Bash uses < and > for some special behavior that when you get the hang of it you love them.

Input redirection

< or input redirection

So you don’t want to keep writing every single time the input for a command?

command input1 input2 input3 ...

You are lucky because < will save you some time

command < inputs.txt

With this you pass the input with a file and not with the keyboard

<< or here document

You can give multiple lines to a command and is going to be read as a document, it will also remove tabs before the text

command <<EOF
	line1
line2
EOF

You can also don’t allow variables in this “file”

command <<'EOF'
	line1
${variable}
EOF

Here the string ${variable} will be used and not the variable value, also tabs will not be ignored, if you want to ignore them you can use <<-

* You can change EOF for whatever delimiter you like

<<< or here string

You pass a string to a command that reads from stdin

for file in *.mp4; do
    duration=$(<long_command>)
    IFS=':' read -r hours minutes seconds <<< $duration
    ...
done

Output redirection

> and >>

You need to have the output of a command written into a file?, you can use > or >>. Both of these commands will create the file if did not exist but if it exists then the first command will wipe the content of the file and write, on the other hand >> will append the data to the file

command > out.txt
command >> out.txt

M>&N and M>N

With M>&N you can redirect the file descriptor M (default 1, or stdout) to N. You can use it, for example, to redirect stderr to stdout.

command input1 2>&1 

You can also remove & and this will redirect that file descriptor to a file

command input 2>err.txt 1>out.txt

A special case of this is &>"filename", with this you are redirecting stdout and stderr to a file named filename.

You can combine them

You don’t have to use only one of the command described in this page, you can use multiple if required

command < input.txt > out.txt

You want to read more? https://tldp.org/LDP/abs/html/io-redirection.html https://tldp.org/LDP/abs/html/x17837.html (Here string)