Command Hooks with the zsh Shell

This post shows a small example how command hooks of the zsh shell can be used to print timestamps before and after a command execution. This can be helpful in postmortems of operations when it can be important to know exactly when you’ve seen a specific output or how long a command took.

Change history:
Date Change description
2022-01-16 The first release

Example

Note the two lines which contain command start and command end which surround the command execution and its output. For postmortems, I copy and paste the whole block into whatever document we use to collect the details.

Implementation

When you add the code below to the ~/.zshrc and call source ~/.zshrc you get what you’ve seen before.

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preexec() {
    echo "=== `date -Is --utc` command start ===\n"
    CMD_EXEC_TS=`date +%s`
    # $2 resolves aliases
    echo "$ $1"
}

precmd() {
    if [[ ! -z "$CMD_EXEC_TS" ]]; then
        CMD_EXEC_D=$((`date +%s`-CMD_EXEC_TS))
        echo "\n=== `date -Is --utc` (${CMD_EXEC_D}s) command end ==="
    fi
    unset CMD_EXEC_TS
}

The zsh shell makes this possible by offering these command hooks:

  • preexec: Gets called before your command gets executed
  • precmd: Gets called before the prompt for the next command gets shown which looks like it’s happening right after your command gets executed.

The rest of the code in short:

  • date -Is --utc: Show the date in ISO 8601 format, with seconds, for UTC-0. Using UTC-0 makes it easier to compare output from different members of your global team.
  • CMD_EXEC_TS=`date +%s`: Save the date in seconds in a variable
  • CMD_EXEC_D=$((`date +%s`-CMD_EXEC_TS)): Calculate the duration of the command in seconds

You can adjust this to whatever you need though.

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