Pipe character ("|")

Use the output of the preceding command as the input of the following command. Multiple commands can be connected using multiple pipe characters.

# Output the string 'hello world' to the wc command to count the number of words
echo "hello world" | wc -w
# output:
2

output redirection character (">")

Redirect the output of the command to a file. Redirection explanation: Generally, the program will output to the standard output stdout, which is usually the terminal being used. The redirection character redirects the standard output to a place other than the standard output.

# Redirect (write) the string hello world to the file hello.txt
# If the file does not exist, create the file; if the file exists, overwrite the file content
echo "hello world" > hello.txt
# Output the content of the hello.txt file
cat hello.txt
# output:
hello world

Append redirection character (">>")

Append the output of the command to a file.

# Append the string hello world to the file hello.txt
# If the file does not exist, create the file; if the file exists, append the content to the end of the file
echo "hello world" >> hello.txt
# Output the content of the hello.txt file
cat hello.txt
# output:
hello world
hello world

The output of the echo command is one line by default, so there is a newline at the end of the output.

input redirection character ("<")

Take a file as input to the command.

echo "hello world" > hello.txt
# Use the file hello.txt as input to the wc command
wc -w < hello.txt
# output:
2

Multi-line input redirector ("<<")

Also known as Here Document, it uses a multi-line string as the input to the command.

# Use a multi-line string as input to the wc command
# It is just a convention to use EOF (end of file) as the end marker, others can be used.
# EOF cannot have any characters or spaces before and after it.
wc -w << EOF
> hello world
> hello world
> EOF
# output:
4

Special redirection symbols

Running the program will open the following three files:

Standard input file (stdin)

The file descriptor of stdin is 0, and the program reads data > from stdin by default.

Standard output file (stdout)

The file descriptor of stdout is 1, and the program outputs data > to stdout by default.

Standard error file (stderr)

The file descriptor of stderr is 2, and the program writes error > information to the stderr stream.

Error output redirector ("2>")

# Redirect the error output of the command to the file error.txt
# If the file does not exist, create the file; if the file exists, overwrite the file content
ls not-exist-directory/ 2> error.txt
# Output the content of the error.txt file
cat error.txt
# output:
ls: cannot access 'not-exist-directory/': No such file or directory

Combine standard output and error output (">&")

1>&2 correct return value is passed to 2 output channels &2 means 2 output channels

2>&1 error return value is passed to 1 output channel, &1 means 1 output channel

Be careful not to have spaces between these characters.

Create file structure

mkdir exist-directory
touch exist-directory/exist-file.txt

# Merge the error output of the command to the standard output channel
# and write to the file out.txt
ls not-exist-directory/ exist-directory/ > out.txt 2>&1
# Output the content of the out.txt file
cat out.txt
# output:
ls: cannot access 'not-exist-directory/': No such file or directory
exist-directory/:
exist-file.txt

# understand first
ls not-exist-directory/ exist-directory/ > out.txt
# equals
ls not-exist-directory/ exist-directory/ 1> out.txt
# This is to redirect standard output to the file out.txt, while error still goes to the terminal
# The same
ls not-exist-directory/ exist-directory/ 2> out.txt
# This is to redirect error output to the file out.txt, while standard output still goes to the terminal

# Merge the standard output of the command into the error output
# and write the combined error output to the file out.txt
ls not-exist-directory/ exist-directory/ 2> out.txt 1>&2
# Output the content of the out.txt file
cat out.txt
# output:
ls: cannot access 'not-exist-directory/': No such file or directory
exist-directory/:
exist-file.txt

Combine output to file ("&>")

# Merge the standard output and error output of the command into the file out.txt.
# is a shorthand for the above two methods
ls not-exist-directory/ exist-directory/ &> out.txt
# Output the content of the out.txt file
cat out.txt
# output:
ls: cannot access 'not-exist-directory/': No such file or directory
exist-directory/:
exist-file.txt

Therefore, common commands such as:

xxx &> /dev/null
xxx > /dev/null 2>&1

It means redirecting both the standard output and error output of the xxx program to the /dev/null file, i.e. discarding the output.