Profiling ZSH startup time

Recently I had a very weird problem with iTerm where new login shells were being created with environment variables already present. Restarting my machine made the issue go away, and I wasn't able to reproduce it again.

But I got curious about how long ZSH spends in various parts of the startup process. A new /bin/bash login shell loads instantaneously and I was wondering why my ZSH startup was so slow, and if there was anything I could do to make it faster. My goal was to get to a command prompt in under 100 milliseconds.

What files run when you start a ZSH login shell?

In order, your machine will load/execute the following files when ZSH starts:


On my machine only /etc/zshenv (which adds paths in /etc/paths* to the $PATH variable) and ~/.zshrc actually existed, which simplified the problem somewhat.

I used the following two scripts to check execution time. First, this snippet logged the execution time of every command run by my startup script in ~/tmp/startlog.<pid>.

    if [[ "$PROFILE_STARTUP" == true ]]; then
        PS4=$'%D{%M%S%.} %N:%i> '
        exec 3>&2 2>$HOME/tmp/startlog.$$
        setopt xtrace prompt_subst
    # Entirety of my startup file... then
    if [[ "$PROFILE_STARTUP" == true ]]; then
        unsetopt xtrace
        exec 2>&3 3>&-

Essentially this prints the command that zsh is running before you run it. The PS4 variable controls the output of this print statement, and allows us to insert a timestamp each time we run it.

I then wrote a short python script to print all lines above a certain threshold. This is okay, but can show a lot of detail and won't show you if one line in your .zshrc is causing thousands of lines of execution, which take a lot of time in total.


At first I used the GNU date program in the PS4 prompt to get millisecond information about the time, however this program consistently used 3 milliseconds to run, so it got costly to run it thousands of times during system profiling.

The first thing you find is that shelling out is very expensive, especially to high-level languages. Running something like brew --prefix takes 50 milliseconds which is half of the budget. I found that Autojump's shell loader ran brew --prefix twice so I submitted this pull request to cache the output of that and run it again.

Another timing method

Ultimately what I want is to time specific lines/blocks of my zshrc, instead of get profiling information for specific lines. I did this by wrapping them in time commands, like this:

    { time (
        # linux
        (( $+commands[gvim] )) && {
            alias vi=gvim;
            alias svi='sudo gvim'
        # set up macvim, if it exists
        (( $+commands[mvim] )) && {
            alias vi=mvim;
            alias svi='sudo mvim'
    ) }

This will time commands in a sub-shell, which means that any environment variables set in the sub-shell won't be set in the environment. However the purpose is to get timing information and it's good enough for that.

Lazy loading

By far the worst offenders were the various "Add this to your .zshrc" scripts that I had added in the past - virtualenvwrapper, travis, git-completion, autojump, pyenv, and more. I wanted to see if there was a way to load these only when I needed them (I don't, frequently). Turns out there is! Most of these set functions in zsh, so I can shadow them with my own functions in a zshrc. Once the file with the actual function definition is sourced, it'll replace the shim and I'll be fine. Here's an example for autojump:

    function j() {
        (( $+commands[brew] )) && {
            local pfx=$(brew --prefix)
            [[ -f "$pfx/etc/" ]] && . "$pfx/etc/"
            j "$@"

Or for pyenv:

pyenv() {
    eval "$( command pyenv init - )"
    pyenv "$@"

Essentially on the first invocation, these functions source the actual definition and then immediately call it with the arguments passed in.


Ultimately I was able to shave my zsh startup time from 670 milliseconds to about 390 milliseconds and I have ideas on how to shave it further (rewriting my weirdfortune program in Go for example, to avoid the Python/PyPy startup cost). You can probably get similar gains from examining your own zshrc.

Liked what you read? I am available for hire.

6 thoughts on “Profiling ZSH startup time

  1. mgroshans

    Lazy loading makes so much sense now that I think of it that it seems crazy that everything doesn’t do this automatically.

    Just wanted to add that if the command you want to load lazily doesn’t replace the function (for example if it adds an alias instead) then you can easily just add “unset -f yourfunctionname” at the top of the function.

  2. Pingback: Profiling zsh shell scripts | Xebia Blog

  3. Charles

    zsh user can use `try` block to handle this
    { . $(brew –prefix)/etc/profile.d/ } || # error handling


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