Streams in Linux: Stdin, Stdout, Stderr, and Operators Explained

The standard input (stdin), standard output (stdout), and standard error (stderr) are the three data streams that are used by Linux and Unix-based operating systems to communicate with the user and programs. These streams are used to redirect data to and from various sources, including files, terminals, and other programs.

In this blog, we'll be exploring the different operators and combinations that can be used to redirect data from these streams.

Definition:

stdin (standard input) is the default input source for commands and programs, typically taking input from the keyboard or a file.

stdout (standard output) is the default output destination for commands and programs, usually sending output to the terminal or a file.

stderr (standard error) is the default error output destination for commands and programs, typically sending error messages to the terminal or a file.

These standard streams allow for a standardized way for commands and programs to receive input, produce output, and report errors, making it easier for different tools and applications to interact and communicate with each other. In bash and other Unix-based shell environments, you can redirect these streams using various operators and combination operators to capture or suppress input, output, and error messages as desired.

Operators:

1. "< " - Input Redirection Operator:

The < operator is used to redirect input from a file to a program. The syntax for this operator is program < file, where program is the name of the program, and file is the name of the file you want to use as input.

For example, the following bash code will redirect the contents of the file "input.txt" to the "cat" command:

cat < input.txt

2. ">" - Output Redirection Operator:

The > operator is used to redirect output from a program to a file. The syntax for this operator is program > file, where program is the name of the program, and file is the name of the file where you want to save the output.

For example, the following bash code will redirect the output of the "ls" command to the file "output.txt":

ls > output.txt

3. "2>" - Error Redirection Operator:

The 2> operator is used to redirect error messages from a program to a file. The syntax for this operator is program 2> file, where program is the name of the program, and file is the name of the file where you want to save the error messages.

For example, the following bash code will redirect the error message of the "ls /non-existent-directory" command to the file "errors.txt":

ls /non-existent-directory 2> errors.txt

4. ">>" - Output Append Operator:

The >> operator is used to append output from a program to a file. The syntax for this operator is program >> file, where program is the name of the program, and file is the name of the file where you want to append the output.

For example, the following bash code will append the output of the "ls" command to the file "output.txt":

ls >> output.txt

5. "2>>" - Error Append Operator:

The 2>> operator is used to append error messages from a program to a file. The syntax for this operator is program 2>> file, where program is the name of the program, and file is the name of the file where you want to append the error messages.

For example, the following bash code will append the error message of the "ls /non-existent-directory" command to the file "errors.txt":

ls /non-existent-directory 2>> errors.txt

Combination Operators:

1. "&>" - All Output Redirection Operator:

The &> operator is used to redirect both the standard output and the standard error from a program to a file. The syntax for this operator is program &> file, where program is the name of the program, and file is the name of the file where you want to save both the standard output and standard error.

For example, the following bash code will redirect both the standard output and error message of the "ls /non-existent-directory" command to the file "output.txt":

ls /non-existent-directory &> output.txt

The "&>" operator is a shorthand that combines the functionality of > (redirect standard output to a file) and 2>&1 (redirect standard error to the standard output). By using &>, you can redirect both standard output and error to the same file in a single step, rather than having to specify both operations individually.

For example, the following command:

ls &> output.txt

Is equivalent to the following command:

ls > output.txt 2>&1

Both of these commands will redirect the standard output and error of the ls command to the file output.txt.

2. "2>&1" - Error Redirection to Standard Output:

The 2>&1 operator is used to redirect the standard error to the standard output. The syntax for this operator is program 2>&1, where program is the name of the program.

For example, the following bash code will redirect both the standard output and error message of the "ls /non-existent-directory" command to the terminal:

ls /non-existent-directory 2>&1

Conclusion

In conclusion, understanding the standard input, standard output, and standard error, as well as the different operators and combinations that can be used to redirect data from these streams, is an essential aspect of working with Linux and Unix-based operating systems. The ability to redirect data from these streams allows you to control and manipulate the flow of data to and from programs, making it easier to manage and analyze your data.