UNIX processes generally take input from standard input (keyboard), write output to standard out (screen) and may write to standard error if there are any error message which need to be displayed. Among the various powerful functions of Linux systems is the ability to redirect both input and output of commands.
The > symbol redirects output of a command. For example, cal command prints the calendar to screen. Using > we can save this calendar into a file.
cal > calendar.txt cat calendar.txt
If the file calendar.txt already exists, its contents would be replaced. If you wish to append content to an existing file, use the >> symbol instead.
ls >> calendar.txt cat calendar.txt
You can also use cat to concatenate files:
cal 2017 > 2017 cal 2018 > 2018 cat 2017 2018 > calendar.txt