Perl CLI code cannot do a string line appended

user_17227456 0
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I'm trying to use a perl -npe one-liner to surround each line with =.

$ for i in {1..4}; { echo $i ;} |perl -npe '...'
=1=
=2=
=3=
=4=

The following is my first attempt. Note that the line feeds are in the incorrect position.

$ for i in {1..4}; { echo $i ;} |perl -npe '$_= "=".$_."=" '
=1
==2
==3
==4
=

I tried using chop to remove them line feeds and then re-add them in the correct position, but it didn't work.

$ for i in {1..4} ;{ echo $i ;} |perl -npe '$_= "=".chop($_)."=\n" '
=
=
=
=
=
=
=
=

Please solve it out, thanks much.

command-line-interface perl
1 Answers

chop returned the removed character, not the remaining string. It modifies the variable in-place. So the following is the correct usage:

perl -npe'chop( $_ ); $_ = "=$_=\n"'

But we can improve this.

  • It's safer to use chomp instead of chop to remove trailing line feeds.
  • -n is implied by -p, and it's customary to leave it out when -p is used.
  • chomp and chop modify $_ by default, so we don't need to explicitly pass $_.
perl -pe'chomp; $_ = "=$_=\n"'

Finally, we can get the same exact behaviour out of -l.

perl -ple'$_ = "=$_="'
user_5899240
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