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POSIX shells

The usual (^{1 2 3 4 5 6 7 8 9 10 11 12 13 14 15} ) 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, ū 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 ū.

bash/zsh alternatives

With 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/akanaga

For 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 = `cmd

or:

output = ``()cmd

In 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.

fish

In 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
  set -l n (count $lines)
  test $n -le 2; or for line in $lines[1..-2]
    set output $output$line\n
  end
  set output $output$lines[-1]
  return $ret
end

Bourne shell

The 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\"
  `"

tcsh

See tcsh preserve newlines in command substitution `...`

(not taking care of the exit status)