I'm using the command printf of bash on Ubuntu under WSL.

I'm trying to use this documented syntax

\xHH byte with hexadecimal value HH (1 to 2 digits)

This works for printf "\x0A" (carriage return) and many other values.

But does not work for printf "\xFF" or printf "\xFE" which I want to use to inject a BOM Utf-16 prefix before piping the content of a Utf-16 file (but without bom) to some treatment.

For this values (past 0xF8 included), I get the error:

-bash: printf: write error: Input/output error

  • Do you want help with to get printf to work as you expect (or to get a description of why it doesn't work), or do you want help with adding a BOM to the data stream (regardless of whether this is done with printf or not)?
    – Kusalananda
    Apr 13, 2022 at 7:23
  • Both, my captain! In priority an help to add a BOM to a data stream. My plan was this { printf "\xFE\xFF"; cat some_utf-16_file.pag ; } | awk -f breakdown_on_invoice_nb.awk. Get printf to work would be nice too. Understanding would be a bonus.
    – Sandburg
    Apr 13, 2022 at 7:32
  • Tested your plan on Ubuntu (not WSL), and it works. So this may be related to WSL.
    – Ole Tange
    Apr 13, 2022 at 7:41
  • If you are faking utf-16 in printf, setting LC_ALL=C temporarily may sidestep the issue. Apr 13, 2022 at 8:06
  • 1
    @terdon I had a quick play on WSL (Ubuntu 20.04) and can reproduce the issue in bash, ksh, zsh and /usr/bin/printf. Curiously it appears to work fine on another machine with (as far as I can tell) the same Windows build version and WSL locale. Apr 13, 2022 at 13:43

1 Answer 1


Thanks to @Steeldriver for originally reproducing this. I wasn't able to myself (at least not at first), but having both you and Steeldriver confirm it lead me to dig a little deeper.

This appears to be a manifestation of the behavior that was reported in this Github issue. It was closed as being not specifically a WSL issue, but likely a problem in the legacy Windows Console Host.

Console Host is the "old" Windows terminal interface, going back at least two decades. I don't know that the issue was ever reported against it, but it would have been unlikely to have been fixed anyway.

Windows Console Host is in the process of being replaced by Windows Terminal, a (relatively) new, open-source terminal developed by Microsoft with modern terminal features. It can currently run as the default terminal on Windows 11. It can also run (just not as the default) on Windows 10.

I have confirmed that the problem does not occur on Windows Terminal.

Since the Github issue was from before WSL2 came out, that gave me a hunch as to why Steeldriver only saw it on one system, and I originally didn't see it. Interestingly, the problem (for me at least) only occurs in WSL1, not WSL2. This means that there's likely some incompatibility in the syscall translation interface in WSL1 that is triggering it. WSL2's Linux kernel does not have the issue.

So to resolve, either:

  • Use Windows Terminal (or any non-Console Host terminal on Windows)
  • Or use a WSL2 instance.

The former is my preference, since you can continue to use both WSL1 and WSL2 while taking advantage of the features of the upgraded Windows Terminal.

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