Mastering Linux ‘time’: Measure, Log, and Customize Command Execution

1. Basic usage of time

The simplest form is time <command>. For example:

$ time ping baidu.com
PING baidu.com (123.125.114.144) 56(84) bytes of data.
64 bytes from 123.125.114.144: icmp_seq=1 ttl=56 time=2.83 ms
...
--- baidu.com ping statistics ---
8 packets transmitted, 8 received, 0% packet loss, time 10818ms
rtt min/avg/max/mdev = 2.765/2.808/2.862/0.039 ms

real    0m11.173s
user    0m0.004s
sys     0m0.002s

In the result, real is the wall‑clock time from start to termination, while user and sys show the CPU time spent in user and kernel space respectively.

2. Writing timing information to a file

Use the -o option to direct the timing summary to a file while the command’s normal output still appears on the terminal:

$ /usr/bin/time -o /home/alvin/time-output.txt ping baidu.com

If you want to append instead of overwriting, use -a:

$ /usr/bin/time -a /home/smart/time-output.txt ping linoxide.com

3. Getting more detailed statistics

Adding the -v flag expands the report with many additional fields such as page faults, context switches, memory usage, and I/O statistics.

$ /usr/bin/time -v ping baidu.com
...
Command being timed: "ping baidu.com"
User time (seconds): 0.00
System time (seconds): 0.00
Percent of CPU this job got: 0%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:03.64
Maximum resident set size (kbytes): 2140
... (other detailed metrics)

4. Customizing the output format

The -f option lets you specify a format string composed of format specifiers. Some common specifiers are: %E – elapsed (wall‑clock) time %I – number of file system inputs %O – number of file system outputs %P – CPU usage percentage %M – maximum resident set size (KB) %x – exit status

Example of a custom format that prints elapsed time, inputs, and outputs:

$ /usr/bin/time -f "Elapsed Time = %E, Inputs %I, Outputs %O" ping baidu.com

To insert line breaks, include \n in the format string:

$ /usr/bin/time -f "Elapsed Time = %E 
 Inputs %I 
 Outputs %O" ping baidu.com

Result:

Elapsed Time = 0:03.92
Inputs 0
Outputs 0

5. Summary of useful options

time <command>

– basic timing -o <file> – write summary to file (overwrite) -a <file> – append summary to file -v – verbose, detailed statistics -f <format> – custom output format using specifiers

These options enable precise performance measurement for scripts and commands, especially in environments where timing accuracy is critical.