On: A disk so full, it couldn't be restored

[pdimitar]

For reasons I am completely unwilling to research, just doing `> filename` has not worked for me in a while.

Since then I memorized this: `cat /dev/null >! filename`, and it has worked on systems with zsh and bash.

[adrianmonk → pdimitar]

That seems to be zsh-specific syntax that is like ">" except that overrides a CLOBBER setting[1].

However, it won't work in bash. It will create file named "!" with the same contents as "filename". It is equivalent to "cat /dev/null filename > !". (Bash lets you put the redirection almost anywhere, including between one argument and another.)

---

[1] See https://zsh.sourceforge.io/Doc/Release/Redirection.html

[pdimitar → adrianmonk]

Yikes, then I have remembered wrong about bash, thank you.

In that case I'll just always use `truncate -s0` then. Safest option to remember without having to carry around context about which shell is running the script, it seems.

[alias_neo → pdimitar]

"truncate -s0 filename"

I believe "> filename" only works correctly if you're root (at least in my experience, if I remember correctly).

EDIT: To remove <> from filename placeholder which might be confusing, and to put commands in quotes.

[pdimitar → alias_neo]

Oh yes, that one also worked everywhere I tried, thanks for reminding me.

[alias_neo → pdimitar]

Pleasure.

It saved me just yesterday when I needed to truncate hundreds of gigabytes of Docker logs on a system that had been having some issues for a while but I didn't want to recreate containers.

"truncate -s 0 /var/lib/docker/containers/**/*-json.log"

Will truncate all of the json logs for all of the containers on the host to 0 bytes.

Of course the system should have had logging configured better (rotation, limits, remote log) in the first place, but it isn't my system.

EDIT: Missing double-star.*

[matja → pdimitar]

Simple to verify with strace -f bash -c "> file":

    openat(AT_FDCWD, "file", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3

man 2 openat:

    O_TRUNC
        If the file already exists and is a regular file and the
        access mode allows writing (i.e., is O_RDWR or O_WRONLY) it
        will be truncated to length 0.
        ...

[pdimitar → matja]

Sure, but I just get an interactive prompt when I type `> file` and I honestly don't care to troubleshoot. ¯\_(ツ)_/¯

[matja → pdimitar]

Probably you are using zsh and need:

    MULTIOS=1 > file

- zsh isn't POSIX compatible by default

[pdimitar → matja]

I see. But in this case it's best to just memorize `truncate -s0` which is shell-neutral.

[matja → pdimitar]

Ok, we'll leave that a mystery then!