Uptime at shutdown

[balanga]

Is there any way to record uptime when a system shuts down when the battery runs out of power?

 

[SirDice]

Why do you care so much about uptime? The datetime stamps in /var/log/messages will tell you when the system started shutting down, assuming it does a 'proper' shutdown, not an instant off kind of thing.

 

[MG]

Replace /sbin/shutdown with a script that writes the uptime to a file and then run the real shutdown command.

 

[balanga]

Why do you care so much about uptime? The datetime stamps in /var/log/messages will tell you when the system started shutting down, assuming it does a 'proper' shutdown, not an instant off kind of thing.

I want to know how long the system will last on a full battery.

 

[ralphbsz]

It's easy enough to figure out when the system booted (that's in the log), and when it shut down. Either the shutdown is in the log, or regular log updates from other subsystems stop happening. Then calculate the difference yourself.

 

[balanga]

It's easy enough to figure out when the system booted (that's in the log), and when it shut down. Either the shutdown is in the log, or regular log updates from other subsystems stop happening. Then calculate the difference yourself.

Does dmesg record the time of a shutdown when a system shuts down when it runs out of power?

 

[Phishfry]

There are probably some ACPI battery signals that throw a message at XX amount of time before battery depletion.
But realistically with no power there is no log entry written.

But it depends on signals received. In some cases the computer may be shutdown by ACPI signals and other case just run to death.

acpi_battery

man.freebsd.org

 

[Spica]

sysutils/tuptime

Not quite what you're after but does report number of startups/shut downs, system life etc

> tuptime

System startups: 111 since 15:22:20 28/06/2025

System shutdowns: 98 ok + 12 bad
System life: 354d 4h 11m 44s

Longest uptime: 14d 12h 7m 38s from 22:34:36 16/09/2025
Average uptime: 1d 22h 24m 54s
System uptime: 60.61% = 214d 16h 4m 34s

Longest downtime: 42d 5h 21m 34s from 12:13:32 14/02/2026
Average downtime: 1d 6h 26m 15s
System downtime: 39.39% = 139d 12h 7m 10s

Current uptime: 9h 40m 42s since 09:53:22 17/06/2026

 

[balanga]

Not quite what you're after but does report number of startups/shut downs, system life etc

Never heard of tuptime before but thanks for drawing my attention to it.

 

[ralphbsz]

Does dmesg record the time of a shutdown when a system shuts down when it runs out of power?

If the system shuts down, the shutdown will be recorded in /var/log/messages.

If the battery is set up correctly, the system should go through shutdown before the battery runs out.

If the system just stops, dmesg and /var/log/messages can't record that. But you get a pretty accurate idea of when it happens by looking at other log entries: What was the last message before the reboot?

 

[balanga]

If the system shuts down, the shutdown will be recorded in /var/log/messages.

If the battery is set up correctly, the system should go through shutdown before the battery runs out.

If the system just stops, dmesg and /var/log/messages can't record that. But you get a pretty accurate idea of when it happens by looking at other log entries: What was the last message before the reboot?

kernel: battery0: critically low charge!

 

[akil]

Write simple rc script that run

uptime > mytery_uptime_book.log

Update your rc.conf accordingly and test it.

 

[richardtoohey2]

I want to know how long the system will last on a full battery.

Won't that depend on the load you put on the battery? Or are you just going to leave it idle to see the "best" it can do with no load? Just curious, not being snarky.

 

[OpenFreeNet]

You can also do something like this

#!/bin/sh

# Path to the output file (change this to your preferred path)
TARGET_FILE="date_log.txt"

# Ensure the file exists before writing to it
touch "$TARGET_FILE"

echo "Starting date logger. Writing to $TARGET_FILE every 2 minutes..."

while true; do
    # Append the current date and time to the file
    date >> "$TARGET_FILE"
    
    # Force flushing of the file's in-core state to the disk
    fsync "$TARGET_FILE"
    
    # Pause for 120 seconds (2 minutes)
    sleep 120
done

and make it a daemon

nohup ./sync_date.sh > /dev/null 2>&1 &

disclosure: the idea is mine, but I asked to Gemini AI to generate the script