Redirection: Sending Output to Files

Every command has three standard streams:

Redirection connects these to files instead.

Output to a file

echo "first line" > notes.txt
echo "second line" >> notes.txt
cat notes.txt
echo "replaced" > notes.txt
cat notes.txt

first line
second line
replaced

• > writes to a file, replacing its contents (or creating it).
• >> appends to the end.

Errors go separately

Error messages go to stderr, so > doesn't capture them:

echo "hello" > notes.txt
ls notes.txt missing.txt > output.txt 2> errors.txt
echo "stdout:"; cat output.txt
echo "stderr:"; cat errors.txt

stdout:
notes.txt
stderr:
ls: cannot access 'missing.txt': No such file or directory

• 2> redirects errors.
• &> redirects both output and errors to the same file (> file 2>&1 does the same, and works in every shell).
• 2> /dev/null throws errors away — /dev/null is a special file that discards everything written to it.

Input from a file

printf '%s\n' banana apple cherry > fruits.txt
sort < fruits.txt

apple
banana
cherry

< feeds a file to a command's standard input. (sort fruits.txt also works, since sort accepts file names — but not every command does.)

Here documents

A here document feeds several lines of text to a command's input:

cat > config.ini <<EOF
[server]
port=8080
EOF
cat config.ini

[server]
port=8080

Everything up to the line EOF becomes the input. It's the standard way for scripts to write small files.

Common mistakes

• sort data.txt > data.txt — the shell empties data.txt before sort reads it, so you lose the data. Write to a new file, then move it.
• Using > when you meant >>, and overwriting a log.