Output Redirection: > and >>

Save what would have hit the screen

cmd > file sends stdout to file, replacing whatever was there. cmd >> file appends to it. The screen stays empty unless the program also writes to stderr.

Truncate vs append

> — overwrite. The file is truncated to zero before the command runs.
>> — append. Existing content stays.

The truncation happens early: cmd > file blanks file even if cmd immediately fails. If you want fresh-write-or-fail, use >| with set -o noclobber (see callout below).

Stdin redirection from a file

cmd < file sends file contents to cmd's stdin. wc -l < lines.txt counts lines without spawning cat. sort < data.txt, tr a-z A-Z < input.txt are the canonical patterns.

Combining redirections

You can stack them: cmd < in.txt > out.txt 2> err.txt. Order doesn't matter for separate fds, but it matters for the same fd: cmd > out.txt 2>&1 sends stderr to wherever stdout currently goes (= out.txt). cmd 2>&1 > out.txt redirects stderr to the original stdout (still terminal!) and only then redirects stdout. Different result.

Code

Append vs overwrite·bash

echo 'first'  > log.txt
echo 'second' > log.txt          # overwrites
echo 'third'  >> log.txt         # appends
cat log.txt                      # second\nthird

Capture both stdout and stderr to one file·bash

# Bash / zsh — order matters
make all > build.log 2>&1
# Bash 4+ shorthand
make all &> build.log
# Append both
make all >> build.log 2>&1

Output Redirection: > and >>

Save what would have hit the screen

Truncate vs append

Stdin redirection from a file

Combining redirections

Code

External links

Exercise

Progress

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