Preserve presence of last newline when assigning to variable [duplicate]

Question

In bash, when an output is captured into variable using $(...), an annoying newline is appended. However, I have an output that sometimes ends in a newline, and sometimes not. I want the output to be captured as-is.

In my real problem, the strings may contain several lines, but the last may or may not close with a newline, and this property should be preserved.

In the style of this answer at Stack Exchange, my minimal working example looks like this:

#!/bin/bash

newlinetest() {
    if [ "$1" = 'with' ]; then
        printf '%s\n' 'Text with newline'
    else
        printf '%s' 'Text without newline'
    fi
}

s="$(newlinetest with ; printf '%s' 'x')"
s="${s%?}"
printf '%s%s%s\n' '(' "${s}" ')'
s="$(newlinetest without ; printf '%s' 'x')"
s="${s%?}"
printf '%s%s%s\n' '(' "${s}" ')'

It does what it should do, but IMHO this looks like an ugly hack. Is there any other elegant way to solve this? Maybe something that involves mapfile or read? A solution without external tools would be very welcome.

Answer 1

Actually, command substitution removes trailing newlines, it doesn't append them. But if you use echo to print the variable later, it adds one by default.

Adding a trailing x or such is indeed somewhat hackish, but it's probably the easiest way, and works in all shells. You can make it somewhat shorter by using echo x instead of printf '%s' 'x'.

Since you mention read, you could use something like this (in Bash), too:

IFS= read -rd '' var < <(newlinetest with)

I'll leave it to you to decide if that's more pretty than this:

var=$(newlinetest with; echo x)
var=${var%x}

Though there is a potential performance difference between the two. In the first one, the process substitution creates a pipe, which read reads from one byte at a time to avoid overrunning a delimiter (That's not relevant here, but the shell doesn't know that.) The command substitution will read in larger blocks, and is therefore likely to be faster with a large output.