Questions[grep](ubuntu)

I was wondering what grep -v "grep" does and what it means?

I want to search for a string of text in all files in a directory (and not its subdirectories; I know the -r option does that, but that is not what I want).

1. Running

   grep "string" /path/to/dir
   

   is supposed to be able to do this, I've read, but it gives me the error:

   grep: dir: Is a directory

2. Next, I tried running grep on multiple files.

   grep "string" .bashrc .bash_aliases works perfectly.

   grep "string" .bash* works as intended too.

   grep "string" * gives me the errors:

   grep: data: Is a directory
   grep: Desktop: Is a directory
   grep: Documents: Is a directory
   grep: Downloads: Is a directory
   ...
   

Only the errors are printed, I don't get the matching lines. I tried using the -s option, but to no avail.

So, my questions:

1. Why am I not being able to use grep on a directory, as in (1), when I should be able to? I've seen that done in plenty examples on the Internet.
   Edit: When I say "using grep on a directory", I mean "search in all the files in that directory excluding its subdirectories". I believe that this is what grep does when you pass a directory to it in place of a file. Am I incorrect?

2. Please give me an explanation on the workings of grep that would explain the behavior of commands in (2).
   Edit: Let me be more specific. Why does using wildcards to specify multiple files to search in for work with .bash* and not with * or even ./*?

3. How can I search all the files in a directory (and not its subdirectories) using grep?

Here's my command that I'm using in a script to grep real-time data. It doesn't seem to pull real-time data correctly as it just misses some lines.

tail -f <file> | fgrep "string" | sed 's/stuff//g' >> output.txt

What would the following command do? What is "line buffering"?

tail -f <file> | fgrep --line-buffered "string" | sed 's/stuff//g' >> output.txt

This

ls -l /var/log | awk '{print $9}' | grep "^[a-z]*\.log."

outputs this:

alternatives.log.1
alternatives.log.10.gz
alternatives.log.2.gz
alternatives.log.3.gz
alternatives.log.4.gz
alternatives.log.5.gz
alternatives.log.6.gz
alternatives.log.7.gz
alternatives.log.8.gz
alternatives.log.9.gz
apport.log.1
apport.log.2.gz
apport.log.3.gz

but this:

ls -l /var/log | awk '{print $9}' | grep "^[a-z]+\.log."

outputs nothing.

Why? I just changed * to +. Isn't it similar? Operator + just needs at least one match, and * zero or more.

How can I recursively search for directory names with a particular string where the string is only part of the directory name?

For example: the directory name is "8.0.3-99966_en", but I want to recursively search for directories with the string "99966".