The format of a symbolic mode is a combination of the letters +-= rwxXstugoa
Multiple symbolic operations can be given, separated by commas.
The full syntax is [ugoa...][[+-=][rwxXstugo...]...][,...] but this is explained below.
A combination of the letters ugoa controls which users' access to the file will be changed:
The user who owns it u
Other users in the file's Group g
Other users not in the file's group o
All users a
If none of these are given, the effect is as if a were given, but bits that are set in the umask are not affected.
All users a is effectively user + group + others
The operator '+' causes the permissions selected to be added to the existing permissions of each file; '-' causes them to be removed; and '=' causes them to be the only permissions that the file has.
The letters 'rwxXstugo' select the new permissions for the affected users:
Execute (or access for directories) x
Execute only if the file is a directory
(or already has execute permission for some user) X
Set user or group ID on execution s
Restricted deletion flag or sticky bit t
The permissions that the User who owns the file currently has for it u
The permissions that other users in the file's Group have for it g
Permissions that Other users not in the file's group have for it o
Deny execute permission to everyone:
chmod a-x file
Allow read permission to everyone:
chmod a+r file
Make a file readable and writable by the group and others:
chmod go+rw file
Make a shell script executable by the user/owner
$ chmod u+x myscript.sh
Allow everyone to read, write, and execute the file and turn on the set group-ID:
chmod =rwx,g+s file