linux I and o redirection USES elaboration

1. Output redirection:

By default, both standard output and error output are terminals, and the standard output and error contents can be redirected:

[~]# echo "hello\!"
hello\!
[~]# echo "hello!" 
-bash: !": event not found "

Redirect standard output to a file

[~]# echo "hello" > test.sh
[~]# cat test.sh
hello
'>' Output mode is equivalent by default '1>'
[~]#  echo "hello" 1> test.sh
[~]# cat test.sh 
hello

But the error is still displayed on the screen:

[~]$cat edit.sql /root/test.sh > temp.sh
cat: /root/test.sh: Permission denied

Error contents can also be output to a file (using the file descriptor) :

[~]$cat edit.sql /root/test.sh 1> temp.sh 2> error.sh
[~]$cat temp.sh 
select           dbms_rowid.rowid_object('AAAZdQAAGAAATxjAAk') data_object_id#,
dbms_rowid.rowid_relative_fno('AAAZdQAAGAAATxjAAk') rfile#,
dbms_rowid.rowid_block_number('AAAZdQAAGAAATxjAAk') block#,
dbms_rowid.rowid_row_number('AAAZdQAAGAAATxjAAk') row# from dual
/
[~]$cat error.sh 
cat: /root/test.sh: Permission denied

Write standard output and error messages to the same file:

[~]$cat edit.sql /root/test.sh > temp.sh 2>&1
[~]$cat temp.sh 
select           dbms_rowid.rowid_object('AAAZdQAAGAAATxjAAk') data_object_id#,
dbms_rowid.rowid_relative_fno('AAAZdQAAGAAATxjAAk') rfile#,
dbms_rowid.rowid_block_number('AAAZdQAAGAAATxjAAk') block#,
dbms_rowid.rowid_row_number('AAAZdQAAGAAATxjAAk') row# from dual
/
cat: /root/test.sh: Permission denied

This might seem like a bit of a hassle, but in practice it's probably used the most:

[~]$cat edit.sql /root/test.sh &>temp.sh 
[~]$cat temp.sh 
select           dbms_rowid.rowid_object('AAAZdQAAGAAATxjAAk') data_object_id#,
dbms_rowid.rowid_relative_fno('AAAZdQAAGAAATxjAAk') rfile#,
dbms_rowid.rowid_block_number('AAAZdQAAGAAATxjAAk') block#,
dbms_rowid.rowid_row_number('AAAZdQAAGAAATxjAAk') row# from dual
/
cat: /root/test.sh: Permission denied
&> Write all the output to the same 1 A file 

If you don't want to output to a file or display it on the screen, you can use /dev/null as a special device file (bit bucket)
[~]$cat edit.sql /root/test.sh & > /dev/null
If you write standard output to a file, you can't use the pipe symbol '|' to pass the contents to the following command. You can use the tee command to solve this problem:

[~]$cat edit.sql /root/test.sh | tee temp.sh | cat -n 
cat: /root/test.sh: Permission denied
select           dbms_rowid.rowid_object('AAAZdQAAGAAATxjAAk') data_object_id#,
             dbms_rowid.rowid_relative_fno('AAAZdQAAGAAATxjAAk') rfile#,
             dbms_rowid.rowid_block_number('AAAZdQAAGAAATxjAAk') block#,
               dbms_rowid.rowid_row_number('AAAZdQAAGAAATxjAAk') row# from dual
/

The tee command is equivalent to writing a copy of stdout to a file and passing stdout to the next command, but the error content cannot be passed using tee, as used > > To append content,tee can append with the -a option:

[~]$cat edit.sql /root/test.sh | tee -a temp.sh | cat -n 
cat: /root/test.sh: Permission denied
select           dbms_rowid.rowid_object('AAAZdQAAGAAATxjAAk') data_object_id#,
             dbms_rowid.rowid_relative_fno('AAAZdQAAGAAATxjAAk') rfile#,
            dbms_rowid.rowid_block_number('AAAZdQAAGAAATxjAAk') block#,
               dbms_rowid.rowid_row_number('AAAZdQAAGAAATxjAAk') row# from dual
/
[~]$cat temp.sh
select           dbms_rowid.rowid_object('AAAZdQAAGAAATxjAAk') data_object_id#,
dbms_rowid.rowid_relative_fno('AAAZdQAAGAAATxjAAk') rfile#,
dbms_rowid.rowid_block_number('AAAZdQAAGAAATxjAAk') block#,
dbms_rowid.rowid_row_number('AAAZdQAAGAAATxjAAk') row# from dual
/
select           dbms_rowid.rowid_object('AAAZdQAAGAAATxjAAk') data_object_id#,
dbms_rowid.rowid_relative_fno('AAAZdQAAGAAATxjAAk') rfile#,
dbms_rowid.rowid_block_number('AAAZdQAAGAAATxjAAk') block#,
dbms_rowid.rowid_row_number('AAAZdQAAGAAATxjAAk') row# from dual
/

2. Input redirection :

Input redirection, in some cases, such as the database monitoring used more often, is the use of inline redirection

[~]$cat < edit.sql 
select           dbms_rowid.rowid_object('AAAZdQAAGAAATxjAAk') data_object_id#,
dbms_rowid.rowid_relative_fno('AAAZdQAAGAAATxjAAk') rfile#,
dbms_rowid.rowid_block_number('AAAZdQAAGAAATxjAAk') block#,
dbms_rowid.rowid_row_number('AAAZdQAAGAAATxjAAk') row# from dual
/

For example, the following operation, put < < EOF > The content between temp.sh and the last EOF is treated as stdin, then stdout is written to the file temp.sh

[~]$cat <<EOF > temp.s
this is my log
EOF
[~]$cat temp.sh 
this is my log

3. Custom file descriptor:

The principle behind the custom file descriptor is to use the basic three file open modes
Read-only ( < )
Truncation ( > )
Additional, > > )
Create a file descriptor 3 to open the file
[~]# exec 3 < test.sh
Now you can open the file directly using the file descriptor, but only once:

[~]# cat <&3
hello
hello

Create file descriptor 4 for writing to files (reusable) :

[~]# exec 4>test.sh
[~]# echo okok >&4
[~]# cat test.sh 
okok

This is actually similar to the previous one:

[~]$cat edit.sql /root/test.sh > temp.sh 2>&1

Create file descriptor 5, which is used to append content to the file (it can also be reused, not used once as input file descriptor) :

[~]# exec 5>>test.sh
[~]# echo okokok >&5 
[~]# cat test.sh     
okokok
[~]# echo okokok >&5
[~]# cat test.sh    
okokok
okokok