Ytdyklly

This queries the uptime from the kernel and displays it in the local timezone:

date -d "`cut -f1 -d. /proc/uptime` seconds ago"

Be careful about other options. The last command will stop working as soon as wtmp has been rotated. The who command depends on the availability and integrity of utmp. And /proc/1 might have the current date instead of the boot time date, and could even be unavailable on a hardened system. Edit: dmesg only has a fixed-length back buffer, so it is unrealiable, too. The kernel logs may be in /var/log but most distributions only keep 8 weeks of them.

I stumbled on this question while looking for a way to get a consistent, parseable boot time, as opposed to time since boot which changes on every call.

It appears that uptime -s will do the trick on most linux systems.

I found the btime line in /proc/stat when poking around a bit

cat /proc/stat | grep btime | awk '{ print $2 }'

and after a quick search, I found this page: /proc/stat explained, which outlines the "Various pieces of information about kernel activity that are available in the
/proc/stat file."

The "btime" line gives the time at which the system booted, in seconds since
the Unix epoch.

• 
  good: uptime -s, who -b or parsing /proc/uptime
  


• 
  bad: ls -ld /proc/1 and variants.

Don't use ls -ld /proc/1 for this purpose. It's sometimes updated after s2disk or s2ram.

In my case, who -b said:

system boot May 2 09:51

While ls -ld /proc/1:

dr-xr-xr-x 7 root root 0 May 3 13:09 /proc/1

ls -ld for /proc or /sys seem to persist after resuming, but it's implementation dependent, and may change in future, so don't use such methods. And if your system clock is in localtime, not UTC, they have negative offset.

(I don't have the privilege to comment to answers yet, so opened a new answer. Sorry.)

EDIT: uptime -s was first answered in this answer by mikegreiling

The simplest way is to look to see when /sbin/init started (that's always the first process to be started after the kernel loads):

# ls -ld /proc/1

dr-xr-xr-x 7 root root 0 2011-03-27 23:54 /proc/1

So I can see that my machine booted up at 6 minutes to midnight on the 27th of March 2011.

If you want to use it in scripting you can use the stat command instead:

# stat --printf='%Y' /proc/1

1301266491

The %Y specifies the time since the directory was last changed (process creation time) in seconds since the epoch (1/1/70) and is a standard unix timestamp.

In Linux,

ls -ld /proc

seems to give me what I need. The post above is odd. /proc/uptime does not contain a date value – it would have to be subtracted from the current time. Maybe he meant:

date -d @$(( $(date +%s) - $(cut -f1 -d. /proc/uptime) ))

Under Bash, without pipes nor other processes; just text:

$ REPLY="$(</proc/uptime)"

$ REPLY="${REPLY%%.*}"

$ echo "$REPLY"

31207

(Just reused the REPLY default variable, but you can choose whatever you need)

date -d @$(sed -n '/^btime /s///p' /proc/stat)

(yet another way to do this, which is useful in certain circumstances)

Command:

(echo ' Currently:' | tr "n" ' ' ; date +"%Y-%m-%d %k:%M:%S" ; echo '  Up Since:' | tr 'n' ' ' ; uptime -s ; echo '  Duration:' | tr 'n' ' ' ; uptime -p)

Output:

 Currently: 2016-05-09  9:06:29

  Up Since: 2016-05-04 12:56:04

  Duration: up 4 days, 20 hours, 10 minutes

Clear and concise with tuptime command:

# tuptime -t

No.             Startup Date                                          Uptime            Shutdown Date   End                    Downtime



1     09:43:39 AM 08/08/2017      41 days, 0 hours, 51 minutes and 2 seconds   10:34:41 AM 09/18/2017    OK                  10 seconds

2     10:34:51 AM 09/18/2017                         1 minute and 16 seconds   10:36:07 AM 09/18/2017    OK                    1 second

3     10:36:08 AM 09/18/2017                       13 minutes and 20 seconds   10:49:28 AM 09/18/2017    OK                   3 seconds

4     10:49:31 AM 09/18/2017       45 days, 0 hours, 1 minute and 20 seconds   09:50:51 AM 11/02/2017    OK                   4 seconds

5     09:50:55 AM 11/02/2017                       27 minutes and 25 seconds   10:18:20 AM 11/02/2017    OK                   4 seconds

6     10:18:24 AM 11/02/2017                                       9 seconds   10:18:33 AM 11/02/2017    OK                   9 seconds

7     10:18:42 AM 11/02/2017      4 days, 5 hours, 41 minutes and 47 seconds   04:00:29 PM 11/06/2017    OK                  44 seconds

8     04:01:13 PM 11/06/2017    15 days, 17 hours, 33 minutes and 48 seconds   09:35:01 AM 11/22/2017   BAD   10 minutes and 40 seconds

9     09:45:41 AM 11/22/2017               8 hours, 9 minutes and 20 seconds   05:55:01 PM 11/22/2017   BAD     7 minutes and 8 seconds

10    06:02:09 PM 11/22/2017                1 hour, 7 minutes and 54 seconds   07:10:03 PM 11/22/2017   BAD   11 minutes and 30 seconds

11    07:21:33 PM 11/22/2017               1 hour, 58 minutes and 32 seconds   09:20:05 PM 11/22/2017    OK                   5 seconds

12    09:20:10 PM 11/22/2017                       14 minutes and 52 seconds   09:35:02 PM 11/22/2017   BAD    5 minutes and 52 seconds

13    09:40:54 PM 11/22/2017                         4 minutes and 6 seconds   09:45:00 PM 11/22/2017   BAD    4 minutes and 51 seconds

14    09:49:51 PM 11/22/2017             11 hours, 15 minutes and 10 seconds   09:05:01 AM 11/23/2017   BAD    7 minutes and 20 seconds

15    09:12:21 AM 11/23/2017      3 days, 2 hours, 17 minutes and 40 seconds   11:30:01 AM 11/26/2017   BAD   27 minutes and 44 seconds

16    11:57:45 AM 11/26/2017   109 days, 19 hours, 12 minutes and 37 seconds   07:10:22 AM 03/16/2018    OK                  17 seconds

17    07:10:39 AM 03/16/2018     25 days, 3 hours, 55 minutes and 59 seconds   12:06:38 PM 04/10/2018    OK                   3 seconds

18    12:06:41 PM 04/10/2018      8 days, 19 hours, 3 minutes and 20 seconds   07:10:01 AM 04/19/2018   BAD    3 minutes and 52 seconds

19    07:13:53 AM 04/19/2018     77 days, 9 hours, 44 minutes and 39 seconds