UNIX for Dummies Questions & Answers

I'm reading about command substitutions and came across this little function in my book:

function lsd
{
date=$1
ls -l |grep -i "^.\{42\}$date"|cut -c55-
}

it's a little example which is supposed to select files by modification date, given as an argument to the function.

I can't figure out what the ^, ., and \{42\} means. The book says that the function tells UNIX to match any line that contains 41 characters followed by the function argument, with the date starting in column 42, then only printing the filenames, which start in column 55, but like I said I don't know what these symbols mean (or how they 'translate'). The only thing I've found about the ^ so far is that it's a 'word designator' but I'm not sure if that applies here.

I'm also not sure how to translate the .\{42\} either.

By the way, how do I find out what the column layout for the ls -l command is? (if I've phrased that correctly), that is, what column each part of the listing begins at?

It's a regular expression

^ means at the beginning of a line in some data
. means Any character
\{42\} means 42 of those any characters

If that . was an A or a B, \{42\} would mean 42 A's or B's.

so that line is looking for a pattern ^ (at the beginning of a line) with \{42\} characters of . (any character).

Can you tell me why the brackets are backslashed?

Oh, and while I'm here, the man pages state that the ^ is for the 'empty string' at the beginning of a line. can you tell me what 'empty string' is and how it relates to the carat here? (Did I ask that right?)

Hi.

The brackets (braces) are backslashed to give them special meaning (they would otherwise be literal braces).

^ matches something at the beginning of the line (an empty string because it doesn't remove anything from the pattern space), but has special meaning if it's the first character after a square braclet [ (in which case it negates the match (i.e. ^[0-9] means match a number at the start of a line, whereas ^[^0-9] means don't match a number at the start of a line, and [^0-9] means don't match a number, wherever [^0-9] appears in your expression).

In your expression ^.\{42\} means match 42 of any character starting at the beginning of the line, as DrSammy described.