3.1 Find, Grep and IO

Input Output and Error

Redirection is a feature in Linux such that when executing a command, you can change the standard input/output devices. The basic workflow of any Linux command is that it takes an input and gives an output. The standard input (stdin) device is the keyboard. The standard output (stdout) device is the screen. These are called streams.

We can control these with redirects and pipes.

Here are all the streams available:

Streams	Usage	stream number
stdin	The keyboard or whatever you type into the terminal	0
stdout	This is the result of a command	1
stderr	If the command or bash makes any error this is put on the error steam	2

Here are some redirects and pipes:

Redirects and pipes	Explanation
|	Pipe this takes the stdout of a command to make it the stdin of the next command
>	Redirect stdout to a file and replace file
>>	Redirect stdout and add it to bottom of file

So you can send input outputs between commands. Let’s examine the following command:

cat textfile | grep word > newtextfile

Let’s deconstruct the command:

command	stdin	pipe	command	stdin	redirect	stdin
cat	textfile	|	grep	word	>	newtextfile

Let’s do some practice with this.

Grep

become root

We are going to use grep to get data from a file. You will find a book at /media/onthegenealogyofmorality. We can read the book with the cat command; this command will paste the content of any text file to stdout, aka the terminal screen.

cat /media/onthegenealogyofmorality

We want to know how many chapters there are in this book…..

cat /media/onthegenealogyofmorality | grep chapter

Results should look like this:

[root@rhcsa media]# cat onthegenealogyofmorality |grep chapter
chapter 1
chapter 2
chapter 3
chapter 4

Let’s make a directory to make the text file in:

mkdir –p /root/found/

Now we wish to send these results to a file to be stored and checked later. Note: this is one line.

cat onthegenealogyofmorality | grep chapter > /root/found/chapterlist.txt

Results inside the file can again be read with cat (or vim). The result of that should look like this:

[root@rhcsa media]# cat /root/found/chapterlist.txt
chapter 1
chapter 2
chapter 3
chapter 4

Find

On your vm, become root and use the following command to find files:

find

We can use find to search from any starting folder on any paramater that identifies the file. For example: we can use “find” here to locate all files from the user nietzche and since we start the search from the / root of the filesystem:

find / -user nietzche

The result should look something like this:

[root@rhcsa ~]# find / -user nietzche
find: ‘/proc/1793/task/1793/fd/5’: No such file or directory
find: ‘/proc/1793/task/1793/fdinfo/5’: No such file or directory
find: ‘/proc/1793/fd/6’: No such file or directory
find: ‘/proc/1793/fdinfo/6’: No such file or directory
/var/spool/mail/nietzche
/var/secret/file2
/home/nietzche
/home/nietzche/file1
/media/onthegenealogyofmorality
/srv/file3
/shared/file4

Now that we found the files, we want to copy them to a found directory, so first we need to make the directory.

mkdir –p /root/found

Now let’s find and copy the result. It is one line! We use xargs for this:

find / -user nietzche | grep file* | xargs cp -t /root/found/

The result should look something like this:

[root@rhcsa found]# ll
total 0
-rwx------. 1 root root 0 Nov 29 16:10 file1
-rwx------. 1 root root 0 Nov 29 16:10 file2
-rwx------. 1 root root 0 Nov 29 16:10 file3
-rwx------. 1 root root 0 Nov 29 16:10 file4