Input-output redirection in Linux

 

stdin, stdout and stderr

stdin   <

stdout 1>

stderr  2>

Redirecting output

$cat file.txt
2
1
4
6
5

$sort file.txt
1
2
4
5
6

$sort file.txt > sortedfile.txt
OR
$sort file.txt 1> sortedfile.txt
$cat sortedfile.txt
1
2
4
5
6

## 1> or > , Overrides the file, sometimes you may not want to override instead you may want to append

$date >> file.txt
$date >> file.txt

$cat file.txt
Mon Jul  4 12:47:13 IST 2022
Mon Jul  4 12:47:15 IST 2022

## tee -a for appending in file instead of overriding .
$echo $SHELL | tee shell.txt
$echo $SHELL | tee -a shell.txt

Redirecting error

$grep -r '^The' /etc/
grep: /etc/cups/ssl: Permission denited

$grep -r '^The' /etc/ 2>/dev/null
It will give clean output

/dev/null is blackhole in the linux file system

### Redirect normal output and error output at the same to 2 different files
## For overwrite
$grep -r '^The' /etc/ 1>output.txt 2>errors.txt

## For appending
grep -r '^The' /etc/ 1>>output.txt 2>>errors.txt

Redirecting error as well as normal output to file

## writing errror text as well as normal text to a file
$grep -r '^The' /etc/ 
grep: /etc/cups/ssl: Permission denied
/etc/sfd/Input/all.txt:The two keys at the left

$grep -r '^The' /etc/ > all_output.txt 2>&1

OR

$grep -r '^The' /etc/ 1>all_output.txt 2>&1

Redirecting Input

$sort file.txt                               FROM FILE

$sendemail someone@example.com               FROM KEYBOARD

$someone@example.com < emailcontent.txt      FROM FILE    

Heredoc and Here String

$sort <<EOF                                  Here document or heredoc
> 4
> 6
> 3
> 1
> EOF
1
3
4
6


$ bc <<<1+2+3+4                             Here string

grep searching, sort for sorting

$grep -v "^#" /etc/login.defs
$grep -v "^#" /etc/login.defs | sort
$grep -v "^#" /etc/login.defs | sort | column -t