GNU Info

(libc.info)Access Permission

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How Your Access to a File is Decided
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   Recall that the operating system normally decides access permission
for a file based on the effective user and group IDs of the process and
its supplementary group IDs, together with the file's owner, group and
permission bits.  These concepts are discussed in detail in Note:
Process Persona.

   If the effective user ID of the process matches the owner user ID of
the file, then permissions for read, write, and execute/search are
controlled by the corresponding "user" (or "owner") bits.  Likewise, if
any of the effective group ID or supplementary group IDs of the process
matches the group owner ID of the file, then permissions are controlled
by the "group" bits.  Otherwise, permissions are controlled by the
"other" bits.

   Privileged users, like `root', can access any file regardless of its
permission bits.  As a special case, for a file to be executable even
by a privileged user, at least one of its execute bits must be set.