Frequently Asked Questions
What does the execute bit on a directory do?
On a directory, the execute bit controls whether you can enter the directory and access files inside it. Without execute, even read permission only lets you know the filenames exist - you cannot open or stat any of them.
What are setuid, setgid, and sticky bits?
These are a fourth octal digit prepended to the standard three (e.g., 4755 for setuid). Setuid causes an executable to run with the owner's privileges. The sticky bit on a directory prevents users from deleting files they don't own, commonly used on /tmp.
What is a common mistake with 777 permissions?
Setting 777 on a directory in a web server root allows any process running as any user to write files there, enabling an attacker who exploits any service to plant malicious files.
How do I apply a chmod value on the command line?
Run `chmod 755 filename` to set permissions to 755, or use symbolic form `chmod u+x filename` to add execute for the owner only. Use `chmod -R` to apply recursively to a directory tree.
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