Use input-output redirection (>, >>, |, 2>, etc.)

1. >
2. >>
3. <
4. <<
5. |
6. 2> or 2>>
7. &> or &>>

1. How to use > in syntax

We could redirect ouput into file with this > symbol that we called as stdout or standard output stream that often symbolize with 1, you could write them like 1> or just >. Like for instanse I would like to print hello world and put the string into file called hello.txt, I could use this command as follows:

┌──(root㉿cfe2a1cc128a)-[/home]
└─# echo "hello world" > hello.txt

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat hello.txt 
hello world

We could also combine with internal function or variable like date to print out the value into a file or many files at once, like for example:

┌──(root㉿cfe2a1cc128a)-[/home]
└─# echo $(date +'%Y-%m-%d %H:%M:%S') > datetime.txt

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat datetime.txt 
2024-02-10 23:15:35

┌──(root㉿cfe2a1cc128a)-[/home]
└─# for i in {1,2,3};do echo $(date +'%Y-%m-%d %H:%M:%S') > datetime-$i.txt;done 

┌──(root㉿cfe2a1cc128a)-[/home]
└─# ls datetime-*
datetime-1.txt  datetime-2.txt  datetime-3.txt

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat datetime-*
2024-02-10 23:29:33
2024-02-10 23:29:33
2024-02-10 23:29:33

2. How to use >> in syntax

While > replaces every content with a new one each time you redirect a string or value into it, instead of replacing, >> added a new line bellow the current one and so on, here is the example how you could implement >> redirection:

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat hello.txt 
hello world

┌──(root㉿cfe2a1cc128a)-[/home]
└─# echo "it should be added bellow the hello world" >> hello.txt 

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat hello.txt 
hello world
it should be added bellow the hello world

┌──(root㉿cfe2a1cc128a)-[/home]
└─# for i in {1..5};do echo "$i says: me too" >> hello.txt ;done

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat hello.txt 
hello world
it should be added bellow the hello world
1 says: me too
2 says: me too
3 says: me too
4 says: me too
5 says: me too

3. How to use < in syntax

This symbol < is stdin or standard input stream that associates with 0 and technically can be called like 0< or just <, commonly uses to redirect input from the right side of the command into the left side, for instanse I have this file called numbers.txt that contains number from 1 to 7 and I want to direct input its contect into a loop statement. It could be as follows

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat numbers.txt 
1
2
3
4
5
6
7

──(root㉿cfe2a1cc128a)-[/home]
└─# while read n;do echo "This is $n";done < numbers.txt 
This is 1
This is 2
This is 3
This is 4
This is 5
This is 6
This is 7

We could also combine with internal command like wc to count how many lines are listed in the file

┌──(root㉿cfe2a1cc128a)-[/home]
└─# wc -l < numbers.txt 
7

4. How to use << in syntax

I tend to use this << command to create or append a file’s conntend live in the screen, which mean we does not need any text editor to do this, let see the example here

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat > live-type.txt << END
> one
> two
> three
> four
> five
> END

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat live-type.txt 
one
two
three
four
five

If I re-do the same action with cat >, it shold replace its content with a new one.

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat > live-type.txt << EOF 
> 1
> 2
> 3
> EOF

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat live-type.txt 
1
2
3

But if I use cat >> it would append a new string after the exsits one

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat >> live-type.txt << EOF
> four
> five
> six
> EOF

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat live-type.txt 
1
2
3
four
five
six

Keep in mind that eigher END and EOF are just a string to tell the shell that that is the end of its content, please end this session.

5. How to use | in syntax

We called this | symbol as pipe, its function kinda similar with <, which redirect output from left side statement to the right side of it. You would use this alot in shell command. Look at this example:

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat numbers.txt 
1
2
3
4
5
6
7

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat numbers.txt |wc -l
7

6. How to use 2> or 2>> in syntax

As we know stdin as 0 and we use to redirect input like 0< or just simply <, we also know stdout as 1 to redirect ouput these statements 1> or just > as well. There is also stderr or standard error stream that helps us to redirect outuput of any error messages that returned from the commands that we have submited to our shell. The following example will best describe how to use this function properly.

┌──(root㉿cfe2a1cc128a)-[/home]
└─# ls Idontexists.txt
ls: cannot access 'Idontexists.txt': No such file or directory

┌──(root㉿cfe2a1cc128a)-[/home]
└─# ls Idontexists.txt 2>errmsg.txt

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat errmsg.txt 
ls: cannot access 'Idontexists.txt': No such file or directory

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat errmsg.txt |wc -l
1

┌──(root㉿cfe2a1cc128a)-[/home]
└─# ls errmsg.txt 2>errmsg.txt 
errmsg.txt

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat errmsg.txt 

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat errmsg.txt |wc -l
0

In this example, we see that I was able redirect stderr to a file called errmsg.txt when there was an error happening in the running command. But when there are no error, it will replace the errmsg.txt with empty or null.

7. How to use &> or &>> in syntax

In order to catch either stdout or stderr we use this symbol to prevent our error catcher replaced by empty or null output stream. If we use &> our target file will always replaced with a new value from the redirected output, if we use &>> it will be append into the file and this is how Linux system manage its own logging file that in general can be accessed in /var/log directory. Let’s have a look at this example:

┌──(root㉿cfe2a1cc128a)-[/home]
└─# ls Idontexists.txt
ls: cannot access 'Idontexists.txt': No such file or directory

┌──(root㉿cfe2a1cc128a)-[/home]
└─# ls Idontexists.txt &>ourerr.txt

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat ourerr.txt 
ls: cannot access 'Idontexists.txt': No such file or directory

┌──(root㉿cfe2a1cc128a)-[/home]
└─# ls ourerr.txt &>ourerr.txt 

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat ourerr.txt 
ourerr.txt

┌──(root㉿cfe2a1cc128a)-[/home]
└─# ls -lh ourerr.txt &>ourerr.txt 

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat ourerr.txt 
-rw-r--r--. 1 root root 0 Feb 11 06:26 ourerr.txt

┌──(root㉿cfe2a1cc128a)-[/home]
└─# ls HowAreYou.txt &>>ourerr.txt 

┌──(root㉿cfe2a1cc128a)-[/home]
└─# cat ourerr.txt 
-rw-r--r--. 1 root root 0 Feb 11 06:26 ourerr.txt
ls: cannot access 'HowAreYou.txt': No such file or directory

Finally, we have covered almost all the subject of input-ouput redirection in Linux shell or bash scripting language. The next section would be refers to topict related to RHCSA Objectives.

This material is refers to the rhca objectives