CentOS-Become System Administrator- Basic Bash Command- I/O Redirection

Basic  BASH Command

I/O Redirection

1. Input/Output Redirection in BASH( Again Shell) - Most command in Linux reads input and write output.By default input is given by Keyboard and output is given  to  terminal(monitor).

Every command has three file descriptor : STDIN, STDOUT, STDERR.
STDIN : 0    - Comes from keyboard
STDOUT  : 1 - goes to console
STDERR  : 2 - goes to console

Not all command listen for STDIN(standard Input) some of them only have argument. for example ls command , it is not waiting for input, it optionally work with argument

Redirection operator

it is possible to redirect STDIN, STDOUT, and STDERR to other places.

Input Redirection  with  0< or <  - for redirect STDIN

Redirection is done using 0< or <(less than symbol) operator. 0 is optional.

[yogesh@lap ~]$ read fromkb

hello i am reading from keyboard and save it fromkb variable, use echo $fromkb to view my content
[yogesh@lap ~]$ echo $fromkb
hello i am reading from keyboard and save it fromkb variable, use echo $fromkb to view my content

Now we use < or 0< for redirect STDIN, I create a message file message.txt using vi editor

$ vi message.txt

Previously we saw that read command read from KEYBOARD we use STDIN redirect , now it read from message.txt file and store it into variable fromfile, we can see the value of fromfile variable using echo command.

[yogesh@lap ~]$ cat message.txt
read command by default read from keyboard now < or 0< is used for redirect STDIN, now it read content form file and store it into variable tempfile
[yogesh@lap ~]$ read fromfile < message.txt
[yogesh@lap ~]$ echo $fromfile
read command by default read from keyboard now < or 0< is used for redirect STDIN, now it read content form file and store it into variable tempfile
[yogesh@lap ~]$

Output Redirection with 1> or > - for redirect STDOUT

> or 1> is used to redirect STDOUT, For example ls command list the content of directory at terminal but if we want to redirect it to a file the we use > or 1>.  In 1> ,No need to type 1 , it is presumed.

[yogesh@lap test]$ ls
file1,  file2  message.txt

[yogesh@lap test]$ ls > ls.out
[yogesh@lap test]$ cat ls.out
file1,
file2
ls.out
message.txt

/*****we can use 1>  produce same result******/

[yogesh@lap test]$ ls 1>ls.out
[yogesh@lap test]$ ls
file1,  file2  ls.out  message.txt
[yogesh@lap test]$ cat ls.out
file1,
file2
ls.out
message.txt
[yogesh@lap test]$ 

Output Redirection 2> - for redirect STDERR

By default all Error message  also shown at Terminal, it is possible to redirect error message to another place like a log file for latter analysis. Following Example show how ls command redirect Error message to file.

[yogesh@lap test]$ ls
file1  file2  ls.out  message.txt
[yogesh@lap test]$ ls file1 file3
ls: cannot access file3: No such file or directory
file1
[yogesh@lap test]$ ls file1 file3 2>errorlog.out
file1
[yogesh@lap test]$ cat errorlog.out 
ls: cannot access file3: No such file or directory

& - Ampersand is  an indication that the number that follows is not a file name, but rather a location that the data stream is pointed to.

[yogesh@lap test]$ ls
file1  file2  message.txt
[yogesh@lap test]$ ls 1>test.out 2>&1
[yogesh@lap test]$ ls
file1  file2  message.txt  test.out
[yogesh@lap test]$ cat test.out
file1
file2
message.txt
test.out
[yogesh@lap test]$ 

>> Operator - >> operator append text to a file instead of overwriting it.

[yogesh@lap test]$ ls
file1  file2  message.txt  test.out
[yogesh@lap test]$ echo hello test file1 > file1
[yogesh@lap test]$ cat file1
hello test file1
[yogesh@lap test]$ echo hello i am test file2 > file2
[yogesh@lap test]$ cat file2
hello i am test file2
[yogesh@lap test]$ echo i am write some content to file1 > file1
[yogesh@lap test]$ cat file1
i am write some content to file1
[yogesh@lap test]$ echo i am append some content to file2 using append operator >> file2
[yogesh@lap test]$ cat file2
hello i am test file2
i am append some content to file2 using append operator
[yogesh@lap test]$ 

By default | (pipe symbol above Enter Key US Keyboard) - Pipe the result of one command to other command. i.e. output of one command become input of another command. for example

[yogesh@lap test]$ cat itemlist
1
2
4
5
6
7
19
3
8
12
9
[yogesh@lap test]$ cat itemlist | sort -n
1
2
3
4
5
6
7
8
9
12
19
[yogesh@lap test]$ 

Writing to STDOUT and File simultaneously

tee command is used in this case, tee command send output to one or more file in one move.

tee - a is used to append.

for example.

[yogesh@lap test]$ date | tee f1 f2 f3
Fri Jan 15 14:21:17 IST 2016
[yogesh@lap test]$ cat f1
Fri Jan 15 14:21:17 IST 2016
[yogesh@lap test]$ cat f2
Fri Jan 15 14:21:17 IST 2016
[yogesh@lap test]$ cat f3
Fri Jan 15 14:21:17 IST 2016
[yogesh@lap test]$ cat teetest | tee -a f1 f2 f3
cat: teetest: No such file or directory
[yogesh@lap test]$ echo content added to f1 f2 and f3 | tee -a f1 f2 f3 
content added to f1 f2 and f3
[yogesh@lap test]$ cat f1 && cat f2 && cat f3
Fri Jan 15 14:21:17 IST 2016
content added to f1 f2 and f3
Fri Jan 15 14:21:17 IST 2016
content added to f1 f2 and f3
Fri Jan 15 14:21:17 IST 2016
content added to f1 f2 and f3
[yogesh@lap test]$ cat f1 f2 f3
Fri Jan 15 14:21:17 IST 2016
content added to f1 f2 and f3
Fri Jan 15 14:21:17 IST 2016
content added to f1 f2 and f3
Fri Jan 15 14:21:17 IST 2016
content added to f1 f2 and f3