Perl 在命令行中： perl -p -i -e “some text” /path

Question

I am not familiar with perl. I am reading an installation guide atm and the following Linux command has come up:

perl -p -i -e "s/enforcing/disabled/" /etc/selinux/config

Now, I am trying to understand this. Here is my understanding so far:

-e simply allows for executing whatever follows

-p puts my commands that follow -e in a loop. Now this is strange to me, as to me this command seems to be trying to say: Write "s/enforcing/disabled/" into /etc/selinux/config. Then again, where is the "write" command? And what is this -i (inline) good for?

Answer 1

-p changes

s/enforcing/disabled/

to something equivalent to

while (<>) {
   s/enforcing/disabled/;
   print;
}

which is short for

while (defined( $_ = <ARGV> )) {
   $_ =~ s/enforcing/disabled/;
   print($_);
}

What this does:

1. It reads a line from ARGV into $_ . ARGV is a special file handle that reads from the each of the files specified as arguments (or STDIN if no files are provided).
2. If EOF has been reached, the loop and therefore the program exits.
3. It replaces the first occurrence of enforcing with disabled .
4. It prints out the modified line to the default output handle. Because of -i , this is a handle to a new file with the same name as the one from which the program is currently reading.*
5. Repeat.

For example,

$ cat a
foo
bar enforcing the law
baz
enforcing enforcing

$ perl -pe's/enforcing/disabled/' -i a

$ cat a
foo
bar disabled the law
baz
disabled enforcing

---

* — In old versions of Perl, the old file has already been deleted at this point, but it's still accessible as long as there's an open file handle to it. In very new versions of Perl, this writes to temporary file that will later overwrite the file from which the program is reading.

Answer 2

To find out exactly what Perl is going to do, you can use the O module

perl -MO=Deparse -p -i -e "s/enforcing/disabled/" file

outputs

BEGIN { $^I = ""; }
LINE: while (defined($_ = readline ARGV)) {
    s/enforcing/disabled/;
}
continue {
    die "-p destination: $!\n" unless print $_;
}
-e syntax OK