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Every file and directory on a Linux system carries a permission mode made of three permission classes — owner, group, and other — each expressed as a string of read (r), write (w),...
The standard rwx bits cover most situations, but Linux also defines three additional flags that change the meaning of executable files and shared directories. The setuid bit, set w...
Permissions only tell half the story; the other half is who owns a file. Ownership is changed with chown user:group path, optionally with -R to descend into subtrees, and with chgr...
A handful of well-known system files have modes that look surprising on first contact, but each has a clear reason. SSH is the strictest case: the ~/.ssh directory must be 700 and...
The classic user/group/other model is sufficient for many systems, but it cannot grant access to a fourth specific user without making the file world-readable. POSIX ACLs solve thi...
When permissions behave unexpectedly, several layered tools help diagnose the cause. To find setuid binaries — both for auditing and for spotting potential privilege-escalation pat...
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