Course/Piping & Redirection/Output Redirection

Output Redirection

Use > to write command output to a file and >> to append without overwriting.

By default, every command's output goes to your screen. But what if you want to save it? The redirect operator > takes stdout and sends it to a file instead. If the file doesn't exist, it creates one. If it does exist, it overwrites the contents completely.

echo "first line" > output.txt     # creates file (or overwrites)
echo "second line" >> output.txt   # appends to the file
cat output.txt                     # see both lines

The single > is like a fresh start — it replaces everything in the file with the new output. The double >> is the append operator — it adds to the end of the file without touching what's already there. This distinction is critical. Using > when you meant >> will destroy the file's previous contents.

Redirection works with any command, not just echo. You could run ls > filelist.txt to save a directory listing, or cat notes.txt > backup.txt to copy a file's contents. Anywhere stdout goes to the screen, you can redirect it to a file instead.

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Be careful with >. Running echo "oops" > important-data.txt will erase everything in that file and replace it with "oops". Always double-check the filename before using >. When in doubt, use >> to append instead.

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A useful trick: > file.txt with no command empties a file without deleting it. This is handy for clearing log files while keeping the file in place so other programs that reference it aren't disrupted.

Let's practice. Use echo with the > operator to create a file called greeting.txt, then use >> to append a second line to it.

echo "Hello, world!" > greeting.txt
echo "Welcome to piping & redirection." >> greeting.txt

WIN

Windows Command Prompt uses the same > and >> operators for redirection. PowerShell also supports them, plus Out-File and Add-Content cmdlets for more control. The concepts transfer directly.

Practice