time - Linux


The time command in Linux measures the duration of time a given command takes to execute. It provides a way to ascertain the performance and efficiency of commands by detailing the real, user CPU, and system CPU times. It is most effective in performance testing, optimizing scripts, and spotting inefficiently running processes.


The basic syntax of the time command is as follows:

time [options] command [arguments...]
  • command: This is the command for which you want to measure the execution time.
  • arguments: These are the arguments passed to the command.


Here are some commonly used options with the time command:

  • -p, --portability: This option ensures the output is in POSIX format.
  • -f FORMAT, --format=FORMAT: Allows you to specify a custom format for the output.
  • -o FILE, --output=FILE: Direct the output to a file instead of standard output.
  • --append: This option is used with -o to append the output to the specified file instead of overwriting it.
  • --verbose: Prints detailed information about the command execution.

Each option modifies the behavior of the time command, allowing for more tailored information regarding the execution times.


  1. Basic Usage:
    Time the duration of the ls command:

    time ls
  2. Using Custom Format:
    Time a script and display the user and system time:

    time -f "User time: %U\nSystem time: %S" ./script.sh
  3. Output to File:
    Save the timing information to a file:

    time -o timing_info.txt ./script.sh
  4. Append Output to File:
    Append output to an existing file:

    time --append -o timing_info.txt ./script.sh

Common Issues

  • Permission Denied: Make sure that the script or command you are timing is executable. Use chmod +x script.sh if necessary.
  • Format Errors: Incorrect format strings in the -f option can cause errors. Ensure the format specifiers are correctly used.


Combine time with other commands to create powerful one-liners or scripts:

  • Comparing Execution Times:
    Compare the execution time of two scripts:

    time ./script1.sh ; time ./script2.sh
  • Using with Loops:
    Time commands executed in a loop:

    time for i in {1..10}; do ./perform_task.sh; done
  • top: Display all running processes in real-time.
  • ps: Report a snapshot of the current processes.
  • cron: Schedule commands to run periodically at fixed times, dates, or intervals.

For further reading and more detailed information, consult the time man page by typing man time in the terminal or visit GNU Time Documentation.