See here for more info
The syntax of the chmod command when using the symbolic mode has the following format:
chmod [OPTIONS] [ugoa…][-+=]perms…[,…] FILE...The first set of flags ([ugoa…]), users flags, defines which users classes the permissions to the file are changed.
u- The file owner.g- The users who are members of the group.o- All other users.a- All users, identical tougo.
If the users flag is omitted, the default one is a and the permissions that are set by umask are not affected.
The second set of flags ([-+=]), the operation flags, defines whether the permissions are to be removed, added, or set:
-Removes the specified permissions.+Adds specified permissions.=Changes the current permissions to the specified permissions. If no permissions are specified after the=symbol, all permissions from the specified user class are removed.
The permissions (perms...) can be explicitly set using either zero or one or more of the following letters: r, w, x, X, s, and t. Use a single letter from the set u, g, and o when copying permissions from one to another users class.
When setting permissions for more than one user classes ([,…]), use commas (without spaces) to separate the symbolic modes.
Examples
Give the members of the group permission to read the file, but not to write and execute it:
chmod g=r filename
Remove the execute permission for all users:
chmod a-x filename
Recursively remove the write permission for other users:
chmod -R o-w dirname
Remove the read, write, and execute permission for all users except the file’s owner:
chmod og-rwx filename
The same thing can be also accomplished by using the following form:
chmod og= filename
Give read, write and execute permission to the file’s owner, read permissions to the file’s group and no permissions to all other users:
chmod u=rwx,g=r,o= filename
Add the file’s owner permissions to the permissions that the members of the file’s group have:
chmod g+u filename
Add a sticky bit to a given directory:
chmod o+t dirname