Hi,
I have query regarding different time taken during command execution.
For example if i execute ls command on the shell like this:

time ls -alh

the output of the command is something like this:
[user@linux]$ time ls -alh
total 48K
drwxr-xr-x 3 junk user 4.0K Jun 27 19:21 .
drwxr-xr-x 18 user user 4.0K Apr 6 11:45 ..
-rw------- 1 user user 51 Jun 27 19:21 .Xauthority

real 0m0.005s
user 0m0.000s
sys 0m0.010s

what does this real , user and sys time means?

any good link to the detailed explanation will be very helpful for me.

TIA

Hi,

Linux comes with so called man pages (short for manual pages). Most commands come with a man page.

man time for example, gives you all sorts of information about the time program, including the answer to your question:

real - Elapsed real time (in [hours:]minutes:seconds).
user - (Not in tcsh.) Elapsed real time (in seconds).
sys - Total number of CPU-seconds that the process spent in kernel mode.

Hope this helps.

I haven't checked the man pages, but there are generally 3 types of "time" recorded for any Linux/Unix process:
time spent in "user space", time spent in "kernel space" and the time duration "on the clock".

User space is the part of memory where normal programs are loaded and executed from.
Kernel space is entered whenever the program does a so-called "system call", which simply means that it calls the operating system (Linux in this case) to perform a standard system operation, like reading a part of a file on disk. When this operation is done, the program will return back to "user space".
The "time on the clock" is simply the time it took for the program to complete, as if it was measured by a stopwatch. This should always be >= the sum of the user and system times, because the program you have time'd is alternated with all other running programs (processes)
(All processes get to run, one after another, for short periods of time, depending on things like priority, niceness, etc.)

Please note also that kernel space has different "rules" than user space. For instance, kernel routines are often made uninterruptable (so, the system can't switch to another process in the middle of the system call), so that the hardware can't get messed up.