bash: Assigning the first line of a variable to a variable

Question

I have a multiline variable, and I only want the first line in that variable. The following script demonstrates the issue:

#!/bin/bash

STRINGTEST="Onlygetthefirstline
butnotthesecond
orthethird"

echo "  Take the first line and send to standard output:"
echo ${STRINGTEST%%$'\n'*}
#   Output is as follows:
# Onlygetthefirstline

echo "  Set the value of the variable to the first line of the variable:"
STRINGTEST=${STRINGTEST%%$'\n'*}

echo "  Send the modified variable to standard output:"
echo $STRINGTEST
#   Output is as follows:
# Onlygetthefirstline butnotthesecond orthethird

Question: Why does ${STRINGTEST%%$'\n'*} return the first line when placed after an echo command, but replace newlines with spaces when placed after assignment?

Answer 1

might not be most efficient but one liner...

firstLine=`echo "${multiLineVariable}" | head -1`

Answer 2

Maybe there is other way to archive what you want to do, but this works

#!/bin/bash

STRINGTEST="
Onlygetthefirstline
butnotthesecond
orthethird
"

STRINGTEST=(${STRINGTEST[@]})
echo "${STRINGTEST[0]}"

Answer 3

That code works for me with all versions of bash I tried between 2.05b and 4.3. More likely you tried to run that script with a different shell that doesn't support the $'...' form of quoting.

That $'...' syntax is not standard sh syntax (yet) and only supported (as of 2015-05-22 and AFAIK) by ksh93 (where it originated), zsh, bash, recent versions of mksh and the sh or recent versions of FreeBSD.

My bet would be that you tried to run that script with sh instead of bash and your sh is based on versions of ash, pdksh, yash or ksh88 that don't support it yet.

If you want to make that code POSIX 2008 compatible, you'd need to write it:

STRINGTEST="Onlygetthefirstline
butnotthesecond
orthethird"

NL='
'
STRINGTEST=${STRINGTEST%%"$NL"*}
printf '%s\n' "$STRINGTEST"

Then, you can have it interpreted by any POSIX compliant shell like bash or any leaner/faster ones like your sh.

(and remember that leaving a variable unquoted in list context has a very special meaning in Bourne-like shells).

Answer 4

This works for me:

STRINGTEST="Some Text 1
Some Text 2
Some Text 3"

readarray -t lines < <(echo "$STRINGTEST")
echo "${lines[0]}"

And it also works for blank lines:

STRINGTEST="
Some Text 1
Some Text 2
Some Text 3"

readarray -t lines < <(echo "$STRINGTEST")
echo "${lines[0]}"

Answer 5

With Bash built-in read and here-string:

#!/usr/bin/env bash

STRINGTEST="
Some Text 1
Some Text 2
Some Text 3"


IFS=$'\n' read -r STRINGTEST <<<"$STRINGTEST"

Using POSIX parameters expansion:

#!/usr/bin/env sh

STRINGTEST="
Some Text 1
Some Text 2
Some Text 3"

# Disables globing
set -f

# Field separator is newline only
IFS="
"

# No quotes, split lines as arguments because of IFS
set -- $STRINGTEST

# First argument is first line
STRINGTEST="$1"

Answer 6

Hey i have found this post with the same problem , but my solution was using the head commnand.

Like as an example:

vfail2ban=$(fail2ban-server --version |head -1)
root@vm:~# echo ${vfail2ban}
Fail2Ban v0.10.2