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I want to test if the output of a function matches the text I envision it should write to standard output. But the code below never prints test PASS and I can't figure out why. I also tried putting various \n characters or deleting them in the variable GROUND_TRUTH but that didn't help.

error_out()
{
    printf "%s\n" "${ERROR}"
    printf "Return to exit\n"
    read -r THROW_AWAY
    exit
}

ERROR="Test output."
OUTPUT_TO_TEST=$(error_out)
GROUND_TRUTH="Test output.\nReturn to exit\n"
{ [ "${OUTPUT_TO_TEST}" = "${GROUND_TRUTH}" ] \
         && printf "error_out test PASS\n"; } \
    || { \
        printf "error_out test FAIL\n"; \
    }
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  • The shell doesn't know that a "\n" should be in fact a LF. Also, when capturing the output of a command, the trailing LF(s) are trimmed so GROUND_TRUTH shouldn't end with a LF... BTW, what's the purpose of read -r THROW_AWAY? Waiting the the user to type ENTER? Commented Dec 8, 2024 at 21:38
  • Well I tried GROUND_TRUTH="Test output.\nReturn to exit" and it doesn't work. And the purpose of read -r THROW_AWAY is that the user should see the message in the ERROR string variable. Commented Dec 8, 2024 at 21:42
  • You need to use a literal LF, i.e. GROUND_TRUTH="Test output. (and in following line) return to exit" Commented Dec 8, 2024 at 21:45
  • Or use an ANSI-C string -- GROUND_TRUTH=$'Test output.\nReturn to exit' Commented Dec 8, 2024 at 21:50
  • Get out of the habit of using ALLCAPS variable names, leave those as reserved by the shell. One day you'll write PATH=something and then wonder why your script is broken: (1) (2) (3). Commented Dec 8, 2024 at 21:50

1 Answer 1

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You have two issues:

  1. The result of a command substitution $(...) will never end with a LF, as they are implicitly trimmed by the shell. So your GROUND_TRUTH variable (that's meant to test the result of a command substitution) shan't end with a LF.

  2. Inside a string literal, the shell doesn't understand \n as an escape sequence for the LF character. As you're targeting a POSIX shell, the most common workaround is to use a real LF in your string.

So, the minimal fix that'll make your code work as intended would be to replace:

GROUND_TRUTH="Test output.\nReturn to exit\n"

with:

GROUND_TRUTH="Test output.
Return to exit"
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2
  • 2
    sh implementations conformant to POSIX 2024 understand \n in the $'...' form of quotes (initially from ksh93 and now found in most other POSIX-like shells, dash (the sh of most Debian derivatives) being a notable exception). Commented Dec 9, 2024 at 8:39
  • @StéphaneChazelas Nice finding! In a few years we'll be able to rely on it even with dash Commented Dec 9, 2024 at 9:48

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