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CentOS 7.0 - man page for systemd-ask-password (centos section 1)

SYSTEMD-ASK-PASSWORD(1) 				       systemd-ask-password					   SYSTEMD-ASK-PASSWORD(1)

systemd-ask-password - Query the user for a system password
systemd-ask-password [OPTIONS...] [MESSAGE]
systemd-ask-password may be used to query a system password or passphrase from the user, using a question message specified on the command line. When run from a TTY it will query a password on the TTY and print it to STDOUT. When run with no TTY or with --no-tty it will query the password system-wide and allow active users to respond via several agents. The latter is only available to privileged processes. The purpose of this tool is to query system-wide passwords -- that is passwords not attached to a specific user account. Examples include: unlocking encrypted hard disks when they are plugged in or at boot, entering an SSL certificate passphrase for web and VPN servers. Existing agents are: a boot-time password agent asking the user for passwords using Plymouth; a boot-time password agent querying the user directly on the console; an agent requesting password input via a wall(1) message; an agent suitable for running in a GNOME session; a command line agent which can be started temporarily to process queued password requests; a TTY agent that is temporarily spawned during systemctl(1) invocations. Additional password agents may be implemented according to the systemd Password Agent Specification[1]. If a password is queried on a TTY, the user may press TAB to hide the asterisks normally shown for each character typed. Pressing Backspace as first key achieves the same effect.
The following options are understood: -h, --help Prints a short help text and exits. --icon= Specify an icon name alongside the password query, which may be used in all agents supporting graphical display. The icon name should follow the XDG Icon Naming Specification[2]. --timeout= Specify the query timeout in seconds. Defaults to 90s. A timeout of 0 waits indefinitely. --no-tty Never ask for password on current TTY even if one is available. Always use agent system. --accept-cached If passed, accept cached passwords, i.e. passwords previously typed in. --multiple When used in conjunction with --accept-cached accept multiple passwords. This will output one password per line.
On success, 0 is returned, a non-zero failure code otherwise.
systemd(1), systemctl(1), plymouth(8), wall(1)
1. systemd Password Agent Specification 2. XDG Icon Naming Specification systemd 208 SYSTEMD-ASK-PASSWORD(1)