command: ls /etc | sort | grep d* is yielding no results but ls /etc | sort | grep p* lists entire directory

Question

I am playing around with piping and grep tonight. I know that grep uses regex and that * means 0 or more occurrences of the preceding character. So the way I understand it is that if I do the following command, the entire directory should be listed... but nothing is listed. All that happens is the command line resets:

[root@LinuxAcademy etc]# ls /etc | sort | grep d*
[root@LinuxAcademy etc]#

However, if I do the command again and replace d with p, the entire directory is listed just as I would expect:

[root@LinuxAcademy ~]# ls /etc | sort | grep p*
ConsoleKit
DIR_COLORS
DIR_COLORS.256color
DIR_COLORS.lightbgcolor
NetworkManager
X11
...........<<rest of listing not pasted in>>

Even though no error was given when i used the grep d*, I redirected stderr to a file just to check and nothing was printed.

I then thought that grep may be interpreting the 'd' in 'grep d*' to be an option or command... but that doesn't seem to be the case either.

Can anyone help me understand what is going on and why when i use the command with

grep d*

nothing is listed?

Answer 1

A regular expression is not a glob.

I am guessing that you want files that start with "d". In that case, you need:

ls /etc | sort | grep '^d'

What went wrong

How the following statement behaves depends on the files in the current directory:

ls /etc | sort | grep p*

Since it is unquoted, the shell will try to expand the glob p*, replacing it with whatever file names from the current directory match.

If no file names match, then a literal p* is passed to grep. Since grep expects regular expressions, it interprets p* to mean zero or more occurrences of the letter p. Since everything either has a p or doesn't, that matches everything.

Why would grep d* list nothing?

That likely means that there was a file name starting with d in the current directory. Just to be specific, let's say that file's name was dichotomy. The shell expands d* to dichotomy and passes that as the argument to grep. Thus, the effective command is:

grep dichotomy

That is likely why there were no results when you ran grep d*.

What happens if multiple files match the d* glob?

Consider the command:

grep d*

If their are multiple files matching that glob, say dmesg daemon.log and dpkg.log. The shell would expand d* to the list of those files in alphabetical order (which depends on your locale). This may result in:

grep daemon.log dmesg dpkg.log

This command searches for the presence of the string daemon.log in the files dmesg and dpkg.log. If that string does not occur in those files, no output results.