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 firstpart=${mapfile[data]%%$mapfile[delimiter]*} 

Or:

zmodload zsh/mapfile 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. If in a locale using a multibyte charmap such as UTF-8, disabling the multibyte option (set +o multibyte) should make it more efficient).

To print that part verbatim, use:

print -rn -- $firstpart 

Or

printf %s $firstpart 

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