly). Permissions: r (read), w (write), x (execute).

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Task 6 — Multiple Permissions (6-multiple_permissions)

Challenge: Set different permissions for different categories in one command — combining owner, group, and other permission changes.

Approach: Use chmod ug+x,o+r hello — add execute for owner and group, add read for others.

New commands introduced:

Key takeaway: chmod accepts comma-separated changes — u+x,g-w,o=r all in one command. Read from left to right; each clause applies independently.

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Task 7 — Execute for Everyone (7-everybody)

Challenge: Grant execute permission to all users — introducing the a (all) shorthand.

Approach: Use chmod a+x hello or equivalently chmod +x hello to add execute for owner, group, and others.

New commands introduced:

Key takeaway: a (all) is the default when no category is specified. chmod +x is the most common way to make a script executable.

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Task 8 — James Bond Permissions (8-James_Bond)

Challenge: Set permissions to 007 — owner and group have no permissions, others have full access — introducing octal notation.

Approach: Use chmod 007 hello. Octal 007 means: owner=0 (---), group=0 (---), others=7 (rwx).

New commands introduced:

Key takeaway: Octal (numeric) chmod is precise and absolute — it sets permissions exactly, not relative to existing ones. Three digits: owner, group, others. Each digit is the sum of r(4) + w(2) + x(1).

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Task 9 — John Doe Permissions (9-John_Doe)

Challenge: Set a file's mode to rwxr-x-wx — reinforcing octal notation with a specific permission pattern.

Approach: Use chmod 753 hello. Octal 753 = owner:7 (rwx), group:5 (r-x), others:3 (-wx).

New commands introduced:

Key takeaway: Octal permissions are a compact way to express any permission combination. Common patterns: 755 (rwxr-xr-x) for executables, 644 (rw-r--r--) for data files, 600 (rw-------) for private files.

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Task 10 — Mirror Permissions (10-mirror_permissions)

Challenge: Copy the exact permissions from one file to another — introducing --reference.

Approach: Use chmod --reference=olleh hello to set hello's mode to match olleh's.

New commands introduced:

Key takeaway: --reference clones permissions without needing to know or type the octal value. Useful for enforcing consistent permissions across related files.

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Task 11 — Directory Execute Permissions (11-directories_permissions)

Challenge: Add execute permission to all subdirectories (not files) — introducing directory-specific operations.

Approach: Use find or chmod with */ glob: chmod a+x */ adds execute to all subdirectories in the current directory.

New commands introduced:

Key takeaway: Execute permission on directories controls traversal (cd), not execution. Without x on a directory, you cannot access files inside it, even if you have read on those files.

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Task 12 — Create Directory with Mode (12-directory_permissions)

Challenge: Create a directory with specific permissions in one step — introducing mkdir -m.

Approach: Use mkdir -m 751 my_dir to create my_dir with mode 751 (rwxr-x--x).

New commands introduced:

Key takeaway: mkdir -m sets permissions at creation time, avoiding the tiny window where a directory exists with default (usually 755) permissions before chmod runs.

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Task 13 — Change Group (13-change_group)

Challenge: Change the group ownership of a file — introducing chgrp.

Approach: Use chgrp school hello to set the group owner to school.

New commands introduced:

Key takeaway: chgrp changes only the group. Use chown :group as an alternative syntax. A user can change group ownership to any group they belong to.

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Task 14 — Change Owner and Group (14-change_owner_and_group)

Challenge: Change both owner and group for all files in the current directory — using a single chown command.

Approach: Use chown vincent:staff * — the user:group syntax changes both simultaneously. The * wildcard applies to all files and directories.

New commands introduced:

Key takeaway: chown user:group combines owner and group changes. The wildcard * applies the command to everything in the current directory (but not hidden files unless .* is used).

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Task 15 — Symbolic Link Ownership (15-symbolic_link_permissions)

Challenge: Change the owner of a symbolic link itself (not its target) — introducing chown -h.

Approach: Use chown -h vincent:staff _hello — the -h flag affects the symlink, not the file it points to.

New commands introduced:

Key takeaway: By default, chown dereferences symlinks and changes the target's owner. -h (or --no-dereference) operates on the link itself. Same behavior applies to chmod and chgrp.

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Task 16 — Conditional Ownership (16-if_only)

Challenge: Change a file's owner only if it's currently owned by a specific user — introducing conditional chown.

Approach: Use chown --from=guillaume vincent hello — only changes owner from guillaume to vincent if the current owner matches.

New commands introduced:

Key takeaway: --from makes chown conditional — a safety guard against changing ownership of the wrong file. If the current owner doesn't match, the command does nothing (no error).

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Technique Inventory

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Resources

• chmod — Linux Manual
• chown — Linux Manual
• Linux File Permissions Explained
• Symbolic vs Octal chmod