Master Linux’s /usr/bin/time: Measure CPU, Memory, and More with Custom Formats

Sometimes you need to analyze a program’s resource usage, such as time spent in user mode, time spent in kernel mode, average memory consumption, and other metrics. Linux provides a dedicated utility called /usr/bin/time that reports this information and can also indicate warnings and exit status when a command fails.

Basic Syntax

The syntax of the time command is: /usr/bin/time [options] program [arguments] In this syntax, options are optional flags that control specific features of the utility.

Common Options

-v, --verbose Show a detailed description of the output.

--quiet Suppress status messages from the time utility.

-f, --format Specify a custom output format.

-p, --portability Produce output conforming to POSIX.

-o FILE, --output=FILE Redirect the output to a file, overwriting it.

-a, --append Append the output to the specified file instead of overwriting.

Default Output Example

# /usr/bin/time ls
0.00user 0.00system 0.00:00.00elapsed 0%CPU (0avgtext+0avgdata 3888maxresident)k 0inputs+0outputs (0major+304minor)pagefaults 0swaps

The last two lines of the output contain the resource usage information.

Format Options

The -f option lets you define a custom format string. Format strings consist of literal text and “resource specifiers” introduced by a percent sign (%). Escape sequences such as \t (tab) and \n (newline) are also supported.

Example:

/usr/bin/time -f "\t%U user,\t%S system,\t%x status" date

Output:

0.00 user,    0.00 system,    0 status

Resources and Specifiers

The utility can report a wide range of resources, each identified by a single‑letter specifier (from the manual page):

C – Command name and arguments.

D – Average size of the process’s non‑shared data area (KB).

E – Elapsed real (wall‑clock) time [hours:]minutes:seconds.

F – Major (requiring I/O) page faults.

I – Number of file system inputs.

K – Average total (data+stack+text) memory usage (KB).

M – Maximum resident set size (KB).

O – Number of file system outputs.

P – Percentage of CPU used (user+system)/elapsed.

R – Minor (reclaimable) 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 amount of shared text (KB).

Z – System page size (bytes).

c – Voluntary context switches.

e – Elapsed real time (seconds).

k – Number of signals delivered.

p – Average non‑shared stack size (KB).

r – Number of socket messages received.

s – Number of socket messages sent.

t – Average resident set size (KB).

w – Voluntary context switches (e.g., waiting for I/O).

x – Exit status of the command.

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

Running time without the full path invokes the Bash built‑in, which does not recognize many of the options described above. For example, using time -f "\t%U user,\t%S system,\t%x status" date without the full path results in a “command not found” error for -f and produces output in the default format rather than the requested custom format.

To access the full-featured utility, use the explicit path /usr/bin/time or consult the manual page with man time. The built‑in help can be viewed with help time.