How to change file permissions in Linux

Change a file permissions with chmod, in octal (755) or symbolic (u+x) form. Run as the owner or root.

chmod -R on the wrong path can break access across the system, and 777 makes a file world-writable, so avoid it. Set the least permission that works.

# Octal: owner rwx, group and other r-x
chmod 755 <file>

# Common file mode: owner rw, others r
chmod 644 <file>

# Symbolic: add execute for the owner
chmod u+x <file>

# Remove write from group and other
chmod go-w <file>

# Recursively (check the path first)
sudo chmod -R 750 /opt/app

# Verify
ls -l <file>