add-shell

add-shell shell_path [options]
(Hypothetical syntax for a non-standard command)

Practical Equivalent:
echo "shell_path" | sudo tee -a /etc/shells

shell_path
    The absolute path to the shell executable to be added to `/etc/shells`. This argument would be mandatory.

--force, -f
    (Hypothetical) Force the addition of the shell path, even if the specified file does not exist or is not executable. Use with extreme caution.

--dry-run
    (Hypothetical) Show what action would be taken without actually modifying `/etc/shells`.

--help, -h
    (Hypothetical) Display usage information and exit.

--version
    (Hypothetical) Display version information and exit.

The command `add-shell` is not a standard Linux command found in typical distributions. If such a command existed, its primary purpose would be to register a new shell executable's path in the system's list of valid login shells. This crucial list is maintained in the `/etc/shells` file. Services like `login(1)`, `chsh(1)`, and `ftpd(8)` consult `/etc/shells` to verify if a user's assigned shell is legitimate and secure. By adding a shell's path to `/etc/shells`, administrators enable users to safely switch to or be assigned alternative shells (e.g., custom builds, non-standard interpreters like `fish` or `zsh` if not installed via package managers). Without an entry in `/etc/shells`, a user might be denied login or face other authentication issues if their assigned shell is not recognized as valid by the system. In practice, new shells are typically added to this file automatically when a package manager installs a new shell, or manually by system administrators using text editors or `echo` commands. This analysis provides a hypothetical structure for what an `add-shell` command would look like, based on its implied functionality.

The command `add-shell` is not a standard utility in most Linux distributions. Functionality to add shells to `/etc/shells` is usually handled implicitly by package managers when installing a new shell (e.g., `apt`, `yum`, `dnf`), or explicitly by system administrators through manual editing of the `/etc/shells` file (e.g., using `echo "path/to/shell" | sudo tee -a /etc/shells`).

Directly manipulating `/etc/shells` requires root privileges. Incorrect modifications can lead to severe security vulnerabilities or prevent users from logging in if their assigned shell is invalid or points to a non-existent or malicious executable. Always ensure the shell path is correct, the executable is legitimate, and its permissions are secure before adding it to `/etc/shells`.

Since `add-shell` is not a standard command, the typical way to add a shell path to `/etc/shells` is manually. You can use the `echo` command combined with `sudo` and `tee` to append the path. For example, to add `/usr/local/bin/myshell`:
echo "/usr/local/bin/myshell" | sudo tee -a /etc/shells

Remember that `tee -a` appends to the file. Always verify the path exists and the shell is executable before adding it to ensure system stability and security. You can verify the contents after modification using `cat /etc/shells`.

Adding arbitrary or unverified executable paths to `/etc/shells` can pose a significant security risk. If an attacker gains control over a path listed in `/etc/shells` and replaces the legitimate shell with a malicious script, they could potentially execute commands as other users or gain unauthorized access. Therefore, only trusted and properly secured shell executables should ever be listed in `/etc/shells`.

The concept of a dedicated `add-shell` command has not gained widespread adoption in mainstream Linux distributions. Historically, the management of valid system shells has been straightforward: either shells are installed via the system's package manager (which typically handles adding the shell's path to `/etc/shells` as part of the installation script), or administrators manually append new shell paths to the `/etc/shells` file. This manual approach, combined with the infrequent need to add custom shells, has likely precluded the development and standardization of a specific `add-shell` utility. Instead, existing tools like `chsh` rely on the integrity of `/etc/shells`, which is maintained either by the package system or direct administrative intervention.

chsh(1), useradd(8), login(1), shells(5), passwd(5)