Linux or UNIX has a ‘sar’ command that “writes to standard output the contents of selected cumulative activity counters in the operating system”, showing system stats for each (by default) 10-minute interval like this
12:00:01 AM CPU %user %nice %system %iowait %steal %idle 12:10:02 AM all 5.29 0.06 0.70 0.29 0.00 93.65 12:20:01 AM all 3.14 0.00 0.51 0.06 0.00 96.28
(Ignore the first timestamp “12:00:01 AM”, which really should be the name of the header such as “timestamp”.) A natural question is, Is the time in the first column the starting or ending moment of the 10-minute interval? For example, does “12:10:02 AM” mean that the CPU usage, I/O Wait, etc. to the right are the average stats beginning at 12:10:02 AM and ending at 12:20:00 AM, or the average for the past 10 minutes up to 12:10:02 AM? Surprisingly, Google searches using various keyword combinations return no webpage talking about it. Then I checked a few UNIX or Linux system admin books I bought many years ago, and searched and found the source code of the ‘sar’ command. None offers a clear answer, although I admit I didn’t spend much time reading the code. Finally, I posed this question to various AI websites:
According to https://github.com/sysstat/sysstat/blob/master/sar.c , can we tell whether the time displayed at the beginning of each row of the Linux sar command output is the beginning or ending moment of the 10-minute time range?
It’s funny that Google, ChatGPT, and DeepSeek all confidently tell me the timestamp is the beginning of the interval. (To trigger Google’s AI, I have to omit the “According to …” clause.) ChatGPT does not say it consulted the source code. DeepSeek says it is “[b]ased on the sar.c source code from the systat GitHub repository” and its “Code Context” section mentions get_time() and get_localtime() functions. But when I asked where the functions are called as I don’t see them in sar.c, it apologized. Perplexity AI, however, tells me the timestamp is the ending of the interval, with no mention of the source code. My question is important because I’m in the middle of writing an email to be sent to my manager and a coworker who respond to our director’s inquiry on a performance problem that happened a few days ago. Getting the time wrong by 10 minutes is unacceptable. For a long time, I’ve interpreted the time as the ending time of the interval. But today I wanted to be cautious so I checked, and yet, what a surprise!, these state-of-the-art AI’s gave me conflicting answers.
Then I had a second thought. Let’s check the output of ‘sar’ against the current time, i.e. by running the following two commands in quick succession.
$ date Fri Mar 14 09:57:43 AM CDT 2025 $ sar | tail -3 09:40:01 AM all 2.45 0.00 1.09 0.11 0.00 96.35 09:50:01 AM all 5.98 0.04 1.10 0.27 0.00 92.62 Average: all 4.93 0.00 0.94 0.47 0.00 93.65
In the above output, the last line above “Average” shows “09:50:01 AM”. It must be the ending time of the interval for this row, representing the time range 09:40–09:50 AM. If it were the beginning time, it would represent the time range 09:50 to 10:00. But the current time is 09:57 as shown by the 'date' command run above, not yet 10:00. How could ‘sar’ possibly know the stats in the time that has not come?
To be extra careful, I thought, why not get a word from the horse’s mouth? The author of sar.c has his email contact right in the C source code. So I emailed him. He quickly replied and confirmed my understanding, and said (to paraphrase) my own natural intelligence may be more powerful than DeepSeek's. Problem solved!
Of the four sophisticated AIs I tested, only Perplexity got the answer right. Obviously, no AI actually ran the command on its side, as a software engineer or a human might do in trying to answer my question. Further, even if AI did, it would have a hard time to interpret the result, or would simply ignore it. To test this hypothesis, I asked ChatGPT and DeepSeek the question again, this time providing my 'date' and 'sar | tail -3' commands and the output (see above), but without my words on reasoning. A person with moderate intelligence, even with no knowledge of Linux or UNIX, should be able to infer that the timestamp in the first column of the 'sar' output must be the ending time of the interval, and yet both ChatGPT and DeepSeek still insist that the first column is the starting time. When can they get smarter?
[Update of Jan 30,2026] The article was written in March 2025. Today I just asked the AIs the second question, i.e. with 'date' and 'sar' output together. ChatGPT still answers wrong (the beginning time). DeepSeek and Google Gemini now answer correctly (ending time). But Perplexity, well, quite perplexingly, says "from this output alone you cannot tell".
Contact me
To my Computer Page