time

Overview

time runs a command and reports how long it took. Bash provides a builtin time (pipeline-aware) and Ubuntu also ships /usr/bin/time (GNU time) with more format options. Use it for quick performance checks of scripts, builds, and one-liners. For deep profiling use perf, language profilers, or application metrics.

Syntax

time command [args...]          # usually bash builtin
/usr/bin/time [options] command [args...]

Common Options

Bash builtin

Controlled by TIMEFORMAT (see help time / bash manual). No traditional flags like GNU time.

GNU /usr/bin/time (package time)

Option	Description
-v	Verbose resource stats
-p	Portable output (real/user/sys)
-f FORMAT	Custom format string
-o file	Write timing to file
-a	Append to file

If type time shows builtin, call /usr/bin/time explicitly for GNU features (sudo apt install time if missing).

Examples with Explanations

Basic timing

time sleep 2
time tar -czf /tmp/t.tgz /var/log

Reports real (wall clock), user (CPU userspace), sys (CPU kernel).

Time a pipeline (bash builtin)

time du -sh /var/* 2>/dev/null | sort -h | tail

Builtin time can attribute the pipeline as a whole depending on shell rules.

GNU verbose

/usr/bin/time -v sha256sum large.iso

Shows max RSS, page faults, voluntary context switches, exit status, etc.

Portable three lines

/usr/bin/time -p make -j"$(nproc)"

Custom format

/usr/bin/time -f 'real %e sec | maxrss %M KB' ./build.sh

Compare two approaches

time gzip -k bigfile
time zstd -k bigfile

Wall clock is what operators feel; user+sys helps spot CPU-bound vs I/O-bound work.

Bash TIMEFORMAT

TIMEFORMAT='real %3R s; user %3U s; sys %3S s'
time ls -R /usr/share/doc >/dev/null

Notes

• real can exceed user+sys when waiting on disk/network/CPU contention.
• user+sys can exceed real on multi-threaded commands (CPU time summed across cores).
• Output of time often goes to stderr, separate from the command’s stdout.
• Alias/functions: time times the resolved command; use command time / full path if shadowed.
• Micro-benchmarks need warm caches and repeats; single runs mislead.

Related Commands

• perf stat — hardware performance counters
• strace -c — syscall time summary
• hyperfine — benchmarking tool (extra install)
• date +%s.%N — manual timestamps around blocks
• timeout — limit runtime, not measure it

Additional Resources

• help time
• man time (GNU)
• man bash (TIMEFORMAT)