Master Linux ‘time’ Command: Custom Formats, Options, and Resource Metrics

When you need to analyze a program’s resource consumption—such as user‑mode time, kernel‑mode time, average memory usage, and more—Linux provides the dedicated utility /usr/bin/time. It accepts a program name and arguments, then reports detailed resource information, warning you if the command exits with a non‑zero status.

Basic Syntax and Common Options

The general form is: /usr/bin/time [options] program [arguments] Key options include: -v or --verbose: output a detailed description. --quiet: suppress the command’s exit status report. -f or --format: let you control the output format. -p or --portability: produce POSIX‑compatible output.

Default Output Example

Running /usr/bin/time ls without options yields something like:

# /usr/bin/time ls
anaconda-ks.cfg  bin  install.log  install.log.syslog  mbox
0.00user 0.00system 0:00.00elapsed 0%CPU (0avgtext+0avgdata 3888maxresident)k
0inputs+0outputs (0major+304minor)pagefaults 0swaps

The last two lines are the resource information generated by time. The default format corresponds to the following specifiers:

%Uuser %Ssystem %Eelapsed %PCPU (%Xtext+%Ddata %Mmax)k
%Iinputs+%Ooutputs (%Fmajor+%Rminor)pagefaults %Wswaps

Custom Format Strings

Using -f (or --format) you can specify exactly which metrics to display. A format string consists of plain text interspersed with “resource specifiers” introduced by a percent sign (%). Escape sequences such as \t (tab) and \n (newline) are supported.

Example:

$ /usr/bin/time -f "\t%U user,\t%S system,\t%x status" date
Sun Jan 22 17:46:58 IST 2012
    0.00 user, 0.00 system, 0 status

Resource Specifiers

Below is a non‑exhaustive list of specifiers taken from the manual page, each followed by a brief description: C: command name and arguments. D: average size of the process’s non‑shared data area (KB). E: elapsed (wall‑clock) time ([hh:]mm:ss). F: major (I/O) page faults. I: file system inputs. K: average total memory (data+stack+text) (KB). M: maximum resident set size (KB). O: file system outputs. P: CPU percentage (user+system)/elapsed. R: minor (recoverable) page faults. S: CPU seconds spent in kernel mode. U: CPU seconds spent in user mode. W: number of times the process was swapped out. X: average shared text size (KB). Z: system page size (bytes). c: involuntary context switches. e: elapsed real time (seconds). k: signals received. p: average non‑shared stack size (KB). r: socket messages received. s: socket messages sent. t: average resident set size (KB). w: voluntary context switches. x: command exit status.

Why Use /usr/bin/time Instead of the Shell Built‑in?

Invoking time without the full path runs the Bash built‑in, which lacks the extended options such as -f. Attempting time -f "..." date results in a “command not found” error for -f and falls back to the default output format. To access the full feature set, always call the external binary via its absolute path, e.g., /usr/bin/time. You can consult the manual with man time for the external command and help time for the Bash built‑in.