chmod 664 Explained — Owner+Group Read-Write, Others Read-Only

What Does chmod 664 Mean?

Permission 664 allows collaborative editing within a group while keeping the file readable by others:

Role	Octal	Symbolic	Permissions
Owner	6	rw-	Read + Write
Group	6	rw-	Read + Write
Others	4	r--	Read only

Where Is 664 Used?

# Shared source code files
chmod 664 /shared/project/main.py

# Collaborative documents
chmod 664 /shared/docs/readme.txt

# Shared configuration files
chmod 664 /etc/myapp/app.conf

664 and umask 002

Permission 664 is the default file permission when umask is set to 002:

666 & ~002 = 664

Many collaborative environments configure umask 002 to enable group writing by default.

Setting Up Group Collaboration

# 1. Create a shared group
sudo groupadd devteam

# 2. Add users to the group
sudo usermod -aG devteam alice
sudo usermod -aG devteam bob

# 3. Set group ownership
sudo chown -R :devteam /shared/project

# 4. Set SGID so new files inherit the group
chmod 2775 /shared/project

# 5. Set umask for group writing
umask 002

# Now new files are automatically 664
touch /shared/project/newfile.txt
ls -l /shared/project/newfile.txt
# -rw-rw-r-- 1 alice devteam ...

664 vs 644

Permission	Group can write?	Default umask
644	No	022 (standard)
664	Yes	002 (collaborative)

Use 664 when team members in the same group need to edit files. Use 644 on single-user systems or when group write access is not needed.