I want to be able to capture the exact output of a command substitution, including the trailing new line characters.I realise that they are stripped by default, so some manipulation may be required to keep them, and I want to keep the original exit code.For example, given a command with a variable number of trailing newlines and exit code:f(){ for i in $(seq $((RANDOM % 3))); do echo; done; return $((RANDOM % 256));}export -f fI want to run something like:exact_output fAnd have the output be:Output: $'\n\n'Exit: 5I'm interested in both bash and POSIX sh.
shell: keep trailing newlines ('\n') in command substitution
bash;shell
POSIX shellsThe usual (123456789101112131415) trick to get the complete stdout of a command is to do:output=$(cmd; ret=$?; echo .; exit $ret)ret=$?output=${output%.}The idea is to add and extra .\n. Command substitution will only strip that \n. And you strip the . with ${output%.}.Note that in shells other than zsh, that will still not work if the output has NUL bytes. With yash, that won't work if the output is not text.Also note that in some locales, it matters what character you use to insert at the end. . should generally be fine, but some other might not. For instance x (as used in some other answers) or @ would not work in a locale using the BIG5, GB18030 or BIG5HKSCS charsets. In those charsets, the encoding of a number of characters ends in the same byte as the encoding of x or @ (0x78, 0x40)For instance,  in BIG5HKSCS is 0x88 0x78 (and x is 0x78 like in ASCII, all charsets on a system must have the same encoding for all the characters of the portable character set which includes English letters, @ and .). So if cmd was printf '\x88' and we inserted x after it, ${output%x} would fail to strip that x as $output would actually contain .Using . instead could lead to the same problem in theory if there were any characters whose encoding ends in the same encoding as ., but for having checked some time ago, I can tell that none of the charsets that may be available for use in a locale in a Debian, FreeBSD or Solaris systems have such characters which is good enough for me (and why I've settled on . which is also the symbol to mark the end of a sentence in English so seems appropriate).A more correct approach as discussed by @Arrow would be to change the locale to C only for the stripping of the last character (${output%.}) which would make sure only one byte is stripped, but that would complicate the code significantly and potentially introduce compatibility issues of its own.bash/zsh alternativesWith bash and zsh, assuming the output has no NULs, you can also do:IFS= read -rd '' output < <(cmd)To get the exit status of cmd, you can do wait $!; ret=$? in bash but not in zsh.rc/es/akanagaFor completeness, note that rc/es/akanga have an operator for that. In them, command substitution, expressed as `cmd (or `{cmd} for more complex commands) returns a list (by splitting on $ifs, space-tab-newline by default). In those shells (as opposed to Bourne-like shells), the stripping of newline is only done as part of that $ifs splitting. So you can either empty $ifs or use the ``(seps){cmd} form where you specify the separators:ifs = ''; output = `cmdor:output = ``()cmdIn any case, the exit status of the command is lost. You'd need to embed it in the output and extract it afterwards which would become ugly.fishIn fish, command substitution is with (cmd) and doesn't involve a subshell.set var (cmd)Creates a $var array with all the lines in the output of cmd if $IFS is non-empty, or with the output of cmd stripped of up to one (as opposed to all in most other shells) newline character if $IFS is empty.So there's still an issue in that (printf 'a\nb') and (printf 'a\nb\n') expand to the same thing even with an empty $IFS.To work around that, the best I could come up with was:function exact_output  set -l IFS . # non-empty IFS  set -l ret  set -l lines (    cmd    set ret $status    echo  )  set -g output ''  set -l line  test (count $lines) -le 1; or for line in $lines[1..-2]    set output $output$line\n  end  set output $output$lines[-1]  return $retendAn alternative is to do:read -z output < (begin; cmd; set ret $status; end | psub)Bourne shellThe Bourne shell did not support the $(...) form nor the ${var%pattern} operator, so it can be quite hard to achieve there. One approach is to use eval and quoting:eval   output='`    exec 4>&1    ret=\`      exec 3>&1 >&4 4>&-      (cmd 3>&-; echo \\$?\ >&3; printf \'\) |        awk 3>&- -v RS=\\\\' -v ORS= -v b='\\\\\\\\' '          NR > 1 {print RS b RS RS}; {print}; END {print RS}'    \`    echo \;ret=\$ret\  `Here, we're generating aoutput='output of cmdwith the single quotes escaped as '\''';ret=Xto be passed to eval. As for the POSIX approach, if ' was one of those characters whose encoding can be found at the end of other characters, we'd have a problem (a much worse one as it would become a command injection vulnerability), but thankfully, like ., it's not one of those, and that quoting technique is generally the one that is used by anything that quotes shell code (note that \ has the issue, so shouldn't be used (also excludes ... inside which you need to use backslashes for some characters). Here, we're only using it after a ' which is OK).tcshSee tcsh preserve newlines in command substitution `...`(not taking care of the exit status, which you could address by saving it in a temporary file (echo $status > $tempfile:q after the command))