redirect to a file:

cmd > file.txt terminal emulator into program, program out to file.txt, error out to terminal emulator

append to a file:

cmd >> file.txt terminal emulator into program, program out to file.txt (append mode), error out to terminal emulator

send a file to stdin:

cmd < file.txt file.txt into program, program out and err to terminal emulator

redirect stderr to a file:

cmd 2 > file.txt terminal emulator into program, program out to nowhere, err out to file.txt

redirect stdout AND stderr:

cmd > file.txt 2>&1 terminal emulator into program, out and err to file.txt

pipe stdout:

cmd1 | cmd2 terminal emulator into program 1, 1 out to program 2 via pipe, 2 out to command line, program 2 out 1 and 2 to terminal emulator

pipe stdout AND stderr:

cmd1 2>&1 | cmd2 terminal emulator into program 1, 1 and 2 out to program 2 via pipe, program 2 out 1 and 2 to terminal emulator

three gotchas

1. cmd file.txt > file.txt will delete the contents of file.txt some people use set -o noclobber (in bash/zsh) to avoid this

but I just have “never read from redirect to the same file” seared into my memory.

2. sudo echo blah > /root/file.txt doesn’t write to /root/file.txt as root. Instead, do:

echo blah | sudo tee /root/file.txt or sudo sh -c 'echo blah > /root/file.txt'

3. cmd 2>&1 > file.txt doesn’t write both stdout and stderr to file.txt. Instead, do: cmd > file.txt 2>&1

cat vs <

I almost always prefer to do:

cat file.txt | cmd

instead of

cmd < file.txt

it works fine & it feels better to me

using cat can be slower if if’s a GIANT file though

&> and &|

some shells support &> and &| to redirect/pipe both stdout and stderr

(also some shells use |& instead of &|)