chmod 664-rw-rw-r--
chmod 664 lets both the owner and the file's group read and write it, while everyone else can only read it — the standard mode for small-team collaboration on a shared file.
Owner
You, the file's creator
Group
Everyone in the file's group
Public
Everyone else on the system
Special permissions(advanced, rarely needed)
Common presets
On a file, 664 means the owner and anyone in the file's group can both edit it, while everyone else can read but not change it. It's common for logs, shared configuration, or working files that more than one trusted account needs to update.
As with the other no-execute modes, 664 doesn't let anyone traverse a directory. For a directory that a group should collaboratively manage — creating and deleting files inside it — use 775 instead, which adds the execute bit needed to enter it.
When to use chmod 664
664 fits situations where two specific accounts need write access to the same file via a shared Unix group — for example, a deploy user and an application user both belonging to a "deploy" group and both needing to update a release artifact.
Security notes
The security of 664 depends entirely on who's in the file's group: anyone added to that group gets full write access. Keep group membership tight and audited, since it's the only thing standing between "a trusted collaborator" and "anyone who can write to this file."