Which of the following symbols takes the output of one command and redirects it to the input of another command?

CONCEPT: Every program you run from the shell opens three files: Standard input, standard output, and standard error. The files provide the primary means of communications between the programs, and exist for as long as the process runs.

The standard input file provides a way to send data to a process. As a default, the standard input is read from the terminal keyboard.

The standard output provides a means for the program to output data. As a default, the standard output goes to the terminal display screen.

The standard error is where the program reports any errors encountered during execution. By default, the standard error goes to the terminal display.

CONCEPT: A program can be told where to look for input and where to send output, using input/output redirection. Unix uses the "less than" and "greater than" special characters (< and >) to signify input and output redirection, respectively.

Redirecting input

EXAMPLE:Use standard input redirection to send the contents of the file /etc/passwd to the more command:

Many Unix commands that will accept a file name as a command line argument, will also accept input from standard input if no file is given on the command line.

EXAMPLE: To see the first ten lines of the /etc/passwd file, the command:

Redirecting output

EXAMPLE: Type the command

Use two "greater-than" signs to append to an existing file. For example:

EXAMPLE: In this example, I list the contents of the /tmp directory, and put it in a file called myls. Then, I list the contents of the /etc directory, and append it to the file myls:

Redirecting error

EXAMPLE: Sort the /etc/passwd file, place the results in a file called foo, and trap any errors in a file called err with the command:

The work of any command is either taking input or gives an output or both. So, Linux has some command or special character to redirect these input and output functionalities. For example: suppose we want to run a command called “date” if we run it will print the output to the current terminal screen. But our requirement is different, we 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 simply means diverting the output or input. 

Similarly, if we have a command that needs input to be performed. Let take 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 have a file called “file.txt” to print the starting some lines of the file we could use the “head”. So let’s see how this all is done on the terminal. 

Types of Redirection 

1. Overwrite  

• “>” standard output
• “<” standard input

2. 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 command, all will be redirected and copied to the “file.txt”. This is standard output redirection.  

cat > file.txt

Now, this is standard input redirection, cat command will take the input from “file.txt” and print it to the terminal screen. This line of code also shows the real working and meaning of the cat command that is copy and paste. Many people have a misconception that the cat is used to create a file, but it is not true, the main 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 understand the real work of cat command  

cat

Just type cat on the terminal and hit enter. It will ask for the input lines, you could write your name and hit enter. You will see your input will be reprinted. 

(base) [root@localhost ~]# cat
Hello this is GeeksForGeeks
Hello this is GeeksForGeeks

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

cat >> file.txt

To see the working of append standard input: 

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

The general form for a here document is −

Syntax:
command << delimiter
document
delimiter

(base) [root@localhost ~]# cat << helo.txt 
> Hello This is 
> GeeksForGeeks
helo.txt
Hello This is 
GeeksForGeeks
(base) [root@localhost ~]#

Note: Here, helo.txt is a delimiter.

The delimiter marks the ending point of the document. Without it, the shell continues to read the input forever. The delimiter must be a single word that does not contain 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 terminal, 3 files are generated: standard input(0), standard output(1), standard error(2). These files are always created whenever a program is run. By default, an error stream is displayed on the screen. 

Examples:

1. In the below-mentioned example, the file descriptor used above is 2(STDERR). Using “2>” re-directs the error output to a file named “error.txt” and nothing is displayed on STDOUT.

$ somerandomcommand 2>error.txt

2.  Here, 2>&1 means that STDERR redirects to the target of STDOUT. More formally, the error message generated by “2” gets merged with the current output “1“. 

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

In the above example, the directory GEEK is not present. The error output is merged with the standard output which in turn is being re-directed to “error.txt“. 

What symbol redirects the output of one to command to be the input of another command?

What characters is used for redirecting input or output?

Which symbol is used for input redirection?

Which symbol allows to take the output of a command and send it to another?