You Can Use Both Input and Output Redirection With the Same Centos 7 Command.

The work of whatever control is either taking input or gives an output or both. Then, Linux has some command or special grapheme to redirect these input and output functionalities. For example: suppose we want to run a command chosen "date" if nosotros run information technology volition print the output to the current terminal screen. Merely our requirement is different, nosotros don't want the output to be displayed on the terminal. We want the output to be saved in a file. This could be done very easily with output redirection. Redirection here but means diverting the output or input.

Similarly, if nosotros have a command that needs input to be performed. Permit have a command "head" this needs input to give output. So either we write input in the form of command directly or redirect input from any other place or file. Suppose we take a file called "file.txt" to print the starting some lines of the file nosotros could use the "head". So let'southward encounter how this all is done on the terminal.

Types of Redirection

ane. Overwrite

• ">" standard output
• "<" standard input

two. Appends

• ">>" standard output
• "<<" standard input

3. Merge

• "p >& q" Merges output from stream p with stream q
• "p <& q" Merges input from stream p with stream q

Implementation: So whatever you will write after running this control, all will exist redirected and copied to the "file.txt". This is standard output redirection.

cat > file.txt

Now, this is standard input redirection, true cat command will have the input from "file.txt" and impress it to the final screen. This line of code besides shows the real working and significant of the true cat command that is copy and paste. Many people have a misconception that the true cat is used to create a file, simply it is not true, the primary work of the cat is to copy the input and give the output to the screen.

cat < file.txt

Let's see an example to empathize the real work of true cat command

true cat

Just type true cat on the terminal and hit enter. It will ask for the input lines, y'all could write your name and striking enter. You will see your input volition exist reprinted.

(base) [root@localhost ~]# cat How-do-you-do this is GeeksForGeeks How-do-you-do this is GeeksForGeeks

This is used when we want to append some lines to the existing content of the file. If y'all utilise only a single angular bracket all the content of the file will be lost.

cat >> file.txt

To come across the working of append standard input:

A here-certificate is used to redirect input into an interactive shell script or program. You tin can run whatever program within a vanquish script without user action by supplying the required input for the interactive programme, or interactive shell script.

The full general class for a hither certificate is −

Syntax: control << delimiter document delimiter  (base of operations) [root@localhost ~]# cat << helo.txt  > Hello This is  > GeeksForGeeks helo.txt Hello This is  GeeksForGeeks (base of operations) [root@localhost ~]#  Note: Here, helo.txt is a delimiter.

The delimiter marks the ending signal of the document. Without information technology, the shell continues to read the input forever. The delimiter must be a unmarried word that does not comprise spaces or tabs.

Error Redirection: Error redirection is transferring the errors generated by some false commands to a file rather than STDOUT.

Whenever a program is executed at the concluding, 3 files are generated: standard input(0), standard output(i), standard error(2). These files are ever created whenever a program is run. By default, an error stream is displayed on the screen.

Examples:

ane. In the below-mentioned instance, the file descriptor used above is 2(STDERR). Using "2>" re-directs the error output to a file named "fault.txt" and nada is displayed on STDOUT.

$ somerandomcommand ii>error.txt

ii.Hither, 2>&ane means that STDERR redirects to the target of STDOUT. More formally, the mistake message generated by "two" gets merged with the current output "1".

$ ls GEEK GFG > mistake.txt 2>&1

In the above example, the directory GEEK is non present. The mistake output is merged with the standard output which in turn is being re-directed to "mistake.txt".

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Source: https://www.geeksforgeeks.org/input-output-redirection-in-linux/

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