etcshells

etcshells refers to the /etc/shells file on Unix-like systems, a plain text listing of valid login shells. The file is used by login-related programs and system utilities to determine whether a requested shell is permitted for an account. If a user's chosen shell is not listed, attempts to switch to that shell can be rejected by the system.

Format and content: /etc/shells typically contains one absolute path to a shell per line. Lines may include comments starting with the hash character and blank lines. Common entries include paths such as /bin/sh, /bin/bash, /bin/tcsh, /bin/ksh, /bin/zsh, and /usr/bin/fish, though the exact contents depend on the distribution and installed shells. The primary purpose is to provide a centralized reference for allowed login shells.

Management and best practices: Editing /etc/shells generally requires root privileges. To add support for a newly installed shell, its binary path should be appended to the file, ensuring the shell is actually installed and executable at that location. After modifying the file, administrators should verify the path and, if necessary, inform users that the new shell is now permitted for login and chsh operations. Conversely, removing an entry from the file can prevent users from setting or using that shell when pam_shells or similar mechanisms are active.

Security implications: Maintaining an up-to-date /etc/shells helps enforce access policies by ensuring only approved shells are usable for logins. It is one layer of defense in account hardening, but it does not replace broader authentication, authorization, and system hardening practices.

See also: chsh, pam_shells, login, sshd.