The sysread builtin is provided by the zsh/system module. You haveNever did that knowingly, anyway that fixes it. I'll keep an eye on when/what might be loading that module without my knowledge.to load the module and enable sysread before you can use it. zmodload -F zsh/system b:sysread This has already been done in the shell that works, either by you or some code you sourced.