Perl Learning note Four--input and output

Reading standard input

Standard input with <STDIN>: Chomp ($line =<stdin>);

If the end of the file is read, the line input operator returns UNDEF. Can use this nature to jump out of the loop.

while (defined ($line =<stdin>)) {

Print "I saw $line";

}

Abbreviated as:

while (<STDIN>) {

Print "I saw $_.";

}

Note: This shorthand works only in the earliest notation, and if you place the line input operator anywhere else (especially in a row), he does not read the line input and automatically saves the default variable $_. This shorthand works only if the line input operator is in the conditional expression in the while loop.

Also, foreach has this usage:

foreach (<STDIN>) {

Print "I saw $_";

}

The difference is that while a row is processed every time a row is read, foreach needs to be read and processed once.

#!/usr/bin/perl UseStrict; Usewarnings; while(<STDIN>) {    Print "I saw $_ \ n";}foreach(<STDIN>) {    Print "I saw $_ \ n";}

Input from the diamond operator (<>)

You can use the diamond operator to write tools such as cat, sed, awk, sort, grep, LPR, and so on.

Program Invocation parameters: $./my_program Fred Barney Betty

The program collects data from the standard input stream if it is more than providing any invocation parameters. Also: If you use a hyphen (-) as a parameter, it means that you want to read the data from the standard input. So if the parameter fred-betty is called, then the program should process the file Fred and then process the data provided by the standard input stream, and finally Betty.

The diamond operator is a special case of the line input operator. But instead of reading the input from the keyboard, he reads from the user-specified location:

while (defined ($line =<>)) {

Chomp ($line);

Print "It is $line that I saw!\n";

}

If the call argument is now Fred, Barney, Betty, then the output will be "It was [a line read from the file] lines that I saw!" Wait until the end of the file is encountered. Note:<> reads a file and then reads the new file, then outputs the content in line. As with the diamond operator, it is reported that these files are merged into a large file, and the diamond operator returns undef and jumps out of the while loop only when it touches the end of all inputs.

The above code shorthand:

while (<>) {

Chomp

Print "It is $_ that I saw!\n";

}

Note: Diamond operators generally handle all inputs, so it is usually wrong to see multiple diamond operators in a program.

# !/usr/bin/perl  while (<>) {    chomp;     Print " $_ \ n " ;}
Note that the following execution instructions in the code are added-and read in from the output pipeline of grep

Perl Learning note Four--input and output