Standard Input

The second data stream is standard input, abbreviated as stdin. While standard output is where commands send their results, standard input is where commands receive data.

Many commands can read from a file by taking the filename as an argument:

cat notes.txt
sort names.txt

But you can also redirect a file's contents directly into a command using the < operator:

sort < names.txt

This feeds the contents of names.txt into the sort command through standard input. The result is the same as sort names.txt, but the mechanism is different — the shell handles the file reading instead of the command itself.

You can combine input and output redirection in a single command:

sort < names.txt > sorted_names.txt

This reads from names.txt, sorts the content, and writes the result to sorted_names.txt. Input redirection becomes especially useful when working with commands that expect data from standard input rather than a filename argument.