Linux Commands Examples

A great documentation place for Linux commands


run programs and summarize system resource usage

see also : printf



[ -apqvV ] [ -f FORMAT ] [ -o FILE ]

[ --append ] [ --verbose ] [ --quiet ] [ --portability ]
[ --format=FORMAT ] [ --output=FILE ] [ --version ]
[ --help ] COMMAND [ ARGS ]

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To run the command ’wc /etc/hosts’ and show the default information:
time wc /etc/hosts

To run the command ’ls -Fs’ and show just the user, system, and total time:
time -f "\t%E real,\t%U user,\t%S sys" ls -Fs

To edit the file BORK and have ’time’ append the elapsed time and number of signals to the file ’log’, reading the format string from the environment variable ’TIME’:
export TIME="\t%E,\t%k" # If using bash or ksh
setenv TIME "\t%E,\t%k" # If using csh or tcsh
time -a -o log emacs bork

Users of the bash shell need to use an explicit path in order to run the external time command and not the shell builtin variant. On system where time is installed in /usr/bin, the first example would become
/usr/bin/time wc /etc/hosts


Network latency measurement (Linux)

Run you program and the meantime capture network traffic with tcpdump or wireshark. Check the time of the request and the reply and do a simple subtraction.


Linux - Forcing 24-hour locale?

Locales can be set for various components separately, so you can set LC_TIME to either "en_GB.utf-8" (Great Britain) or "C" (no locale at all).

(Don't forget /etc/locale.gen.)


Fastest booting desktop linux distro?

PuppyLinux, xPUD and DSL should be fast.
Ext4 support needs to be checked though.
Also consider things like XFCE.


how long to zero a drive with dd?

I'm guessing, but my guess is that it would depend on the drive controller, the controller on the motherboard, and what else is soaking up CPU/IO.

My guess, on the order of hour or hours. Days seems long. Depending on how your machine is set up, running both at the same time may actually slow things down if you create contention for the drive controller. Even though you're pumping out zeros, nothing in your drive knows that and it needs to write every byte.


How can I permanently fix my date synchronize problem in linux?

I had the same problem yesterday but under Slackware 8. To make a long story short, I read a lot on google to finally reimage the computer. My manager sent me that link but it did not fixed my issue.

I changed the local to UTC time. I changed the timezome to have the good one I ln both.

Hope this will help you!

Also, you can try this:

hwclock --systohc

Timezone conversion by command line

This example is from

It gives the local time corresponding to 9AM on the west coast of the US, accounting for differing day light savings transitions.

date --date='TZ="America/Los_Angeles" 09:00 next Fri'

Use tzselect to get the TZ. The PST format is ambiguous. IST = Indian Standard Time and Irish Summer Time for example.


Is there any command like time, but for memory usage?

GNU time can report a bit more information than the version built into Bash; use command time rather than just time to invoke it, and see the man page or info for details.


Time changes when using two OSs

It's probably because Ubuntu is configured to assume that the system clock is GMT/UTC (or whatever you want to call it) and it adjusts the time zone for the location of the user that is logging in. Windows changes the actual system clock to match the time zone. You can reconfigure Ubuntu to assume that the system clock is local time, but I can't remember how to do it at the moment (google will tell you!).


How to set Debian to automatically update time and date?

Install ntp and made it run on runlevels 2,3,5. Then it will pull the date and time from well-known sources and will adjust your system gradually.


Get the time it took to execute the previous command in linux(without re-executing that command)

If you run around the block and don't measure the amount of time it took, didn't watch any clocks before, during or after and no people saw you do it, you also don't know how long it takes on average; would you be able to tell how long it took you to run around the block?


At best, you can know when you launched a certain application.


But that doesn't tell you when it has ended, which you will need to deduce, if at all possible.


time run the program COMMAND with any given arguments ARG.... When COMMAND finishes, time displays information about resources used by COMMAND (on the standard error output, by default). If COMMAND exits with non-zero status, time displays a warning message and the exit status.

time determines which information to display about the resources used by the COMMAND from the string FORMAT. If no format is specified on the command line, but the TIME environment variable is set, its value is used as the format. Otherwise, a default format built into time is used.

Options to time must appear on the command line before COMMAND. Anything on the command line after COMMAND is passed as arguments to COMMAND.


-o FILE, --output=FILE

Write the resource use statistics to FILE instead of to the standard error stream. By default, this overwrites the file, destroying the file’s previous contents. This option is useful for collecting information on interactive programs and programs that produce output on the standard error stream.

-a, --append

Append the resource use information to the output file instead of overwriting it. This option is only useful with the ’-o’ or ’--output’ option.

-f FORMAT, --format FORMAT

Use FORMAT as the format string that controls the output of time. See the below more information.


Print a summary of the command line options and exit.

-p, --portability

Use the following format string, for conformance with POSIX standard 1003.2:
real %e
user %U
sys %S

-v, --verbose

Use the built-in verbose format, which displays each available piece of information on the program’s resource use on its own line, with an English description of its meaning.


Do not report the status of the program even if it is different from zero.

-V, --version

Print the version number of time and exit.


The elapsed time is not collected atomically with the execution of the program; as a result, in bizarre circumstances (if the time command gets stopped or swapped out in between when the program being timed exits and when time calculates how long it took to run), it could be much larger than the actual execution time.

When the running time of a command is very nearly zero, some values (e.g., the percentage of CPU used) may be reported as either zero (which is wrong) or a question mark.

Most information shown by time is derived from the wait3(2) system call. The numbers are only as good as those returned by wait3(2). On systems that do not have a wait3(2) call that returns status information, the times(2) system call is used instead. However, it provides much less information than wait3(2), so on those systems time reports the majority of the resources as zero.

The ’%I’ and ’%O’ values are allegedly only ’real’ input and output and do not include those supplied by caching devices. The meaning of ’real’ I/O reported by ’%I’ and ’%O’ may be muddled for workstations, especially diskless ones.


The time command returns when the program exits, stops, or is terminated by a signal. If the program exited normally, the return value of time is the return value of the program it executed and measured. Otherwise, the return value is 128 plus the number of the signal which caused the program to stop or terminate.

formatting the output

The format string FORMAT controls the contents of the time output. The format string can be set using the ’-f’ or ’--format’, ’-v’ or ’--verbose’, or ’-p’ or ’--portability’ options. If they are not given, but the TIME environment variable is set, its value is used as the format string. Otherwise, a built-in default format is used. The default format is:
%Uuser %Ssystem %Eelapsed %PCPU (%Xtext+%Ddata %Mmax)k
%Iinputs+%Ooutputs (%Fmajor+%Rminor)pagefaults %Wswaps

The format string usually consists of ’resource specifiers’ interspersed with plain text. A percent sign (’%’) in the format string causes the following character to be interpreted as a resource specifier, which is similar to the formatting characters in the printf(3) function.

A backslash (’\’) introduces a ’backslash escape’, which is translated into a single printing character upon output. ’\t’ outputs a tab character, ’\n’ outputs a newline, and ’\\’ outputs a backslash. A backslash followed by any other character outputs a question mark (’?’) followed by a backslash, to indicate that an invalid backslash escape was given.

Other text in the format string is copied verbatim to the output. time always prints a newline after printing the resource use information, so normally format strings do not end with a newline character (or ’\n’).

There are many resource specifications. Not all resources are measured by all versions of Unix, so some of the values might be reported as zero. Any character following a percent sign that is not listed in the table below causes a question mark (’?’) to be output, followed by that character, to indicate that an invalid resource specifier was given.

The resource specifiers, which are a superset of those recognized by the tcsh(1) builtin ’time’ command, are:


A literal ’%’.


Name and command line arguments of the command being timed.


Average size of the process’s unshared data area, in Kilobytes.


Elapsed real (wall clock) time used by the process, in [hours:]minutes:seconds.


Number of major, or I/O-requiring, page faults that occurred while the process was running. These are faults where the page has actually migrated out of primary memory.


Number of file system inputs by the process.


Average total (data+stack+text) memory use of the process, in Kilobytes.


Maximum resident set size of the process during its lifetime, in Kilobytes.


Number of file system outputs by the process.


Percentage of the CPU that this job got. This is just user + system times divided by the total running time. It also prints a percentage sign.


Number of minor, or recoverable, page faults. These are pages that are not valid (so they fault) but which have not yet been claimed by other virtual pages. Thus the data in the page is still valid but the system tables must be updated.


Total number of CPU-seconds used by the system on behalf of the process (in kernel mode), in seconds.


Total number of CPU-seconds that the process used directly (in user mode), in seconds.


Number of times the process was swapped out of main memory.


Average amount of shared text in the process, in Kilobytes.


System’s page size, in bytes. This is a per-system constant, but varies between systems.


Number of times the process was context-switched involuntarily (because the time slice expired).


Elapsed real (wall clock) time used by the process, in seconds.


Number of signals delivered to the process.


Average unshared stack size of the process, in Kilobytes.


Number of socket messages received by the process.


Number of socket messages sent by the process.


Average resident set size of the process, in Kilobytes.


Number of times that the program was context-switched voluntarily, for instance while waiting for an I/O operation to complete.


Exit status of the command.

see also

tcsh, printf


time was written by David MacKenzie. This man page was added by Dirk Eddelbuettel <edd[:at:]debian[:dot:]org>, the Debian GNU/Linux maintainer, for use by the Debian GNU/Linux distribution but may of course be used by others.

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