Unix file permissions come in threes. There are three numbers per file, each of which represents the permissions for one of these roles: - user (u) (i.e., owner),
- group (g),
- other (o).
There are three permissions: - read (r),
- write (w),
- execute (x).
The numbers range from 1 to 7. (A zero means "no permissions.") And because of how the numbers are constructed, each number uniquely identifies a set of permissions for the party it refers to. Specifically, the each permission corresponds to these numbers: - read = 4
- write = 2
- execute = 1
By combining these, we get every possible combination of permissions (common ones in bold): - 1 = execute only (not read or write)
- 2 = write only (not execute or read)
- 3 = write + execute (but not read)
- 4 = read only (not write or execute)
- 5 = read + execute (but not write)
- 6 = read + write (but not execute)
- 7 = read + write + execute (i.e., everything)
So here are the meaning of some common permission sets: - 644 = user can read/write, group and others can only read.
- 755 = user has full permissions; group and others cannot write.
- 775 = user and group have full permissions; others cannot write.
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