Emitting binary from POSIX shell scripts

comparison.md

Emitting binary from POSIX shell scripts

Different (supposedly) POSIX compliant shell environments yield varied results when asked to emit non-printable characters. But there is one method that works consistently across a wide variety of shells.

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Comparison

Given

hex='\x40\x00\x40'
hex2='\\x40\\x00\\x40'
oct='\100\000\100'
oct2='\0100\0000\0100'

you get this:

	dashᵃ	dashᵇ	ksh	bash	BusyBox sh	zsh
echo -n "$hex"	$hex¹	$hex	$hex	$hex	$hex	@␀@¹
printf "$hex"²	$hex	$hex	@␀@	@␀@	@␀@	@␀@
printf '%b' "$hex"³	$hex	$hex	$hex	@␀@	@␀@	@␀@
echo -n "$hex2"	$hex	$hex	$hex2	$hex2	$hex2	$hex
printf "$hex2"	$hex	$hex	$hex	$hex	$hex	$hex
printf '%b' "$hex2"	$hex	$hex	$hex	$hex	$hex	$hex
echo -n "$oct"	@␀@	@	$oct	$oct	$oct	\100␀\100¹
printf "$oct"*	@␀@	@␀@	@␀@	@␀@	@␀@	@␀@
printf '%b' "$oct"	@␀@	@	\100␀\100	@␀@	@␀@	\100␀\100
echo -n "$oct2"	@␀@	@	$oct2	$oct2	$oct2	@␀@
printf "$oct2"	␈0␀0␈0¹	␈0␀0␈0	␈0␀0␈0	␈0␀0␈0	␈0␀0␈0	␈0␀0␈0
printf '%b' "$oct2"	@␀@	@	@␀@	@␀@	@␀@	@␀@

• ᵃ These results are from running dash in an Ubuntu Docker container.
• ᵇ This is from running dash on my host system, where it apparently interpreted the octal sequence as a C style (zero-terminated) string.
• ¹ Italicized $var name stands for the unchanged literal value of the corresponding variable;
  @␀@ represents the expected byte sequence;
  ␀ stands for the null character (00) and ␈ stands for the backspace character (08);.
• ² A POSIX-compliant printf is not supposed to recognize "hexadecimal character constants as defined in the ISO C standard".
• ³ The b conversion specifier character is supposed to only convert \0ddd octal sequences, according to the POSIX spec.
• * The only one that yields consistent results — printf "$oct".

Conclusion

The only approach that works consistently across all shells is using the octal escape sequence as the printf format string (printf '\100\000\100').