Yes I know there are a number of questions (e.g. (0) or (1)) which seem to ask the same, but AFAICS none really answers what I want.

What I want is, to replace any occurrence of a newline (LF) with the string \n, with no implicitly assumed newlines... and this with POSIX only utilities (and no GNU extensions or Bashisms) and input read from stdin with no buffering of that is desired.

So for example:

• printf 'foo' | magic should give foo
• printf 'foo\n' | magic should give foo\n
• printf 'foo\n\n' | magic should give foo\n\n

The usually given answers, don't do this, e.g.:

• awk
  printf 'foo' | awk 1 ORS='\\n gives foo\n, whereas it should give just foo
  so adds an \n when there was no newline.
• sed
  would work for just foo but in all other cases, like:
  printf 'foo\n' | sed ':a;N;$!ba;s/\n/\\n/g' gives foo, whereas it should give foo\n
  misses one final newline.
  Since I do not want any sort of buffering, I cannot just look whether the input ended in an newline and then add the missing one manually.
  And anyway... it would use GNU extensions.
  sed -z 's/\n/\\n/g'
  does work (even retains the NULs correctly), but again, GNU extension.
• tr
  can only replace with one character, whereas I need two.

The only working solution I'd have so far is with perl:
perl -p -e 's/\n/\\n/'
which works just as desired in all cases, but as I've said, I'd like to have a solution for environments where just the basic POSIX utilities are there (so no Perl or using any GNU extensions).

Thanks in advance.

CodePudding user response：

Here is a tr sed solution that should work on any POSIX shell as it doesn't call any gnu utility:

printf 'foo' | tr '\n' '\7' | sed 's/\x7/\\n/g'
foo

printf 'foo\n' | tr '\n' '\7' | sed 's/\x7/\\n/g'
foo\n

printf 'foo\n\n' | tr '\n' '\7' | sed 's/\x7/\\n/g'
foo\n\n

Details:

• tr command replaces each line break with \x07
• sed command replace each \x07 with \\n

CodePudding user response：

One way would be to use Awk to modify the default value of input RS to some random character, so that the \n character (otherwise default value RS) is available for replacement with literal \\n

printf 'foo'      |  awk '{ gsub("\n", "\\n") }1' RS="#"
foo
printf 'foo\n'    |  awk '{ gsub("\n", "\\n") }1' RS="#"
foo\n
printf 'foo\n\n'  |  awk '{ gsub("\n", "\\n") }1' RS="#"
foo\n\n