如果没有指定文件，为什么grep不起作用？

Question

I have some problem with the Linux grep command, it don't work !!!

I am trying the following test on my Ubuntu system:

1. I have create the following folder: /home/andrea/Scrivania/prova

2. Inside this folder I have created a txt file named prova.txt and inside this file I have write the string test and I have save it

3. In the shell I have first access the folder /home/andrea/Scrivania/prova and so I have launched the grep command in the following way:

    ~/Scrivania/prova$ grep test 

The problem is that the cursor continues to blink endlessly and cannot find NOTHING! Why? What is the problem?

Answer 1

You've not provided files for the grep command to scan

grep "test" *

or for recursive

grep -r "test" *

Answer 2

Because grep searches standard input if no files are given. Try this.

grep test *

Answer 3

You are not running the command you were looking for.

grep test * will look for test in all files in your current directory.

grep test prova.txt will look for test specifically in prova.txt

( grep test will grep the test string in stdin , and will not return until EOF .)

Answer 4

You need to pipe in something to grep - you cant just call grep test without any other arguments as it is actually doing nothing. try grep test *

Another use for grep is to pipe in a command

eg This is my home directory:

drwx------+  3 oliver  staff    102 12 Nov 21:57 Desktop
drwx------+ 10 oliver  staff    340 17 Nov 18:34 Documents
drwx------+ 17 oliver  staff    578 20 Nov 18:57 Downloads
drwx------@ 12 oliver  staff    408 13 Nov 20:53 Dropbox
drwx------@ 52 oliver  staff   1768 11 Nov 12:05 Library
drwx------+  3 oliver  staff    102 12 Nov 21:57 Movies
drwx------+  5 oliver  staff    170 17 Nov 10:40 Music
drwx------+  3 oliver  staff    102 20 Nov 19:17 Pictures
drwxr-xr-x+  4 oliver  staff    136 12 Nov 21:57 Public

If i run

l | grep Do

I get the result

drwx------+ 10 oliver  staff    340 17 Nov 18:34 Documents
drwx------+ 17 oliver  staff    578 20 Nov 18:57 Downloads

remember to pipe the grep command

Answer 5

From grep man page:

Grep searches the named input FILEs (or standard input if no files are named, or the file name - is given) for lines containing a match to the given PATTERN.

If you don't provide file name(s) for it to use, it will try to read from stdin.

Try grep test *

Answer 6

As per GNU Grep 3.0

A file named - stands for standard input. If no input is specified, grep searches the working directory . if given a command-line option specifying recursion; otherwise, grep searches standard input.

So for OP's command, without any additional specification, grep tries to search in standard input, which is not actually provided there.

A simple approach is grep -r [pattern] , as per the above, to specify recursion with -r and search in current directory and sub-directories.

Also note that wildcard * only includes files, not directories . If used, a prompt might be shown for hint:

grep: [directory_name]: Is a directory