Interview Q&A / Linux & Shell

1. What does SGID do on a directory, and when is it useful?

   On a directory, SGID makes new files and sub-folders inherit the directory's group instead of the creator's primary group. It is used for shared team folders so everyone's files stay accessible to the same group. It shows as s in the group execute slot.

   IntermediateQuick answer1 min read
2. What does the execute permission mean on a directory?

   On a directory, execute (x) means permission to enter it and access items by name. Read (r) lets you list names, and write (w) lets you create or delete entries. Without x on a directory you cannot cd into it or open files inside, even if the files themselves are readable.

   IntermediateQuick answer1 min read
3. What is the sticky bit, and why does /tmp have it?

   The sticky bit on a directory means only a file's owner, the directory's owner or root may delete or rename files inside, even if others have write permission on the directory. It shows as a t at the end of the permissions, for example drwxrwxrwt.

   IntermediateQuick answer1 min read
4. What is the SUID bit, and how can passwd change root-owned files?

   SUID makes an executable run with the permissions of its file owner instead of the user who launched it. passwd is owned by root with SUID set, so a normal user can update /etc/shadow through it. The bit shows as s in the owner execute slot.

   IntermediateQuick answer1 min read
5. What is umask, and how does it decide default file permissions?

   umask is a mask of permission bits that are removed from the defaults for newly created files and directories. Files start from 666 and directories from 777, so a umask of 022 gives files 644 and directories 755.

   IntermediateQuick answer1 min read