bash-tutorial/tutorial/redirect_output.org

Redirect input, output and standard error

Everything in GNU systems is a file. The keyboard and the terminal are also a file. The keyboard is used for input and the terminal is used by commands as output.

The files can be found in directory "fd", which stands for file descriptor)

  cd /dev/fd/
  ls -l

total	0
lr-x——	1	user	user	64	dec	15	10:31	0	->	/tmp/babel-yl1RTd/ob-input-izpXuk
l-wx——	1	user	user	64	dec	15	10:31	1	->	pipe:[42063]
lr-x——	1	user	user	64	dec	15	10:31	17	->	/dev/pts/0
l-wx——	1	user	user	64	dec	15	10:31	2	->	/tmp/emacsA1M18o

• standard input stream is bound to /dev/fd/0
• standard output (stout) stream is bound to /dev/fd/1
• standard errror (stderr) stream is bound to /dev/fd/2

During interaction a terminal gets written to stdin (dev/fd/0), which a command read. The command then do an action and writes the output (stout or stderr) where it will read by the terminal and is displayed to the user.

To redirect use the operator `>` as follows:

• `command > file_to_redirect`

The operator `>` redirects stout, i.e., is the same that 1>. This code creates a new file with the list of files present in the actual directory:

  ls -l > file_redirect.txt
  cat file_redirect.txt

total	32
-rw-r–r–	1	user	user	15782	dec	15	10:49	bash_tutorial.org
-rw-r–r–	1	user	user	0	dec	15	10:49	file_redirect.txt
-rw-r–r–	1	user	user	11895	dec	9	17:44	markdown.md
-rw-r–r–	1	user	user	121	dec	9	17:17	README.org

Use "2>&1" to redirect stderr with the stout. Using an arbitrary word and redirect it to the file should print a warning message that is redirected to the file:

  ls -l > file_redirect.txt
  paco >> file_redirect.txt 2>&1
  cat file_redirect.txt

total	32
-rw-r–r–	1	user	user	15995	dec	15	12:39	bash_tutorial.org
-rw-r–r–	1	user	user	0	dec	15	13:41	file_redirect.txt
-rw-r–r–	1	user	user	11895	dec	9	17:44	markdown.md
-rw-r–r–	1	user	user	121	dec	9	17:17	README.org
bash:	line	2:	paco:	command	not	found

Use the "<" operator to pass commands to the file:

  ls -l > file_redirect.txt
  paco >> file_redirect.txt 2>&1
  sort < file_redirect.txt
  cat file_redirect.txt

bash:	line	2:	paco:	command	not	found
-rw-r–r–	1	user	user	0	dec	15	13:44	file_redirect.txt
-rw-r–r–	1	user	user	121	dec	9	17:17	README.org
-rw-r–r–	1	user	user	16501	dec	15	13:41	bash_tutorial.org
total	24
total	24
-rw-r–r–	1	user	user	16501	dec	15	13:41	bash_tutorial.org
-rw-r–r–	1	user	user	0	dec	15	13:44	file_redirect.txt
-rw-r–r–	1	user	user	121	dec	9	17:17	README.org
bash:	line	2:	paco:	command	not	found