Command Line Input Output Redirection

In shell programs such as Bash, input output redirection is a powerful mechanism that allows control over where the input comes from and where the output goes. By default, commands read input (stdin) from the keyboard and send output (stdout) to the terminal. However, redirection makes it possible to read from files, write to files, or even discard output altogether.

Special operators are used for redirection:

• < redirects input.
• > redirects output.

Output Redirection

echo "This is a redirection example." > write_file.txt
cat write_file.txt

# Output:
# This is a redirection example.

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In the example above, the > operator redirects stdout to write_file.txt, instructing the echo command to write the supplied string to the file. Redirection occurs before echo is executed.

Note: If write_file.txt does not exist, it is created. If it already exists, its content is overwritten.

Input Redirection

Input redirection allows a program to read from a file instead of the keyboard.

If there is a file named read_file.txt containing a string “Showing how input redirection works!”:

cat read_file.txt

# Output:
# Showing how input redirection works!

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If < is used:

cat < read_file.txt

# Output:
# Showing how input redirection works!

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In both cases, the output is same but both approaches are very different fundamentally. In the first case, a read_file.txt is opened and its contents are printed out. In the second case, the < operator redirects stdin to read_file.txt and the cat command reads from stdin instead of the file directly.

Suppressing Output

To suppress an output, it is redirected to /dev/null, which is a virtual device that is always empty:

echo "This is a redirection example." > /dev/null

# No output is printed

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