With the IO redirection feature, we can easily manipulate the input and output of files or other information.

Standard Output

The standard output stream is usually used to print the output of a program to a file, its file descriptor is 1.

Output Text to a File

1
2

echo 'This is a text for test' > fresh_file
cat fresh_file

Output Result to a File

1
2

ll -a > dir_list_file
cat dir_list_file

Append Text to a File

1
2

echo 'This is a text for test' >> fresh_file
cat fresh_file

Append Result to a File

1
2

ll -a >> dir_list_file
cat dir_list_file

Standard Input

Standard input streams are usually used to print the contents of a file to a screen, its file descriptor is 0.

Read Text from a File

1

cat < fresh_file

Copy Text from a File to Another File

1

cat > new_fresh_file < fresh_file

Multi-line Text to a File Using Here Documents

1
2
3
4
5
6

cat > new_here_document_file << EOF
First line
Second line
Third line
EOF
cat new_here_document_file

Standard Error

Standard error streams are typically used to print error content from the output of a program to a file, its file descriptor is 2.

Redirects Error Messages to a File

1
2

cat non_existing_file 2> error_log
cat error_log

Suppress the Error Messages

1

cat non_existing_file 2> /dev/null

Show Error Messages and Output Standard Results to a File

1
2

cat non 2>&1 > std_output_result    # file "non" not found
cat std_output_result    # no standard output

1
2

cat fresh_file 2>&1 > std_output_result    # file fresh_file found
cat std_output_result    # no standard error

References 3.6 Redirections

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