bash: Read from stdin until a string delimiter

Perl to the rescue!

perl -e 'local $/; open $de, "<", "delimiter" or die $!; $/ = <$de>; open $da, "<", "data" or die $!; chomp( $first = <$da> ); print $first;' 

The special variable $/ sets the input record separator, by localising it, we will read the whole file (also called "slurping"). We then use the diamond operator to read from the delimiter file, and set the separator to its content. We then read the first record from the data file, chomping the record separator from it.

With zsh (the only shell that can store arbitrary byte sequences in its variables), assuming data and delimiter are regular (or at least mmap()able) files, you can do:

zmodload zsh/mapfile set +o multibyte # necessary so sequences of bytes that # happen to form valid characters may be # broken in the middle if necessary. firstpart=${mapfile[data]%%$mapfile[delimiter]*} 

Or:

zmodload zsh/mapfile set +o multibyte # necessary so sequences of bytes that # happen to form valid characters may be # broken in the middle if necessary. delimiter=$mapfile[delimiter] parts=( ${(ps[$delimiter])mapfile[data]} ) firstpart=$parts[1] 

(don't expect it to be very efficient nor to scale well to files larger than a few hundred megabytes).

To print that part verbatim, use:

print -rn -- $firstpart 

Or

printf %s $firstpart