Question

It seems like my shell is taking way too long to start up. Is there any way to profile it so I can figure out what’s slowing it down so much?

Answer 1

Try adding this at the beginning of the file:

# set the trace prompt to include seconds, nanoseconds, script name and line number
# This is GNU date syntax; by default Macs ship with the BSD date program, which isn't compatible
PS4='+$(date "+%s:%N") %N:%i> '
# save file stderr to file descriptor 3 and redirect stderr (including trace 
# output) to a file with the script's PID as an extension
exec 3>&2 2>/tmp/startlog.$$
# set options to turn on tracing and expansion of commands contained in the prompt
setopt xtrace prompt_subst

and this at the end:

# turn off tracing
unsetopt xtrace
# restore stderr to the value saved in FD 3
exec 2>&3 3>&-

And you should get a detailed log showing the epoch_second.nanosecond time of the execution of each line. Note that GNU date (and OS support) is required to have nanosecond output.

Edit:

added comments

Edit 2:

If you have zsh 4.3.12 or later, you should be able to set PS4 like this instead of using the date command:

zmodload zsh/datetime
setopt promptsubst
PS4='+$EPOCHREALTIME %N:%i> '

which should work on both Linux and OS X to give you nanosecond precision.