Perl: Input and output

Input and output:

 

<STDIN> operator: used to read data from STDIN:

If you read the input character at the end of the file, it will return undef.

 

$ line = <STDIN>; #Read the next line.

chomp ($ line = <STDIN>); #Remove the last line break of the input line

while (<STDIN>) {

    print "$ _";

}

foreach (<STDIN>) {

    print "$ _";

}

 

Print function: used to output information to STDOUT.

$ _ = "Str1 str2…";

print; #print will print the value of $ _ by default.

print @arr; #Print array elements, but no spaces in between.

print "@arr"; #Print array elements with spaces in between.

 

printf function: formatted output:

%%: percent sign

% c: character

% u: decimal

% o: octal

% x: hexadecimal

% s: string

% d: decimal integer, round off the integer directly

% g: Floating point number, automatically select scientific notation or decimal.

% e: floating point number, scientific notation

% f: floating point number, decimal

% m.nf: m is the total number of digits, n is the number of digits after the decimal point

space: Add a space before the non-negative number.

+: Add a plus sign in front of non-negative numbers, the default is not.

-: Left aligned, the default is right aligned.

0: Right-aligned with 0 to fill, the default is a space.

#: Enable the prefix, 0b / 0B means binary, 0x / 0X means hexadecimal, and 0 means octal.

printf (“% s \ n” x @arr), @arr; #print array

 

say: output

say will automatically add a newline character at the end of each line.

The other syntax of say is the same as print.

 

<> Diamond operator:

Read data from file or standard input

perl program file1 file2…

while (<>) {

    chomp;

    print "$ _";

}

print <>; # Similar to cat command.

print sort <>; # Similar to the sort command.

 

@ARGV array parameters:

Perl's built-in array @ARGV is used to store parameters, you can also specify this array in the program.

@ARGV = qw # file1 file2 –options file3 #;

 

select function: select output

After using select, the default output to STDOUT is output to the specified output handle.

select FH; # This is the default before the next select and the specified handle

$ | = 1; #Refresh the buffer immediately

if (STDERR / STDOUT / STDIN .., ">> filename") {#equivalent to copying a standard file handle.

    …

}

If the standard file handle fails to copy, Perl uses the original standard handle by default.

 

die function: handle errors

Die will terminate the program and print an error message to stderr.

The $! Special variable indicates a readable system error message.

if (! open FH, ">> filename") {

    die "Can not create file description: $!"; #Output information about yourself and the system.

}

if (@ARGV <2) {

    die "not enough arguments \ n"; #Adding a line break will not output a specific error line number.

}

 

warn function: process warnings and print warning messages to stderr.

 

Close the handle:

If you reopen a handle perl will close the previous handle;

Perl will automatically close the handle when the program ends.

close FH;

 

read

readline

write

tell

truncate

binmode

dbmclose

dbmopen

eof

fileno

flock

format

getc

seek

sysopen

syscall

sysread

sysseek

syswrite

perl: input and output