Line continuation and pipes within command substitution and process substitution

[Robin A. Meade]

I ran into an interesting difference in behavior between bash-4.4 and 
bash 5.0 involving line continuation and pipes within command/process 
substitutions.

Consider this script:

#!/bin/bash
echo "$(cat <<'EOF' | cat | cat | cat
hello
EOF
)"

It works fine (prints `hello`) on both bash-4.4 and bash 5.0.

Now introduce line continuation:

#!/bin/bash
echo "$(cat <<'EOF' \
| cat \
| cat \
| cat
hello
EOF
)"


Running it on bash-4.4 still gives the expected result:

bash-4.4$ ./myscript.sh
hello


But running it on bash-5.0 gives an unexpected result:

bash-5.0$ ./myscript.sh
./myscript.sh: line 14: warning: here-document at line 12 delimited by 
end-of-file (wanted `EOF')
./myscript.sh: line 14: warning: here-document at line 12 delimited by 
end-of-file (wanted `EOF')
./myscript.sh: line 13: warning: here-document at line 11 delimited by 
end-of-file (wanted `EOF')
hello
EO)


## Workaround: Use curly braces (preferred*) or parentheses to group the 
pipeline

#!/bin/bash
echo "$({ cat \
| cat \
| cat \
| cat
} <<'EOF'
hello
EOF
)"

*Parentheses would needlessly introduce a subshell.


## This applies to process substitution as well

The following process substitution example works on bash-4.4 but not 
bash-5.0:

#!/bin/bash
cat < <(
cat <<'EOF' \
| cat \
| cat \
| cat
hello
EOF
)

bash-5.0$ ./myscript.sh
./myscript.sh: line 9: warning: here-document at line 7 delimited by 
end-of-file (wanted `EOF')
./myscript.sh: line 8: warning: here-document at line 6 delimited by 
end-of-file (wanted `EOF')
hello
EO

Grouping the commands with curly braces or parentheses again is a 
workaround:

cat < <(
{ cat \
| cat \
| cat \
| cat
} <<'EOF'
hello
EOF
)


Robin