Determine last time Mac OS X was rebooted

How can I determine the last time that a Mac OS X computer was rebooted or shut down?
IT GuyNetwork EngineerAsked:
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johnsoneSenior Oracle DBACommented:
Use the uptime command.  That will tell you how long it has been up, you can back into when it was started from there.

Typical UNIX command would also be who -b, but Mac OS X doesn't seem to support that option.

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OSX does support the who -b option to show the date and time of the most recent reboot.

The last command will also show a list of reboot and shutdown times, including the most recent reboot of who -b.

uptime works better for showing how long it's been up if you prefer that over who -b.
johnsoneSenior Oracle DBACommented:
Good to know that who -b works.  I found this man page which doesn't list it for OS X.
That's a really old man page.

Here's the current one:

WHO(1)                    BSD General Commands Manual                   WHO(1)

     who -- display who is logged in

     who [-abdHlmpqrsTtu] [file]
     who am i

     The who utility displays a list of all users currently logged on, showing for each user the login name,
     tty name, the date and time of login, and hostname if not local.

     Available options:

     -a    Same as -bdlprTtu.

     -b    Time of last system boot.

     -d    Print dead processes.

     -H    Write column headings above the regular output.

     -l    Print system login processes (unsupported).

     -m    Only print information about the current terminal.  This is the POSIX way of saying who am i.

     -p    Print active processes spawned by launchd(8) (unsupported).

     -q    ``Quick mode'': List only the names and the number of users currently logged on.  When this
           option is used, all other options are ignored.

     -r    Print the current runlevel.  This is meaningless on Mac OS X.

     -s    List only the name, line and time fields.  This is the default.

     -T    Print a character after the user name indicating the state of the terminal line: `+' if the ter-
           minal is writable; `-' if it is not; and `?' if a bad line is encountered.

     -t    Print last system clock change (unsupported).

     -u    Print the idle time for each user, and the associated process ID.

     am I  Returns the invoker's real user name.

     file  By default, who gathers information from the file /var/run/utmpx.  An alternative file may be


     last(1), mesg(1), users(1), getuid(2), utmpx(5)

     The who utility conforms to IEEE Std 1003.1-2001 (``POSIX.1'').

     A who utility appeared in Version 6 AT&T UNIX.

BSD                            January 17, 2007                            BSD

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