What is the difference between “perl -n” and “perl -p”?

Question

What is the difference between the perl -n and perl -p options?

What is a simple example to demonstrate the difference?

How do you decide which one to use?

Answer 1

How do you decide which one to use?

You use -p if you want to automatically print the contents of $_ at the end of each iteration of the implied while loop. You use -n if you don't want to print $_ automatically.

An example of -p . Adding line numbers to a file:

$ perl -pe '$_ = "$.: $_"' your_file.txt

An example of -n . A basic grep replacement.

$ perl -ne 'print if /some search text/' your_file.txt

Answer 2

-p is short for -np , and it causes $_ to be printed for each pass of the loop created by -n .

---

perl -ne'...'

executes the following program:

LINE: while (<>) {
    ...
}

while

perl -pe'...'

executes the following program:

LINE: while (<>) {
    ...
}
continue {
    die "-p destination: $!\n" unless print $_;
}

---

See perlrun for documentation about perl 's command-line options.

Answer 3

What is the difference between the perl -n and perl -p options?

-p causes each line to be printed; equivalent to:

    while (<>) { ... } continue { print }

-n does not automatically print each line; equivalent to:

    while(<>) {...}

---

What is a simple example to demonstrate the difference?

eg, replace foo with FOO :

$ echo 'foo bar' | perl -pe 's/foo/FOO/'
FOO bar
$ echo 'foo bar' | perl -ne 's/foo/FOO/'
$

---

How do you decide which one to use?

One example where -n is useful is when you don't want every line printed, and there is a conditional print in the code, eg, only show lines containing foo :

$ echo -e 'foo\nbar\nanother foo' | perl -ne 'print if /foo/;'
foo
another foo
$

The command-line options are documented in perlrun documentation

Answer 4

perl -n 等价于 while(<>){...} perl -p 等价于 while(<>){...;print;}