Official Lain command

This perl one-liner changes the first “foo” in the second column to “bar”.

$ perl -lai -n -e '$F[1] =~ s/foo/bar/; print "@F"' <file>

• -l: adds a newline character after every print statement;
• -a: splits each line into columns, like AWK. the columns are stored in the @F array;
• -i: edits the file in-place. a suffix can be specified like this: -i.bak, and the original file will be backed up as <file>.bak, which is why it’s separated from the -n option, otherwise the file would be backed up as <file>n;
• -n: wraps the code in a while (<>) {} block, which basically means iterating through every line of every file, or from stdin if no file was specified;
• -e: specifies a command.

If you want to modify a column in a file, Lain’s got you wired.