• Profile slow bashrc
  • The core idea
  • Tracing script
  • Top slowest commands
  • Interpreting the results

I have been growing my bashrc bits by bits for years now, and coming back to an older computer made me realize how slow spawning a new terminal is.

Here is how I was able to bring down my 2 seconds terminal boot time to below 200ms.

With bash, you have:

• set -x to tells what runs and where,
• PS4 to inject text for debuglines,
• ${EPOCHREALTIME} to display timestamps in PS4

So basically, the profiling happens by:

1. emitting timestamps, and
2. computing deltas.

Save this as /tmp/profile-bashrc-xtrace.sh:

#!/usr/bin/env bash
# Open a dedicated FD for xtrace output (robust against FD reuse)
exec {__XFD}>"/tmp/bashrc-xtrace.$$".log
export BASH_XTRACEFD=$__XFD
# Prefix each traced command with:
#   +<timestamp> <file>:<line>:
export PS4='+${EPOCHREALTIME} ${BASH_SOURCE}:${LINENO}: '
# Enable tracing and source bashrc
set -x
source ~/.bashrc
set +x

# Close trace FD
exec {__XFD}>&-

Run it with a clean shell:

bash --noprofile --norc -i /tmp/profile-bashrc-xtrace.sh

This produces a log like:

+1702901234.421391 /home/me/.bashrc:1380: stow ...
+1702901234.567842 /home/me/.bashrc:1471: curl ...

This analyzer:

• extracts only the numeric timestamp,
• handles , VS . defensively,
• computes delta time spent
• attributes the delta to the previous command,

  awk '
    /^\+/ {
      line=$0
      sub(/^\+/, "", line)
      # Extract timestamp (first token only)
      t=line
      sub(/[ \t].*$/, "", t)
      gsub(/,/, ".", t)
      loc=$2
      cmd=""
      for (i=3; i<=NF; i++) cmd = cmd $i (i<NF ? " " : "")
      if (prev_t != "") {
        dt = (t - prev_t) * 1000
        if (dt >= 0)
          printf "%.1f ms\t%s\t%s\n", dt, prev_loc, prev_cmd
      }
      prev_t=t; prev_loc=loc; prev_cmd=cmd
    }
  ' "/tmp/bashrc-xtrace.*.log" | sort -nr | head -30
  

What showed up at the top:

• "curl --max-time …" -> network probes I don't needed any more
• version managers ("nvm", "pyenv", etc.) -> those I need daily unfortunately
• useless "stow" call -> it was a cleanup command, but now all laptops are clean, I can safely remove it

Things to try to solve them (YMMV):

• move heavy work out of startup,
• cache results,
• background probes,
• lazy-load toolchains.