Modifying a File in Place with -i Switch

Problem

You need to modify a file in place from the command line, and you're too lazy[5] for the file manipulation of .

[5] Lazy-as-virtue, not lazy-as-sin.

Solution

Use the -i and -p switches to Perl. Write your program on the command line:

% perl -i.orig -p -e 'FILTER COMMAND' file1 file2 file3 ...

Or use the switches in programs:

#!/usr/bin/perl -i.orig -p # filter commands go here

The -i command-line switch modifies each file in place. It creates a temporary file as in the previous recipe, but Perl takes care of the tedious file manipulation for you. Use it with -p (explained in ) to turn:

% perl -pi.orig -e 's/DATE/localtime/e'

into:

while (<>) {
 if ($ARGV ne $oldargv) {
 # are we at the next file? rename($ARGV, $ARGV . '.orig'); open(ARGVOUT, ">$ARGV"); # plus error check select(ARGVOUT); $oldargv = $ARGV;
}
s/DATE/localtime/e;
}
continue{ print;
}
select (STDOUT); # restore default output

The -i switch takes care of making a backup (say -i instead of -i.orig to discard the original file contents instead of backing them up), and -p makes Perl loop over filenames given on the command line (or STDIN if no files were given).

The preceding one-liner would turn a file containing the following:

Dear Sir/Madam/Ravenous Beast, As of DATE, our records show your account is overdue. Please settle by the end of the month. Yours in cheerful usury, --A. Moneylender

into:

Dear Sir/Madam/Ravenous Beast, As of Sat Apr 25 12:28:33 1998, our records show your account is overdue. Please settle by the end of the month. Yours in cheerful usury, --A. Moneylender

This switch makes in-place translators a lot easier to write and to read. For instance, this changes isolated instances of "hisvar" to "hervar" in all C, C++, and yacc files:

% perl -i.old -pe 's{\bhisvar\b}{hervar}g' *.[Cchy]

Turn on and off the -i behavior with the special variable $^I. Set @ARGV, and then use <> as you would with -i on the command line:

# set up to iterate over the *.c files in the current directory, # editing in place and saving the old file with a .orig extension local $^I = '.orig'; # emulate -i.orig local @ARGV = glob("*.c"); # initialize list of files while (<>) {
 if ($. == 1) {
 print "This line should appear at the top of each file\n";
}
s/\b(p)earl\b/${1}erl/ig; # Correct typos, preserving case print;
}
continue {close ARGV if eof}

Beware that creating a backup file under a particular name when that name already exists clobbers the previously backed up version.