Guide Linux Intermediate

Permissions & User Commands

chmod, chown, useradd, usermod, sudo, and the everyday commands for managing file permissions, ownership, and user accounts.

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linuxcommandspermissionsusers

Permissions: chmod

chmod 644 file.txt          # owner rw-, group r--, others r-- (standard for files)
chmod 755 script.sh           # owner rwx, group r-x, others r-x (standard for executables)
chmod u+x script.sh             # add execute for owner only
chmod g-w file.txt                # remove write for group
chmod -R 755 directory/             # apply recursively
chmod +t /shared/                     # sticky bit — only owner can delete their own files

Ownership: chown / chgrp

sudo chown deploy file.txt                 # change owner
sudo chown deploy:developers file.txt        # change owner AND group in one command
sudo chown -R www-data:www-data /var/www/       # recursive ownership change
sudo chgrp developers file.txt                    # change group only

Inspecting Permissions & Ownership

ls -l file.txt              # rwx string + owner + group
stat file.txt                 # full metadata including octal permission mode
namei -l /var/www/html/index.html   # shows permissions of EVERY directory in the path (great for debugging "permission denied")
getfacl file.txt                       # view ACL entries (finer-grained than owner/group/other)

User Management

sudo useradd -m -s /bin/bash deploy    # create user, home directory, default shell
sudo passwd deploy                       # set/change password
sudo userdel -r deploy                     # delete user AND their home directory
sudo usermod -aG docker deploy               # add to a supplementary group (note: -a is critical, appends rather than replaces)
sudo usermod -L deploy                         # lock an account (disable password login)

Group Management

sudo groupadd developers
sudo groupdel developers
groups deploy                # list all groups a user belongs to
id deploy                      # UID, GID, and all group memberships

Switching Users & Privilege Escalation

sudo whoami                   # run a single command as root
sudo -u postgres psql            # run a command as a specific (non-root) user
su - deploy                        # full login shell as another user
visudo                                # safely edit /etc/sudoers (validates syntax before saving)

Password & Account Policy

chage -l deploy                 # show password expiry/aging info
sudo chage -M 90 deploy            # force password change every 90 days
sudo passwd -l deploy                # lock the password (alternative to usermod -L)

Production Considerations

  • Always edit /etc/sudoers with visudo, never a plain text editor — a syntax error there can lock out sudo access system-wide.
  • usermod -aG — forgetting the -a (append) flag replaces all existing supplementary groups instead of adding one, a classic and dangerous mistake.
  • Service accounts should be created with --system (or a nologin shell) and no usable password — they should only own processes/files, never be logged into interactively.

Quick Interview Answer

“chmod sets read/write/execute permissions numerically (chmod 644) or symbolically (chmod u+x); chown/chgrp change ownership. User accounts are managed with useradd/usermod/userdel, and group membership with usermod -aG — the -a matters, since omitting it replaces rather than appends group memberships. sudo runs a single command with elevated privileges and logs it; /etc/sudoers should only ever be edited via visudo.”

Common Mistakes

  • Running usermod -G group user without -a, wiping out the user’s other group memberships.
  • Editing /etc/sudoers directly instead of through visudo.
  • Using chmod 777 to “fix” a permission error instead of diagnosing the actual ownership/group mismatch.

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