time command in Linux with examples

The time command in Linux is used to measure the execution time of a command or program. It provides the amount of time the program took to execute, broken down into real time, user CPU time, and system CPU time.

Syntax

time <command>

Output Breakdown

• real: Total elapsed time (wall clock time) it took to execute the command.
• user: The time spent in user-mode (the time the CPU was working on the command itself).
• sys: The time spent in kernel-mode (system calls and operating system tasks).

Examples

1. Basic Usage

time ls

This will show how long it took to run the ls command.

Output Example:

real    0m0.002s
user    0m0.000s
sys     0m0.002s

• real is the wall-clock time it took to list the directory.
• user and sys times are very low since ls doesn’t require a lot of processing.

2. Using time with a Complex Command

time find / -name "*.log" > result.txt

This command will search for all .log files on the system and redirect the output to result.txt. It will also show how much time was spent to complete the task.

3. Using time with a Program

time ./my_program

This will show how long it took to run a program named my_program in the current directory.

4. Using time with Pipe

time cat largefile.txt | grep "error" > error_logs.txt

This runs cat and grep in a pipeline, and time measures the total time spent running the whole pipeline.

5. Using time for a Command with Multiple Processes

time tar -czf archive.tar.gz folder/

This command creates a compressed tarball from a folder and measures how long it takes.

Additional Options

1. -p (POSIX output format)

time -p ls

This gives a more concise, POSIX-compliant output format:

real 0.01
user 0.00
sys 0.00

2. -v (Verbose output)

time -v ls

This provides a more detailed output:

Command being timed: "ls"
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): 0m0.01s

Summary

The time command is a useful tool for measuring the performance of commands and programs. It gives insights into how much CPU time was spent in user mode, system mode, and the overall real time (elapsed wall clock time).