Redirection and Pipelining

Redirection with <

Used like command < file. Normally commands like cat will use the file of “stdin” (one of your O Streams!) when run as just cat and nothing else. But if you want to pass another file, you do cat < file.txt or something like that.

Also cat 0< file.txt explicitly shows that 0 (stdio) is what we are shoving the file into

2> errors.log   # redirect stderr (fd 2) to a file
1> output.log   # redirect stdout (fd 1)
0< input.txt    # redirect stdin

Redirection with >

Takes the output (by default, stdin) and redirects it to another file. Used like command -options all the args > out.txt

Redirecting with I/O streams

If you specifically want stderr to go to a file, or stdio to output to stderr instead, you do this: echo "hihihi" 1>&2 Now "hihihi" is sent to stderr. The &2 represents the address of stderr’s file. That 1 is optional since by default we redirect from stdin anyway

Redirection is NOT ordered!

You can do this: echo "hi!" >&2 > out.txt

• This will just take the oldest redirection, > out.txt, and ignore >&2.

What if I want to dump an output into nowhere?

Simple! Redirect into /dev/null

echo "garbage sentence" > /dev/null

Piping

• Is done concurrently with the program. I.e. as soon as something is outputted in the first command, it is piped into the second command

Appending with >>

Sends output to a file by adding onto of the file

Other misc things

goes-in-cheat-sheet-DONE All that is below command > output.txt 2> error.txt

• Redirects two streams : stdin and stderr! { echo hello ; exit 1; echo world ; } > out.txt
• This only prints hello. Why? Redirection happens first! First, the line will be like “kay, we are redirecting” and whatever output is outputted will be redirected. If we exit, well frick the program is dead! But we were redirecting happily before exit was ever called
• The pipe operator technically does this too. BUT, for most commands, the pipe operator WAITS for the previous commands to be finished before it can store anything into the next command.
  • Counterexample: yes nonono | head --lines=42

sort < random.txt > random.txt

• First, we know all stdin is going into sort, and all stdout is going into random.txt
• Since we are outputting into random.txt and not appending, we open for writing. What this does is clears the contents of random.txt
  • However this happens before anything else runs. So sort sorts nothing and, yeah, rip.