Here's what I've got in ~/.zshenv (/etc/zshenv is blank)

print -P "before %D{%H:%M:%S}"
/usr/bin/time /bin/cat /dev/null
print -P "after %D{%H:%M:%S}"

If I try to start zsh, it will occasionally hang on the cat command for up to 30 seconds:

bash$ zsh -c exit
before 16:04:08
  0.00s user 0.00s system 0% cpu 0.001 total
after 16:04:29

If I open another new zsh immediately afterwards, the hanging never happens. If I wait at least two minutes first, it almost always happens.

Why is this happening? How can I stop it?

This is zsh-4.3.11 on CentOS 6.9.

  • 3
    Any clue from strace -f zsh -c exit? – Stéphane Chazelas Aug 24 '18 at 20:34
  • 2
    Is it only cat? And only when passed /dev/null? What about /bin/true or paste /dev/null? – Stéphane Chazelas Aug 24 '18 at 20:43
  • It originally was cating a different (empty) file, and I switched to /dev/null to see if the choice of file made a difference. Looking at the output of time above, I think that's the command that's hanging now, not cat. Maybe it's the first non-native call? Originally I had multiple cats, and it was always only ever the first one that hung. I'll try strace when I get in Monday. – rampion Aug 25 '18 at 16:17
  • make it strace -fTtt zsh -c exit for timing information. – Stéphane Chazelas Aug 25 '18 at 21:37

The slowdown was due to hash. One of the default directories the local SAs had put in my path had 8k+ entries in it.

Using the strace command suggested by Stéphane Chazelas revealed that the stat() calls used by hash were causing the slowdown.

If I opened another shell immediately afterwards, the each stat() call was up to 180x faster (presumably due to caching), which explains why the slowdown didn't happen every time.

As a fix, I removed the directory from my PATH. If I need any executables from there, I'll just put symlinks to them in a smaller directory in my PATH.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.