test
Using 13.3-RELEASE and "pkg install gsed" (version 4.9). For the a\ command of sed(1), whether or not specifying the -e option, produces different results. sed ' ... ' (without the -e option) seems to be in error. $ cat t 1 2 $ cat sed-add.sh #!/bin/sh sed '/1/a\ -new line-' <t printf "\n>> sed -e '... '\n" sed -e '/1/a\ -new line-' <t $ sh sed-add.sh 1 -new line-2 >> sed -e ' ... ' 1 -new line- 2 Using variants with double quotes, as in: sed "/1/a\\ -new line-" <t display the same differences. gsed behaves as FreeBSD sed -e ' ... ' I couldn't find any specific POSIX guidance at: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/sed.html but the behaviour of sed -e ' ... ' seems the correct one.
In addition to previous comment: sed '/1/a\ -new line- ' <t results in: 1 -new line- 2 as do all variations over: - double double quotes vs single quotes, - sed vs gsed, - option -e present vs not present. Implicit description of its intended <EOL> behaviour, is mentioned in: sed & awk, 2nd edition; Arnold Robbin, Dale Dougherty at the "Command Summary for sed": " a [address]a\ text Append text following each line matched by address. If text goes over more than one line, newlines must be "hidden" by preceding them with a backslash. The text will be terminated by the first newline that is not hidden in this way. The text is not available in the pattern space and subsequent commands cannot be applied to it. The results of this command are sent to standard output when the list of editing commands is finished, regardless of what happens to the current line in the pattern space. "