UNIX for Dummies Questions & Answers

I'm taking Unix Scripting course and we've been given the following command

Code:

grep -l "`echo '\t'`" foo

What this command is basically doing is giving the lines in the ordinary file foo that contains the letter 't'.

From my understanding, command substation occurs first. So echo '\t' is executed first.

Now, the command looks like this

Code:

grep -l "\t" foo

However, the double quotes don't save backslashes so the \t turns into just the character t which means now it is simply looking for that.

Now, this is what the question was about; to answer what the command is really doing.

However, I'm lost when I try to find a way to change this command to show simply \t

If I change the code to this

Code:

grep -l "`echo '\\t'`" foo

it still does the same thing.

However, when I change it to this

Code:

grep -l "`echo '\\\t'`" foo

then it tries to find \t but I don't see why it needs three backslahes.

Can anyone explain the logic in why it needs three and not two backslahes?

From: Bash Guide Guide for Beginners - Quoting characters

answer in bold:

Using double quotes the literal value of all characters enclosed is preserved, except for the dollar sign, the backticks (backward single quotes, ``) and the backslash.

The dollar sign and the backticks retain their special meaning within the double quotes.

The backslash retains its meaning only when followed by dollar, backtick, double quote, backslash or newline. Within double quotes, the backslashes are removed from the input stream when followed by one of these characters. Backslashes preceding characters that don't have a special meaning are left unmodified for processing by the shell interpreter.

A double quote may be quoted within double quotes by preceding it with a backslash.

What turk451 said is correct. To add just a bit more.....

You need to ultimately pass \\t to grep. If the search string given to grep has \t then grep will match a tab. Passing the double slant to grep is necessary to escape it such that grep treats it as a two character pattern.

In this case the shell is passing the backslant, but it is grep that is interpreting it as a tab rather than two characters.

As for why you need three backslants...
When the shell sees \\\t it replaces the first two with a single slant, and because \t isn't a special case it leaves it alone resulting in \\t being passed to grep which is exactly what you need.