Linux login command

Updated: 03/13/2021 by Computer Hope
login command

On Unix-like operating systems, the login command begins a new login session on the system.

This page covers the Linux version of login.


The login program is used to establish a new session with the system. It is normally invoked automatically by responding to the "login:" prompt on the user's terminal. login may be special to the shell and may not be invoked as a sub-process. When called from a shell, login should be executed as exec login which causes the user to exit from the current shell (and thus prevents the new logged in user to return to the session of the caller). Attempting to execute login from any shell but the login shell produces an error message.

The user is then prompted for a password, where appropriate. Echoing is disabled to prevent revealing the password. Only a small number of password failures are permitted before login exits and the communications link is severed.

If password aging is enabled for your account, you may be prompted for a new password before proceeding. You will be forced to provide your old password and the new password before continuing; refer to our passwd for more information.

Your user and group ID will be set according to their values in the /etc/passwd file. The value for $HOME, $SHELL, $PATH, $LOGNAME, and $MAIL are set according to the appropriate fields in the password entry. ulimit, umask and nice values may also be set according to entries in the GECOS field.

On some installations, the environment variable $TERM will be initialized to the terminal type on your tty line, as specified in /etc/ttytype.

An initialization script may also be executed; check the documentation of your command interpreter for information on init scripts.

A subsystem login is indicated by the presence of a "*" as the first character of the login shell. The given home directory will be used as the root of a new file system which the user is actually logged into.


login [-p] [-h host] [username] [ENV=VAR...]
login [-p] [-h host] -f username
login [-p] -r host


-f Do not perform authentication; user is pre-authenticated. In that case, username is mandatory.
-h Name of the remote host for this login.
-p Preserve environment.
-r Perform autologin protocol for rlogin.


The following configuration variables in /etc/login.defs change the behavior of this tool:

name type description
CONSOLE_GROUPS string List of groups to add to the user's supplementary groups set when logging in on the console (as determined by the CONSOLE setting). Default is none.

Use with caution - it is possible for users to gain permanent access to these groups, even when not logged in on the console.
DEFAULT_HOME boolean Indicate if login is allowed if we can't cd to the home directory. Default is no.

If set to yes, the user will login in the root (/) directory if it's not possible to cd to the user's home directory.
ENV_PATH string If set, it will be used to define the PATH environment variable when a regular user login. The value is a colon separated list of paths (for example /bin:/usr/bin) and can be preceded by PATH=. The default value is PATH=/bin:/usr/bin.
ENV_SUPATH string If set, it will be used to define the PATH environment variable when the superuser logs in. The value is a colon separated list of paths (for example /sbin:/bin:/usr/sbin:/usr/bin) and can be preceded by PATH=. The default value is PATH=/sbin:/bin:/usr/sbin:/usr/bin.
ERASECHAR number Terminal Erase character (010 = backspace, 0177 = Del).

The value can be prefixed "0" for an octal value, or "0x" for a hexadecimal value.
FAIL_DELAY number Delay in seconds before being allowed another attempt after a login failure.
FAKE_SHELL string If set, login will execute this shell instead of the users' shell specified in /etc/passwd.
HUSHLOGIN_FILE string If defined, this file can inhibit all the usual chatter during the login sequence. If a full pathname is specified, then hushed mode will be enabled if the user's name or shell are found in the file. If not a full pathname, then hushed mode will be enabled if the file exists in the user's home directory.
KILLCHAR number Terminal KILL character (025 = ^U).

The value can be prefixed "0" for an octal value, or "0x" for a hexadecimal value.
LOGIN_RETRIES number Maximum number of login retries in case of bad password.

This will most likely be overridden by PAM since the default pam_unix module has its own built-in of 3 retries. However, this is a safe fallback in case you are using an authentication module that does not enforce PAM_MAXTRIES.
LOGIN_TIMEOUT number Max time in seconds for login.
LOG_OK_LOGINS boolean Enable logging of successful logins.
LOG_UNKFAIL_ENAB boolean Enable display of unknown usernames when login failures are recorded.

Note! Logging unknown usernames may be a security issue if a user enters their password instead of their login name, as the typo would then be logged.
TTYGROUP, TTYPERM string The terminal permissions: the login tty will be owned by the TTYGROUP group, and the permissions will be set to TTYPERM.

By default, the ownership of the terminal is set to the user's primary group and the permissions are set to 0600.

TTYGROUP can be either the name of a group or a numeric group identifier.

If you have a write program which is "setgid" to a special group which owns the terminals, define TTYGROUP to the group number and TTYPERM to 0620. Otherwise, leave TTYGROUP commented out and assign TTYPERM to either 622 or 600.
TTYTYPE_FILE string If defined, file which maps the tty line to the TERM environment parameter. Each line of the file is in a format such as "vt100 tty01".
USERGROUPS_ENAB boolean If set to yes, userdel removes the user's group if it contains no more members, and useradd creates (by default) a group with the name of the user.


/var/run/utmp List of current login sessions.
/var/log/wtmp List of previous login sessions.
/etc/passwd User account information.
/etc/shadow Secure user account information.
/etc/motd System message of the day file.
/etc/nologin Prevent non-root users from logging in.
/etc/ttytype List of terminal types.
$HOME/.hushlogin Suppress printing of system messages.
/etc/login.defs Shadow password suite configuration.

The -r, -h and -f options are only used when login is invoked by root.



Attempts to log in to the host

csh — The C shell command interpreter.
exit — Exit the command shell.
init — The parent of all processes on the system.
ksh — The Korn shell command interpreter.
mail — Read, compose, and manage mail.
mailx — Process mail messages.
newgrp — Log into a new group.
passwd — Change a user's password.
rlogin — Begin a session on a remote system.
rsh — Execute a command on a remote shell.
sh — The Bourne shell command interpreter.
telnet — Connect to a remote system using the telnet protocol.
umask — Get or set the file mode creation mask.