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can a perl script be written to receive data from any of (1) file, (2) stdin, (3) redirect?

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1

Several unix utilities, such as fmt, head, and cat, can receive data in any of 3 ways: a filename; a pipe from standard input; or a redirect "<". For instance:

printf '%b' 'dog ncat nratn' > dogcatrat
fmt dogcatrat
cat dogcatrat | fmt
fmt < dogcatrat

Can one write a perl script that will behave with the same versatility? Or is there a good reason not to attempt this? And is "pipe from standard input" the right way to refer to the line of code that starts with cat?

I want to write myfmt.pl, to be used in any of these three ways.

perl redirect pipe filenames

share|improve this question

asked Mar 27 at 17:34

Jacob WegelinJacob Wegelin

9144 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  uh, yes? If you have an argument, read from that file, otherwise read from stdin.
  
  – Tanktalus
  Mar 27 at 17:40
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes, this is exactly the use case that the special filehandle ARGV addresses.
  
  – mob
  Mar 27 at 17:58
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Nit: cat dogcatrat | fmt and fmt < dogcatrat both feed fmt's STDIN. There's no difference between the two as far as fmt is concerned.
  
  – ikegami
  Mar 27 at 18:36
  
  

  
  
  
  

add a comment | 

1

Several unix utilities, such as fmt, head, and cat, can receive data in any of 3 ways: a filename; a pipe from standard input; or a redirect "<". For instance:

printf '%b' 'dog ncat nratn' > dogcatrat
fmt dogcatrat
cat dogcatrat | fmt
fmt < dogcatrat

Can one write a perl script that will behave with the same versatility? Or is there a good reason not to attempt this? And is "pipe from standard input" the right way to refer to the line of code that starts with cat?

I want to write myfmt.pl, to be used in any of these three ways.

perl redirect pipe filenames

share|improve this question

asked Mar 27 at 17:34

Jacob WegelinJacob Wegelin

9144 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  uh, yes? If you have an argument, read from that file, otherwise read from stdin.
  
  – Tanktalus
  Mar 27 at 17:40
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes, this is exactly the use case that the special filehandle ARGV addresses.
  
  – mob
  Mar 27 at 17:58
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Nit: cat dogcatrat | fmt and fmt < dogcatrat both feed fmt's STDIN. There's no difference between the two as far as fmt is concerned.
  
  – ikegami
  Mar 27 at 18:36
  
  

  
  
  
  

add a comment | 

Several unix utilities, such as fmt, head, and cat, can receive data in any of 3 ways: a filename; a pipe from standard input; or a redirect "<". For instance:

printf '%b' 'dog ncat nratn' > dogcatrat
fmt dogcatrat
cat dogcatrat | fmt
fmt < dogcatrat

Can one write a perl script that will behave with the same versatility? Or is there a good reason not to attempt this? And is "pipe from standard input" the right way to refer to the line of code that starts with cat?

I want to write myfmt.pl, to be used in any of these three ways.

perl redirect pipe filenames

share|improve this question

asked Mar 27 at 17:34

Jacob WegelinJacob Wegelin

9144 bronze badges

printf '%b' 'dog ncat nratn' > dogcatrat
fmt dogcatrat
cat dogcatrat | fmt
fmt < dogcatrat

share|improve this question

asked Mar 27 at 17:34

Jacob WegelinJacob Wegelin

9144 bronze badges

share|improve this question

asked Mar 27 at 17:34

Jacob WegelinJacob Wegelin

9144 bronze badges

asked Mar 27 at 17:34

Jacob WegelinJacob Wegelin

9144 bronze badges

asked Mar 27 at 17:34

Jacob WegelinJacob Wegelin

9144 bronze badges

2 Answers
2

active

oldest

votes

5

The ARGV special filehandle will do this by default. It is also the handle used by readline (aka the <> and <<>> operators) when not given a handle. So this is actually pretty common in Perl scripts.

#!/usr/bin/env perl
use 5.022;
use warnings;
while (my $line = <<>>) 
 # $line from one of the filenames passed as an argument, otherwise STDIN
 # $ARGV is the current filename, or - when reading from STDIN

You can use the <> operator instead to support older versions of Perl, but the <<>> operator added in Perl 5.22 is a better option for this task if available, because the standard <> operator allows passing strange things like date| to run processes rather than read files.

For safer filename-only operation while supporting older versions of Perl, you could use ARGV::readonly or emulate the <<>> operator like the following:

#!/usr/bin/env perl
use strict;
use warnings;
unshift @ARGV, '-' unless @ARGV;
while (my $file = shift) 
 my $fh;
 if ($file eq '-') 
 $fh = *STDIN;
 else 
 open $fh, '<', $file or die "open $file failed: $!";
 
 while (my $line = <$fh>) 
 # ...
 

(Technically the <<>> operator also does not allow passing - as an argument to read STDIN, but it is your choice if you want to allow that.)

share|improve this answer

edited Mar 27 at 21:20

answered Mar 27 at 17:58

GrinnzGrinnz

5,24955 silver badges1515 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It will even read from multiple files if multiple files are provided, just like cat, grep, etc.
  
  – ikegami
  Mar 27 at 18:37
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What the answer doesn't make it clear is that you can use use strict; use warnings; while (my $line = <>) ... if you want to be compatible with older versions of Perl. But don't make setuid scripts (or scripts used by setuid scripts) that use <>.
  
  – ikegami
  Mar 27 at 18:39
  
  

  
  
  
  

add a comment | 

1

It appears that the following script fills the bill.

#!/usr/bin/perl
use strict;
use warnings;
use 5.18.2;
local $/ = ""; # input record separator: one paragraph at a time
while (<>) 
 print;
 print "n";
 say '-' x 30;

Example:

printf '%b' 'dog ncat nratn' > aaa
try.pl aaa
cat aaa | try.pl
try.pl < aaa

share|improve this answer

answered Mar 27 at 18:09

Jacob WegelinJacob Wegelin

9144 bronze badges

add a comment | 

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5

The ARGV special filehandle will do this by default. It is also the handle used by readline (aka the <> and <<>> operators) when not given a handle. So this is actually pretty common in Perl scripts.

#!/usr/bin/env perl
use 5.022;
use warnings;
while (my $line = <<>>) 
 # $line from one of the filenames passed as an argument, otherwise STDIN
 # $ARGV is the current filename, or - when reading from STDIN

You can use the <> operator instead to support older versions of Perl, but the <<>> operator added in Perl 5.22 is a better option for this task if available, because the standard <> operator allows passing strange things like date| to run processes rather than read files.

For safer filename-only operation while supporting older versions of Perl, you could use ARGV::readonly or emulate the <<>> operator like the following:

#!/usr/bin/env perl
use strict;
use warnings;
unshift @ARGV, '-' unless @ARGV;
while (my $file = shift) 
 my $fh;
 if ($file eq '-') 
 $fh = *STDIN;
 else 
 open $fh, '<', $file or die "open $file failed: $!";
 
 while (my $line = <$fh>) 
 # ...
 

(Technically the <<>> operator also does not allow passing - as an argument to read STDIN, but it is your choice if you want to allow that.)

share|improve this answer

edited Mar 27 at 21:20

answered Mar 27 at 17:58

GrinnzGrinnz

5,24955 silver badges1515 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It will even read from multiple files if multiple files are provided, just like cat, grep, etc.
  
  – ikegami
  Mar 27 at 18:37
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What the answer doesn't make it clear is that you can use use strict; use warnings; while (my $line = <>) ... if you want to be compatible with older versions of Perl. But don't make setuid scripts (or scripts used by setuid scripts) that use <>.
  
  – ikegami
  Mar 27 at 18:39
  
  

  
  
  
  

add a comment | 

5

The ARGV special filehandle will do this by default. It is also the handle used by readline (aka the <> and <<>> operators) when not given a handle. So this is actually pretty common in Perl scripts.

#!/usr/bin/env perl
use 5.022;
use warnings;
while (my $line = <<>>) 
 # $line from one of the filenames passed as an argument, otherwise STDIN
 # $ARGV is the current filename, or - when reading from STDIN

You can use the <> operator instead to support older versions of Perl, but the <<>> operator added in Perl 5.22 is a better option for this task if available, because the standard <> operator allows passing strange things like date| to run processes rather than read files.

For safer filename-only operation while supporting older versions of Perl, you could use ARGV::readonly or emulate the <<>> operator like the following:

#!/usr/bin/env perl
use strict;
use warnings;
unshift @ARGV, '-' unless @ARGV;
while (my $file = shift) 
 my $fh;
 if ($file eq '-') 
 $fh = *STDIN;
 else 
 open $fh, '<', $file or die "open $file failed: $!";
 
 while (my $line = <$fh>) 
 # ...
 

(Technically the <<>> operator also does not allow passing - as an argument to read STDIN, but it is your choice if you want to allow that.)

share|improve this answer

edited Mar 27 at 21:20

answered Mar 27 at 17:58

GrinnzGrinnz

5,24955 silver badges1515 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It will even read from multiple files if multiple files are provided, just like cat, grep, etc.
  
  – ikegami
  Mar 27 at 18:37
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What the answer doesn't make it clear is that you can use use strict; use warnings; while (my $line = <>) ... if you want to be compatible with older versions of Perl. But don't make setuid scripts (or scripts used by setuid scripts) that use <>.
  
  – ikegami
  Mar 27 at 18:39
  
  

  
  
  
  

add a comment | 

The ARGV special filehandle will do this by default. It is also the handle used by readline (aka the <> and <<>> operators) when not given a handle. So this is actually pretty common in Perl scripts.

#!/usr/bin/env perl
use 5.022;
use warnings;
while (my $line = <<>>) 
 # $line from one of the filenames passed as an argument, otherwise STDIN
 # $ARGV is the current filename, or - when reading from STDIN

You can use the <> operator instead to support older versions of Perl, but the <<>> operator added in Perl 5.22 is a better option for this task if available, because the standard <> operator allows passing strange things like date| to run processes rather than read files.

For safer filename-only operation while supporting older versions of Perl, you could use ARGV::readonly or emulate the <<>> operator like the following:

#!/usr/bin/env perl
use strict;
use warnings;
unshift @ARGV, '-' unless @ARGV;
while (my $file = shift) 
 my $fh;
 if ($file eq '-') 
 $fh = *STDIN;
 else 
 open $fh, '<', $file or die "open $file failed: $!";
 
 while (my $line = <$fh>) 
 # ...
 

(Technically the <<>> operator also does not allow passing - as an argument to read STDIN, but it is your choice if you want to allow that.)

share|improve this answer

edited Mar 27 at 21:20

answered Mar 27 at 17:58

GrinnzGrinnz

5,24955 silver badges1515 bronze badges

#!/usr/bin/env perl
use 5.022;
use warnings;
while (my $line = <<>>) 
 # $line from one of the filenames passed as an argument, otherwise STDIN
 # $ARGV is the current filename, or - when reading from STDIN

#!/usr/bin/env perl
use strict;
use warnings;
unshift @ARGV, '-' unless @ARGV;
while (my $file = shift) 
 my $fh;
 if ($file eq '-') 
 $fh = *STDIN;
 else 
 open $fh, '<', $file or die "open $file failed: $!";
 
 while (my $line = <$fh>) 
 # ...
 

share|improve this answer

edited Mar 27 at 21:20

answered Mar 27 at 17:58

GrinnzGrinnz

5,24955 silver badges1515 bronze badges

answered Mar 27 at 17:58

GrinnzGrinnz

5,24955 silver badges1515 bronze badges

answered Mar 27 at 17:58

GrinnzGrinnz

5,24955 silver badges1515 bronze badges

1

It appears that the following script fills the bill.

#!/usr/bin/perl
use strict;
use warnings;
use 5.18.2;
local $/ = ""; # input record separator: one paragraph at a time
while (<>) 
 print;
 print "n";
 say '-' x 30;

Example:

printf '%b' 'dog ncat nratn' > aaa
try.pl aaa
cat aaa | try.pl
try.pl < aaa

share|improve this answer

answered Mar 27 at 18:09

Jacob WegelinJacob Wegelin

9144 bronze badges

add a comment | 

1

It appears that the following script fills the bill.

#!/usr/bin/perl
use strict;
use warnings;
use 5.18.2;
local $/ = ""; # input record separator: one paragraph at a time
while (<>) 
 print;
 print "n";
 say '-' x 30;

Example:

printf '%b' 'dog ncat nratn' > aaa
try.pl aaa
cat aaa | try.pl
try.pl < aaa

share|improve this answer

answered Mar 27 at 18:09

Jacob WegelinJacob Wegelin

9144 bronze badges

add a comment | 

It appears that the following script fills the bill.

#!/usr/bin/perl
use strict;
use warnings;
use 5.18.2;
local $/ = ""; # input record separator: one paragraph at a time
while (<>) 
 print;
 print "n";
 say '-' x 30;

Example:

printf '%b' 'dog ncat nratn' > aaa
try.pl aaa
cat aaa | try.pl
try.pl < aaa

share|improve this answer

answered Mar 27 at 18:09

Jacob WegelinJacob Wegelin

9144 bronze badges

#!/usr/bin/perl
use strict;
use warnings;
use 5.18.2;
local $/ = ""; # input record separator: one paragraph at a time
while (<>) 
 print;
 print "n";
 say '-' x 30;

printf '%b' 'dog ncat nratn' > aaa
try.pl aaa
cat aaa | try.pl
try.pl < aaa

share|improve this answer

answered Mar 27 at 18:09

Jacob WegelinJacob Wegelin

9144 bronze badges

answered Mar 27 at 18:09

Jacob WegelinJacob Wegelin

9144 bronze badges

answered Mar 27 at 18:09

Jacob WegelinJacob Wegelin

9144 bronze badges