time

Measure execution time of a command

$ time [command]

$ /usr/bin/time -v [command]

$ /usr/bin/time -o [time.log] [command]

$ /usr/bin/time -f "Real: %e User: %U Sys: %S" [command]

$ /usr/bin/time -a -o [time.log] [command]

time [command [arguments...]]
/usr/bin/time [-v] [-o file] [-f format] command [arguments...]

time measures the execution time of a command, reporting real (wall clock), user (CPU time in user mode), and system (CPU time in kernel mode) times.
There are two versions: the shell builtin (bash, zsh) and GNU time (/usr/bin/time). The builtin is simpler; GNU time provides more detailed resource statistics and formatting options.
Real time is the actual elapsed time. User time is CPU time executing user code. System time is CPU time in kernel calls. User + System may exceed Real on multi-core systems with parallel execution, or be less if the process waits for I/O.

-v, --verbose

Display detailed resource usage

Write timing statistics to file

Append to output file instead of overwriting

Use specified output format

Use POSIX output format

%e: Elapsed real time (seconds)
%E: Elapsed real time (h:mm:ss)
%U: User CPU time (seconds)
%S: System CPU time (seconds)
%P: Percentage of CPU ((U+S)/E)
%M: Maximum resident set size (KB)
%x: Exit status of command
%C: Command and arguments

The shell builtin time takes precedence. Use /usr/bin/time or \time for the external command with full features.
For accurate benchmarking, run multiple iterations and consider system load. Use tools like hyperfine for statistical benchmarking.
Resource measurements (memory, I/O) are only available with GNU time, not the shell builtin.

timeout(1), watch(1), hyperfine(1), perf(1)