Master Linux Shell Commands and Permissions: A Practical Guide

Understanding the Linux Shell

The Linux kernel cannot be accessed directly by users; instead, a shell (command interpreter) such as bash acts as an intermediary, translating user commands for the kernel and returning results. This is analogous to using a graphical interface on Windows to interact with the OS kernel.

Shells are a generic term for command‑line interpreters; bash is one specific implementation.

Linux User Types and Switching

Linux defines two user categories: superuser (root) , who can perform any operation, and regular users , who have limited privileges. The command prompt reflects the user type ( # for root, $ for regular users).

To switch users, use the su command: su [username] Examples:

Switch from root to a regular user: su user Switch to root (password prompt may be omitted): su or

su -

File Permission Basics

Permissions determine whether a user or process can read ( r), write ( w), or execute ( x) a file or directory. Permissions are assigned to three classes:

Owner (u)

Group (g)

Others (o)

File types are indicated by the first character in ls -l output: d – directory - – regular file l – symbolic link b – block device p – FIFO/pipe c – character device s – socket

Symbolic Permission Representation

rwx   → read, write, execute

Examples: r-- – read‑only -wx – write and execute only --- – no permissions

Octal Permission Representation

r = 4, w = 2, x = 1

Combine values for each class (owner, group, others). For instance, 755 means:

Owner: rwx (7)

Group: r-x (5)

Others: r-x (5)

Changing Permissions

The chmod command modifies file permissions:

# Numeric mode

chmod 777 file.txt   # rwx for everyone

# Symbolic mode

chmod u+x,g-w,o=r file.txt   # add execute for owner, remove write for group, set read for others

Common options: -R – apply changes recursively to directories

Only the file owner or root can change a file’s permissions.

Changing Ownership

Use chown to change the file owner and optionally the group:

sudo chown root file.c               # change owner to root

sudo chown user:group file.c          # change owner and group

Changing Group

Use chgrp to modify the group association:

sudo chgrp developers project/

Umask – Default Permission Mask

The umask value masks out permission bits when new files or directories are created. Default values are 0022 for root and 0002 for regular users.

umask 022   # results in default 755 for directories, 644 for files

Directory Permissions and Sticky Bit

Directory access requires:

Read (r) – list contents

Write (w) – create or delete entries

Execute (x) – enter the directory

The sticky bit ( chmod +t) restricts deletion within a directory: only the file’s owner or root can delete a file, even if others have write permission on the directory.

chmod +t /tmp   # enable sticky bit on /tmp

Typical use: shared temporary directories where users may create files but cannot remove each other’s files.

Summary

Shells translate user commands for the kernel; bash is a common shell.

Linux distinguishes superuser and regular users; su switches contexts.

Permissions are expressed symbolically (rwx) and numerically (octal); they apply to owner, group, and others.

Use chmod, chown, chgrp, and umask to manage access.

Directory execute permission ( x) is required to cd into it; read permission ( r) allows listing; write permission ( w) allows creating or deleting entries.

The sticky bit ( +t) protects files in shared directories from being removed by non‑owners.