How can I disable variable interpolation with the Perl substitution operator?

Question

I'm trying to replace a particular line in a text file on VMS. Normally, this is a simple one-liner with Perl. But I ran into a problem when the replacement side was a symbol containing a VMS path. Here is the file and what I tried:

Contents of file1.txt:

foo
bar
baz
quux

Attempt to substitute 3rd line:

$ mysub = "disk$data1:[path.to]file.txt"
$ perl -pe "s/baz/''mysub'/" file1.txt

Yields the following output:

foo
bar
disk:[path.to]file.txt
quux

It looks like Perl was overeager and replaced the $data1 part of the path with the contents of a non-existent variable (i.e., nothing). Running with the debugger confirmed that. I didn't supply /e, so I thought Perl should just replace the text as-is. Is there a way to get Perl to do that?

(Also note that I can reproduce similar behavior at the linux command line.)

In Perl, you can disable variable-interpolation within `s///` by using the delimiter `'` instead of `/` (e.g. `s'baz'disk$data1:[path.to]file.txt'`). Unfortunately, judging from what you've posted, it looks like VMS uses `'` to perform its own variable substitution? So, it might be easier-said-than-done to get the single-quotes into the Perl script. :-/ — ruakh, Oct 01 '12 at 18:14
Yep, I tried various flavors of that before posting here. You need exactly two `''` and one `'` to do variable substitution on VMS, and its interpreter is easily confused. — Michael Kristofik, Oct 01 '12 at 18:17
When in doubt, use a script file instead of a one-liner. And use `use strict; use warnings;` If you had defined `my $mysub = 'disk$data1:[path.to]file.txt'` (note the single quotes to avoid variable interpolation) or even `my $mysub = $ENV{mysub]` (if you insist on using shell variables) inside a perl script, you would not have this problem. — TLP, Oct 01 '12 at 18:18
@TLP: Actually, `$ENV{mysub}` would completely address the problem; that's the minimal fix. Instead of trying to disable the interpolation in Perl, the OP should disable the interpolation in VMS, by writing `perl -pe "s/baz/$ENV{mysub}/" file1.txt`! — ruakh, Oct 01 '12 at 18:32
`mysub` would have to be defined as a logical in that case, but yep, I think you're right. — Michael Kristofik, Oct 01 '12 at 18:33

score 4 · Answer 1 · edited May 23 '17 at 12:00

4

As ruakh discovered when reading my comment, the problem of perl interpolation can be solved by accessing the %ENV hash, rather than using a shell variable:

perl -pwe "s/baz/$ENV{mysub}/" file1.txt

Added -w because I do not believe in not using warnings even in one-liners.

edited May 23 '17 at 12:00

Community

1
1

answered Oct 01 '12 at 18:40

TLP

66,756
10
92
149

Yes, this works (just tried it). You need to use a logical instead of a symbol: `define mysub "disk$data1:[path.to]file.txt"`. – Michael Kristofik Oct 01 '12 at 18:49

score 4 · Accepted Answer · edited Oct 02 '12 at 13:21

With just the right mix of quotes and double quotes you can get there:

We create a tight concatenation of "string" + 'substitute symbol' + "string". The two double quoted strings contain single quotes for the substitute. Contrived... but it works.

$ perl -pe "s'baz'"'mysub'"'" file1.txt

That's a DCL level solution.

For a Perl solution, use the q() operator for a non-interpolated string and execute that. Stuff the symbol into the parens using simple DCL substitution. This is my favorite because it (almost) makes sense to me and does not get too confusing with quotes.

$ perl -pe "s/baz/q(''mysub')/e" file1.txt

score 1 · Answer 3 · answered Oct 01 '12 at 18:10

1

Haven't even seen a VMS system in decades, but ... escape your sigil?

$ mysub = "disk\$data1:[path.to]file.txt"

or maybe

$ mysub = "disk\\$data1:[path.to]file.txt"

?

answered Oct 01 '12 at 18:10

mob

117,087
18
149
283

+1, that will work for this contrived case. But what if I don't control `mysub`? – Michael Kristofik Oct 01 '12 at 18:35
Not familiar with how the VMS shell works - TLP's answer is something I would try. – mob Oct 01 '12 at 19:05

ikegami · Answer 4 · 2012-10-01T19:06:55.817

1

For sh or a derivative, I'd use

perl -pe'BEGIN { $r = shift(@ARGV) } s/baz/$r/' "$mysub" file1.txt

Otherwise, you have to somehow covert the value of mysub into a Perl string literal. That would be more complicated. Find the equivalent for your shell.

Edit by OP:

Yes, this works. The VMS equivalent is

perl -pe "BEGIN { $r = shift(@ARGV) } s/baz/$r/" 'mysub' file1.txt

edited Oct 01 '12 at 19:06

answered Oct 01 '12 at 18:18

ikegami

367,544
15
269
518

score 0 · Answer 5 · answered Oct 01 '12 at 18:06

0

You have to escape the $ with \$. Otherwise Perl sees a variable reference and replaces the string $data1 with the content of $data1 before the regular expression is evaluated. As you didn't define $data1 it is, of course, empty.

answered Oct 01 '12 at 18:06

Moritz Bunkus

11,592
3
37
49

score 0 · Answer 6 · answered Oct 01 '12 at 18:12

0

You can use the following code :

$ mysub='disk\$data1:[path.to]file.txt'
$ perl -pe 's/baz/'$mysub'/' FILE
foo
bar
disk$data1:[path.to]file.txt
quux

answered Oct 01 '12 at 18:12

Gilles Quénot

173,512
41
224
223

The single quotes are used for variable substitution on VMS, so that won't work. It looks like it works on linux though. – Michael Kristofik Oct 01 '12 at 18:52

score 0 · Answer 7 · edited Oct 24 '12 at 16:45

0

You could try using a logical name instead of a DCL symbol:

$ define mysub "disk$data1:[path.to]file.txt"
$ $ perl -pe "s/baz/mysub/" file1.txt

edited Oct 24 '12 at 16:45

Brian Mains

50,520
35
148
257

answered Oct 24 '12 at 16:27

bonzothecat

1

score -1 · Answer 8 · answered Oct 01 '12 at 18:02

-1

Use sed.

Sed won't try to interpolate $data1 as a variable, so you should get what you want.

$ sed -e "s/baz/''mysub'/" file1.txt

answered Oct 01 '12 at 18:02

Michael Kristofik

34,290
15
75
125

How can I disable variable interpolation with the Perl substitution operator?

8 Answers8