Looping through a variable with newlines without a sub shell

Question

I am trying to loop over a variable which contains newlines in a POSIX compatible way.

The following works:

echo "$REQUIRE_PURPOSE" | while IFS= read -r line ; do
  echo "$line"
done

but the piping spawns another shell.

I then tried:

OLDIFS="$IFS"
IFS='
'
for line in "$REQUIRE_PURPOSE"; do
  echo "$line"
done
IFS="$OLDIFS"

But it just prints the content of "$REQUIRE_PURPOSE"

I also tried:

while IFS= read -r line ; do
  echo "$line"
done <<< "$REQUIRE_PURPOSE"

but also here it does not work.

Example:

echo "$REQUIRE_PURPOSE"
echo "---"
while IFS= read -r line ; do echo "$line"; done <<< "$REQUIRE_PURPOSE"

generates:

uuu
iii
Digital Signature
---
uuuniiinDigital Signature

or

00000000  75 75 75 0a 69 69 69 0a  44 69 67 69 74 61 6c 20  |uuu.iii.Digital |
00000010  53 69 67 6e 61 74 75 72  65 0a 2d 2d 2d 0a 31 20  |Signature.---.1 |
00000020  75 75 75 6e 69 69 69 6e  44 69 67 69 74 61 6c 20  |uuuniiinDigital |
00000030  53 69 67 6e 61 74 75 72  65 0a 75 75 75 0a 69 69  |Signature.uuu.ii|
00000040  69 0a 44 69 67 69 74 61  6c 20 53 69 67 6e 61 74  |i.Digital Signat|
00000050  75 72 65 0a                                       |ure.|
00000054

Any idea?

The code while ... do ... done <<< "$REQUIRE_PURPOSE" works for me. What problem do you have? — Bodo
– Bodo, Commented Jun 17, 2022 at 12:27
@bodo I get just one loop iteration. macOS with bash 5.1.16(1)-release (I added the example to the question) — Matteo
– Matteo, Commented Jun 17, 2022 at 12:36
I successfully tested it with Git Bash on Windows and bash on Linux (armv7l, Raspberry PI). How do you set the value of REQUIRE_PURPOSE? Can you add code that sets the variable? Did you intentionally use read -a instead of read -r in the code below "Example:"? — Bodo
– Bodo, Commented Jun 17, 2022 at 12:44
Does your variable contain a sequence of \ and n instead of real linefeed characters? — Bodo
– Bodo, Commented Jun 17, 2022 at 12:45

White Owl · Accepted Answer · 2022-06-17 12:24:24Z

0

The option to read you need is -a. It will parse the input by IFS into the array:

read -a lines <<<"$REQUIRE_PURPOSE"
for i in "${lines[@]}"
do
    echo "$i"
done

answered Jun 17, 2022 at 12:24

White Owl

5,2871 gold badge9 silver badges16 bronze badges

Is there a POSIX compatible way?

Matteo
– Matteo

2022-06-17 12:27:27 +00:00
Commented Jun 17, 2022 at 12:27
ITYM IFS=$'\n' read -a arr -d '' <<< "$var", since you need to have the -d separator distinct from the IFS separator. Though you'd probably be better off using readarray/mapfile there, just mapfile -t arr <<< "$var" would do.

ilkkachu
– ilkkachu

2022-06-17 13:42:58 +00:00
Commented Jun 17, 2022 at 13:42
@Matteo, <<< already isn't standard. Also note that it adds a trailing newline, so you might get an extra one at the end.

ilkkachu
– ilkkachu

2022-06-17 13:43:42 +00:00
Commented Jun 17, 2022 at 13:43

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Will Chen · Accepted Answer · 2024-12-16 21:01:57Z

0

You can do this with parameter expansion (S.2.6.2) and command substitution (S.2.6.3) inside an unquoted HereDoc (S2.7.4):

VAR="$(echo Line1; echo Line2; echo EOF; echo "HereDoc continues."; echo Line5)"
while read line; do
 printf "Line-%s-END\n" "$line"
done << EOF
$VAR
EOF

while read line; do
 printf "Line-%s-END\n" "$line"
done << EOF
$(
 echo Line1
 echo Line2
 echo EOF
 echo "The HereDoc continues."
 echo Line5
)
EOF

answered Dec 16, 2024 at 21:01

Will Chen

1193 bronze badges

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