Explore Perl Learn Perl Part 5 one liner files

2. Processing files (read)

2.1 Create

Basically, all code below is an extension of


$ perl -e 'expression' file

by adding switches, multiple expressions and files, e.g.:


$ perl switches -e 'expression-1; expression-2; ... expression-n;' file-1 file-2 ... file-n

2.2 Access

The switch -n add loops to the -e code, so you can process files a line at a time. And each line will be put into the special variable $_

For demonstration purposes, let's first create two text files with some content.

The first file name_age_1.txt contains four lines (format is name:age)

Sebastian:41
Daniel:39
Florence:37
Hannah:8

The second file name_age_2.txt contains also four lines (format is name:age)

Kirsten:42
Bibian:40
Niki:36
Elle:5

To loop through the content of the file name_age_1.txt, write


$ perl -n -e '$_;' name_age_1.txt # each line of name_age_1.txt will be put into $_

To loop through the content of multiple file, e.g. name_age_1.txt and name_age_2.txt:


$ perl -n -e '$_;' name_age_1.txt name_age_2.txt

So, now both filenames are added as argument.

2.3 Print

2.3.1 Print contents of a file

To print all lines from name_age_1.txt, type


$ perl -n -e 'print("$_");' name_age_1.txt # each line of name_age_1.txt will be put into $_


$ perl -n -e 'print;' name_age_1.txt

The commandline switches can be combined: -ne


$ perl -ne 'print("$_");' name_age_1.txt (1)

However, I prefer the first option (with separated commandline switches). It is completely clear what it does:


$ perl -n -e 'print("$_");' name_age_1.txt (1)

2.3.2 Print contents of multiple files

To print all records from both files, type


$ perl -n -e 'print("$_");' name_age_1.txt name_age_2.txt

2.3.3 Add a linenumber

Put a line number to the lines of the file via the special variable $.


$ perl -n -e 'print("$. : $_");' name_age_1.txt

2.4 Iterate

Iteration has already been discussed.

2.5 Operate

2.5.1 Filtering contents of a file

To show only the lines that contain the name daniel, use a regex:


$ perl -n -e 'print("$_") if (/daniel/);' name_age_1.txt

To show only the lines with an even linenumber:


$ perl -n -e 'print("$_") if (! $. % 2);' name_age_1.txt

To show only the first two lines:


$ perl -n -e 'print($_) if ++$count < 3;' name_age_1.txt

If App::pl is installed, it's still easier:


$ pl -rp2 '' name_age_1.txt

or the last two lines (knowing the file has 4 lines):


$ pl -P '3..eof' name_age_1.txt

or the last line:


$ pl -e Echo '' name_age_1.txt

alternatively:


$ pl -P 'eof' name_age_1.txt

which outputs the last line also.

2.5.2 Print names of a file name_age_1.txt

You've to use the -a switch (with option –n) to split the lines of the file into sets of words, assuming white space (default) as the field separators. To specify another pattern as field separator, the –F option is used followed by the delimiter (here -F:). In all cases, the resulting list is stored in the special array @F. $F[0] refers to first 'column' which contains the name.


$ perl -F: -l -a -n -e 'print($F[0]);' name_age_1.txt

outputs:


Sebastian
Daniel
Florence
Hannah

2.5.3 Print names which are a palindrome in name_age_1.txt and name_age_2.txt


$ perl -F: -l -a -n -e 'print() if (length($F[0]) > 3 and (uc($F[0])) eq reverse(uc($F[0])));' name_age_1.txt name_age_2.txt

outputs:


Hannah
Elle

2.5.4 Print the average age in name_age_1.txt

Here you've to use END to indicate the last action (after calculating the variables sum and count), i.e. printing the average:


$ perl -F: -l -a -n -e '$sum = $sum + $F[1];$count++;END{print($sum/$count)};' name_age_1.txt

outputs:


31.25

2.5.5 Use module

With Mmodule you can load a module. With Statistics::Lite (https://metacpan.org/pod/Statistics::Lite) you can calculate statistical values such as min, max, mean, median in an array. Let's rewrite the last code. I make first an array with the age data and apply then the statistical function mean:


$ perl -MStatistics::Lite=:all -F: -l -a -n -e 'push(@ages,$F[1]);END{print(mean @ages);};' name_age_1.txt

outputs (of course):


31.25

Notice that -MStatistics::Lite=:all is equivalent to use Statistics::Lite qw(:all); which you would use in a script file.

Footnotes

(1) There is a shorter one: $ perl -p -e '' name_age_1.txt The switch -p loops and prints.