􀀂􀀟􀀍􀀆 􀀂􀀖􀀌􀀆 Shell startup precomputation

A month ago, Thorsten Ball wrote How fast is your shell?, encouraging users to profile their shells to make sure they start up quickly. His results are about 70ms - better than mine, but I still think mine are OK.

> /usr/bin/time zsh -i -c exit
        0.18 real         0.05 user         0.04 sys

A couple of days later, Laurence Tratt wrote Faster Shell Startup With Shell Switching, which suggests using different shells for scripts and interactive use. (A real callback to arguments¹ about the C shell² on Usenet!)

This post is about my suggestion for improving shell startup time: precomputation.

I have a script called dhd-shdetect that detects a bunch of stuff I want to account for in my shell that rarely changes, caching it in the file $HOME/.shdetect_dhd.sh. Anything that it can precompute is code that doesn’t have to run at every shell startup. Then, as much as possible, .zshrc and .profile files use the precomputed values rather than running code themselves. I’m not fanatic about this; if the best thing to do is run code at shell startup, then I do.

A few examples:

1. I have a script called pathsetup which takes a list of paths that may exist, and returns all those in the list that actually do exist. It takes a while to check all these paths, so dhd-shdetect caches the result.

   # Stuff that we want to use, only if it exists on the filesystem
   POSSIBLE_PATHS="
   ${HOME}/opt/bin
   ${HOME}/opt/sbin
   ${HOME}/.cabal/bin
   ${HOME}/.cabal/sbin
   ... dozens more entries
   "
   # Stuff we want to ignore even if it does exist
   IGNORE_PATHS="
   ${HOME}/.nvm/*
   "
   export PATH=$("$DHD/opt/bin/pathsetup" "$POSSIBLE_PATHS" "$IGNORE_PATHS")
   

   This is saved directly to the $HOME/.shdetect_dhd.sh file, which is dot-sourced in the .profile script.

2. Detecting the ls(1) implementation in dhd-shdetect requires actually running the binary:

   DHD_LS_TYPE=
   if ls --version 2>/dev/null | grep -q GNU; then
       DHD_LS_TYPE=gnu
   elif ls -h -G >/dev/null 2>&1; then
       DHD_LS_TYPE=bsd
   fi
   

   This means that in .zshrc, we only have to read the cached value:

   case "$DHD_LS_TYPE" in
       gnu) alias ls="ls -FhN --color=always";;
       bsd) alias ls="ls -FhG";;
       *) alias ls="ls -F";;
   esac
   

3. dhd-shdetect looks for specific commands that the profile scripts use if they are available. It saves them to a variable called $DHD_CMDAVAIL, and provides a function for fast lookups into that variable, avoiding the need to scan $PATH during shell startup.

   In dhd-shdetect:

   # Available commands
   # Can test for them with
   #    test "${DHD_CMDAVAIL#*cmd_name}" != "$DHD_CMDAVAIL"
   DHD_CMDAVAIL=
   cmdavail fortune && DHD_CMDAVAIL="$DHD_CMDAVAIL fortune"
   cmdavail kubectl && DHD_CMDAVAIL="$DHD_CMDAVAIL kubectl"
   # ... several more
   

   The result saved to $HOME/.shdetect_dhd.sh:

   export DHD_CMDAVAIL="$DHD_CMDAVAIL"
   
   # ...
   
   # Very fast way to test precomputed command availability
   dhd_cmdavail() {
       test "\${DHD_CMDAVAIL#*\$1}" != "\$DHD_CMDAVAIL"
       return \$?
   }
   

   Called in .zshrc:

   dhd_cmdavail kubectl && source <(kubectl completion zsh)
   

I can run as many programs as I want to influence my shell configuration, but I don’t have to pay the time cost every time my shell starts up. It’s a significant time savings, as running dhd-shdetect takes over a second:

> /usr/bin/time dhd-shdetect
Generated data saved to /Users/mrled/.shdetect_dhd.sh
Now log in again, or run the '.z' alias
        1.03 real         0.43 user         0.32 sys

That’s time I don’t have to spend waiting for new terminals ✨.