Say we have some arbitrary literals in a file that we need to replace with some other literal.
Normally, we'd just reach for sed(1) or awk(1) and code something like:
sed "s/$target/$replacement/g" file.txt
But what if the $target and/or $replacement could contain characters that are sensitive to sed(1) such as regular expressions. You could escape them but suppose you don't know what they are - they are arbitrary, ok? You'd need to code up something to escape all possible sensitive characters - including the '/' separator. eg
t=$( echo "$target" | sed 's/\./\\./g; s/\*/\\*/g; s/\[/\\[/g; ...' ) # arghhh!
That's pretty awkward for such a simple problem.
perl(1) has \Q ... \E quotes but even that can't cope with the '/' separator in $target.
perl -pe "s/\Q$target\E/$replacement/g" file.txt
I just posted an answer!! So my real question is, "is there a better way to do literal replacements in sed/awk/perl?"
If not, I'll leave this here in case it comes in useful.
Me again!
Here's a simpler way using xxd(1):
t=$( echo -n "$target" | xxd -p | tr -d '\n')
r=$( echo -n "$replacement" | xxd -p | tr -d '\n')
xxd -p file.txt | sed "s/$t/$r/g" | xxd -p -r
... so we're hex-encoding the original text with xxd(1) and doing search-replacement using hex-encoded search strings. Finally we hex-decode the result.
EDIT: I forgot to remove \n from the xxd output (| tr -d '\n') so that patterns can span the 60-column output of xxd. Of course, this relies on GNU sed's ability to operate on very long lines (limited only by memory).
EDIT: this also works on multi-line targets eg
target=$'foo\nbar' replacement=$'bar\nfoo'