User, file and group permissions

Info

Show all groups a user belongs to

groups <user>

Show primary groups for user

id <user>

Sudo

Perform a command as the super user

sudo <command>

Switch users

Switch to specific user

su <user>

Switch to root or specific user and load user profile (CTRL + d to exit user mode)

su -
su - <user>

Managing users

Create a new user

adduser <username>

Change password for active user

passwd

File permissions

Change file mode and access control

chmod <permissions> <file>

Permission types

r = read
w = write
x = execute

Numerical values

r = 4
w = 2
x = 1

Permission groups

u = user
g = group
o = other
a = all (sets u, g and o)

Set permissions with assigment

Set user to rwx, group to rw and other to r

chmod u=rwx,g=rw,o=r <file>

Set u to rwx, group and other to r

chmod u=rwx,go=r <file>

Alter some permission with plus and minus

Add write permissions to user group

chmod g+w <file>

Remove read and write permission from other group

chmod o-rw <file>

Add execute for user, remove write for group, remove rwx for other

chmod u+x,g-w,o-rwx

Use -R to set permissions recursively

Set group to read and write for the directory and all sub files and folders

chmod -R g=rw <dir>

Set user / group for new files

Set group automatically when creating new files within <dir> via the g+s SETGID (SET Group ID) flag

chmod g+s <dir>

Remove setting group automatically for new files within <dir> by removing set group flag

chmod g-s <dir>

Managing groups and owners

List available groups

less /etc/group

Modify a user (usermod) and add (-a) them to a group (-G)

usermod -a -G <group> <username>

Change file group associated with the file permissions

chgrp <group> <file>

Change file owner

chown <user> <file>

Cleaning up file permissions

Find files with permission set to 777 and change them to default 644

find . -type f -perm 777 -print0 | xargs -0 chmod 644

Find directories with permission set to 777 and change them to default 755

find . -type d -perm 777 -print0 | xargs -0 chmod 755

Unlock all locked files found under the current directory

chflags -R nouchg *