Fixing Corrupt ZSH History

Today, when I logged into my console, I saw following error message

zsh: corrupt history file /home/rishi/.zsh_history

doing a quick search, I was able to reach a stackoverflow link¹ that recommended I run following set of commands

mv .zsh_history .zsh_history_bad
strings .zsh_history_bad > .zsh_history
fc -R .zsh_history

I ran them and everything was handy-dandy again. This did made me wonder what these commands do.

Rename

mv .zsh_history .zsh_history_bad

Well mv should be familiar to anyone who's toyed with linux. It moves files from one path to another. Also often used as a cheap was to rename files as we did here. More details on move in the man page.

Sanitize

strings .zsh_history_bad > .zsh_history

Strings is interesting. I had not come upon it before. Based on the man page:

... strings prints the printable character sequences that are at least 4 characters long (or the number given with the options below) and are followed by an unprintable character...

It seems strings is a nice way to actually get the printable characters in a file. In our case, this means if the .zsh_history was corrupted due to non-printable characters², this will help parse them out. It also means if you ran any commands with non-printable characters, you've now lost them from your history as well. I'm very unlikely to run printable characters, so this wasn't as big of an issue but this may make past commands for QAs and pen-testers a tad-annoying.

Parse

fc -R .zsh_history

fc was interesting. It doesn't show up in the manpage. I ended up in a searching a fair amount to realize that this is a zsh-built in feature. There's some information about it within zsh documentation.

...'fc -R’ reads the history from the given file

It seems fc will interact with your zsh_history. In this case, we are reading from the file and, since no second file was mentioned, putting it back into the same history file. i.e. re-writing history by parsing the same file.

Cleanup

rm .zsh_history_bad 

This wasn't part of the original script but you should always clean-up after yourself. This command deletes the tmp "bad" file that you created in the first step.

Conclusion

If I power shutdown my VM / linux box while zsh is still writing content, I can still corrupt the zsh_history. As long as I don't care about any other non-printable characters in the past and open to keep using zsh, I can run the above commands to get back to a happier stage

1. Technically not stackoverflow but I hope you know what I mean. ↩

2. Probably because I did a power shutdown while carrying out a load test on a tiny VM. ↩