permissions
File and user permissions
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chmod
Tested chmod examples: numeric and symbolic modes, recursive changes, setuid/setgid/sticky bits, permission audits, and troubleshooting.
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chown
Tested chown examples: setting owner and group, -R over a tree, why a glob misses the dotfiles, and the reason you cannot give away a file you own.
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rm
Tested rm examples: -r and -rf, why a directory's permissions decide what you may delete, and three ways a command removes more than the files you named.
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sudo
Tested sudo and su examples: running one command as root, what the environment loses on the way, the redirect that still runs as you, and reading sudoers.
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File permissions explained
The rwx/owner-group-other model, numeric vs symbolic chmod, umask, and why setuid/setgid on directories behave differently than you'd expect.
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Find files without the permission denied noise
Why 2>/dev/null is the wrong fix for find's permission denied errors, and what -readable -prune does instead, including what happens to the exit status.
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sudo: command not found
On Debian, sudo is often simply not installed, and your user is not in the sudo group. Why the installer does that, and how to fix it from a root shell.