heredoc syntax for multi-line perl using stdin from bash

Question

How to pass file to perl script for processing and also use heredoc syntax for multi-line perl script? I have tried those but no luck:

cat ng.input | perl -nae <<EOF
if (@F==2) {print $F[0] . "\t". $F[1] . "\n"} else { print "\t" . $F[0] . "\n" }
EOF

cat ng.input | perl -nae - <<EOF
if (@F==2) {print $F[0] . "\t". $F[1] . "\n"} else { print "\t" . $F[0] . "\n" }
EOF

All you need is perl -ple'$_="\t$_"if!/\t/' (if @F!=2 when @F<2, and if the input is specifically tab separated rather than whitespace separated) — ikegami
– ikegami, Commented Apr 23, 2013 at 1:55

ikegami · Accepted Answer · 2013-04-23 01:30:54Z

3

There's really no need for here-docs. You could simply use a multi-line argument:

perl -nae'
    if (@F==2) {
       print $F[0] . "\t". $F[1] . "\n"
    } else {
       print "\t" . $F[0] . "\n"
    }
' ng.input

Clean, more portable than Barmar's, and only uses one process instead of Barmar's three.

Note that your code could be shrunk to

perl -lane'unshift @F, "" if @F!=2; print "$F[0]\t$F[1]";' ng.input

or even

perl -pale'unshift @F, "" if @F!=2; $_="$F[0]\t$F[1]";' ng.input

edited Apr 23, 2013 at 1:30

answered Apr 23, 2013 at 1:21

ikegami

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3 Comments

Barmar Over a year ago

I assume cat ng.input in the question is just a placeholder for some other process that will pipe input to the script. Otherwise there wouldn't be a problem using the here-doc.

ikegami Over a year ago

@Barmar, That's completely besides the point. A multi-line argument can also be used with ... | perl. (But to address your comment anyway, you underestimate how many people use cat file |.)

Jonathan Leffler Over a year ago

That's why there's a UUOC award, though in actuality, it is hard to find a novel UUOC — they're mainly mundane repetitions of the same old problem, over and over.

Barmar · Accepted Answer · 2013-04-23 01:30:31Z

1

Use process substitution:

cat ng.input | perl -na <(cat <<'EOF'
if (@F==2) {print $F[0] . "\t". $F[1] . "\n"} else { print "\t" . $F[0] . "\n" }
EOF
)

Also put single quotes around the EOF tag so that $F in the perl script won't be expanded as shell variables.

edited Apr 23, 2013 at 1:30

answered Apr 23, 2013 at 0:12

Barmar

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1 Comment

Barmar Over a year ago

@JonathanLeffler Thanks, I didn't even notice the -e in the original question.

Ale · Accepted Answer · 2021-11-02 08:52:29Z

0

Can also pass a heredoc as a "-" first argument to perl:

perl -lna - ng.input <<'EOF'
if (@F==2) {print $F[0] . "\t". $F[1] . "\n"} else { print "\t" . $F[0] . "\n" }
EOF

answered Nov 2, 2021 at 8:52

Ale

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