Illustrated Redirection Tutorial
This tutorial is not a complete guide to redirection, it will not
cover here docs, here strings, name pipes etc…I just hope it'll make
you understand better what things like 3>&2, 2>&1 or 1>&3- do.
stdin, stdout, stderr
When Bash starts, normally, 3 file descriptors are opened, 0, 1 and 2
also known as standard input (stdin), standard output (stdout) and
standard error (stderr).
For example, in Bash, in a terminal emulator, on Linux you'll see:
$ lsof -a -p $$ -d0,1,2 COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME bash 24507 root 0u CHR 136,5 7 /dev/pts/5 bash 24507 root 1u CHR 136,5 7 /dev/pts/5 bash 24507 root 2u CHR 136,5 7 /dev/pts/5
This /dev/pts/5 pts is a pseudo terminal used to simulate a real
terminal connected to the computer. Bash reads (stdin) from this
terminal and prints via stdout and stderr to this terminal.
--- +-----------------------+
standard input ( 0 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| /dev/pts/5 |
--- +-----------------------+
When a command, a compound command, a subshell etc… is executed, it inherits
these file descriptors. For instance echo foo will send the text foo to the file
descriptor 1 inherited from the shell, which is connected to /dev/pts/5.
Simple Redirections
Output Redirection "n> file"
> is probably the simplest redirection.
echo foo > file
the > file after the command alter the files descriptors of the
command foo. It changes the file descriptor 1 (> file is the same as
1>file) so that it points to the file file. They will look like:
--- +-----------------------+
standard input ( 0 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| file |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| /dev/pts/5 |
--- +-----------------------+
Now characters written by our command, echo, that are sent to the
standard output ie the file descriptor 1, end up in the file
named file.
In the same way command 2> file will change the standard error and
will make it point to file. Standard error is often use by
applications to prints… errors.
What command 3> file will do? It will open a new file descriptor
pointing to file. The command will then starts with:
--- +-----------------------+
standard input ( 0 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
new descriptor ( 3 ) ---->| file |
--- +-----------------------+
What will the command do with this descriptor? It depends, often nothing. We will see later why we might want other file descriptors.
Input Output "n< file"
I hope that the following will be obvious, when you run a command
using command < file, it changes the filedescriptor 0 so that
it looks like:
--- +-----------------------+
standard input ( 0 ) ---->| file |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| /dev/pts/5 |
--- +-----------------------+
If the command reads from stdin, it now will read from file and not
from the terminal.
As with >, < can be used to open a new file descriptor for reading,
command 3<file. We will see how this can be useful.
Pipes |
What this | do? Among other things, it connects the standard output of
the left command to the standard input of the right command.
That is it creates a special file, a pipe which is opened for writing
for the left command, and opened for reading for the command on the right.
echo foo | cat
--- +--------------+ --- +--------------+
( 0 ) ---->| /dev/pts/5 | ------> ( 0 ) ---->|pipe (read) |
--- +--------------+ / --- +--------------+
/
--- +--------------+ / --- +--------------+
( 1 ) ---->| pipe (write) | / ( 1 ) ---->| /dev/pts |
--- +--------------+ --- +--------------+
--- +--------------+ --- +--------------+
( 2 ) ---->| /dev/pts/5 | ( 2 ) ---->| /dev/pts/ |
--- +--------------+ --- +--------------+
This is possible because the redirections are set up, by the shell, before the commands are executed and the commands inherit the file descriptors.
More On File Descriptors
Duplicating File Descriptor 2>&1
We have seen how to open (or redirect) file descriptors. Let us see
how to duplicate them, starting with the classic 2>&1. What does this
mean? That something written on the file descriptor 2 will go where
file descriptor 1 goes. In a shell command 2>&1 is not a very
interesting example so we will use ls /tmp/ doesnotexist 2>&1 | less
ls /tmp/ doesnotexist 2>&1 | less
--- +--------------+ --- +--------------+
( 0 ) ---->| /dev/pts/5 | ------> ( 0 ) ---->|from the pipe |
--- +--------------+ / ---> --- +--------------+
/ /
--- +--------------+ / / --- +--------------+
( 1 ) ---->| to the pipe | / / ( 1 ) ---->| /dev/pts |
--- +--------------+ / --- +--------------+
/
--- +--------------+ / --- +--------------+
( 2 ) ---->| to the pipe | / ( 2 ) ---->| /dev/pts/ |
--- +--------------+ --- +--------------+
Why is it call duplicating? Because after 2>&1, we have 2 file
descriptor pointing to the same file. Take care that this is not
called “File Descriptor Aliasing”, if we redirect stdout after 2>&1 to a
file B, filedescriptor 2 will still be opened on the file A where it was. This is
often misunderstood by people wanting to redirect both standard input
and standard output to the file. Continue reading for more on this.
So if you have a file descriptor like:
--- +-----------------------+
a descriptor ( n ) ---->| /some/file |
--- +-----------------------+
Using a m>&n (where m is a number) you got a copy of this descriptor:
--- +-----------------------+
a descriptor ( m ) ---->| /some/file |
--- +-----------------------+
Note that the positions are also duplicated. If you have allready
read a line of n, then after n>&m if you read a line from m, you will
get the second line of the file.
Order Of Redirection, ie "> file 2>&1" vs "2>&1 >file"
While it doesn't matter where the redirections appears on the command line, there order does matter. They are setup from left to right.
2>&1 >file
A common error, is to do command 2>&1 > file to redirect both stderr
and stdout to file. Let's see what's going on. First we type the
command in our typical terminal, the descriptors look like this:
--- +-----------------------+
standard input ( 0 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| /dev/pts/5 |
--- +-----------------------+
Then our shell, Bash sees 2>&1 so it duplicates 1, and the file
descriptor look like this:
--- +-----------------------+
standard input ( 0 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| /dev/pts/5 |
--- +-----------------------+
That's right, nothing has changed, 2 was already pointing to the same
place as 1. Now Bash sees > file and thus change stdin:
--- +-----------------------+
standard input ( 0 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| file |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| /dev/pts/5 |
--- +-----------------------+
And that's not what we want.
>file 2>&1
Now let's look at the correct command >file 2>&1. We start as in the
previous example, and Bash sees > file:
--- +-----------------------+
standard input ( 0 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| file |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| /dev/pts/5 |
--- +-----------------------+
Then it sees our duplication 2>&1:
--- +-----------------------+
standard input ( 0 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| file |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| file |
--- +-----------------------+
And voila, both 1 and 2 are redirected to file.
Why sed 's/foo/bar/' file >file Doesn't Work
This is a common error, we want to modify a file using something that
reads from a file and write the result to stdout. To do this we redirect stdout
to the file we want to modify. The problem here is that, as we
have seen, the redirections are setup before the command is actually
executed.
So BEFORE sed starts standard input has already been redirected, with
the additional side effect that, because we use >, “file” is truncated. When sed starts to
read the file, it contains nothing.
exec
In Bash the exec built-in replaces the shell with the specified
program. So what does this has to do with redirection?
exec also allow us to manipulate the file descriptors.
If you don't specify a program the redirection after exec modify the
file descriptors of the current shell.
For example, all the command after exec 2>file will have file
descriptor like:
--- +-----------------------+
standard input ( 0 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| file |
--- +-----------------------+
All the the errors sent to stderr by the commands after the exec 2>file
will go to the file, just as if you had the command in a script and
run myscript 2>file.
exec can be used for instance if you want to log the errors that the
commands in your script produce, just add exec 2>myscript.errors at
the beginning of your script.
Let's see another use case. We want to read a file line by line, this is easy we just do:
while read -r line;do echo "$line";done < file
Now, we want after each printing each line, to do a pause, waiting for the user to press a key:
while read -r line;do echo "$line"; read -p "Press any key" -n 1;done < file
And, surprise this doesn't work. Why ? because the shell descriptor of the while loop looks like:
--- +-----------------------+
standard input ( 0 ) ---->| file |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| /dev/pts/5 |
--- +-----------------------+
and our read inherits these descriptors, and our command (read -p “Press any key” -n 1)
inherits them and thus read from file and not from our terminal.
A quick look at help read tells us that we can specify a file
descriptor from which read should read. Cool. Now let's use exec to
get another descriptor:
exec 3<file while read -u 3 line;do echo echo "$line"; read -p "Press any key" -n 1;done
Now the file descriptors look like:
--- +-----------------------+
standard input ( 0 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard error ( 3 ) ---->| file |
--- +-----------------------+
and it works.
Closing The File Descriptors
Closing a file a file descriptor is easy, just make it a duplicate of -.
For instance let's close stdin <&- and stderr 2>&-:
bash -c '{ lsof -a -p $$ -d0,1,2 ;} <&- 2>&-'
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
bash 10668 pgas 1u CHR 136,2 4 /dev/pts/2
we see that inside the {} that only 1 is still here.
Though the OS will probably clean up the mess, it can perhaps be a
good idea to close the file descriptors you open. For instance if you
open a file descriptor with exec 3>file, all the command afterward
will inherit it. It's probably nicer to do something like:
exec 3>file ..... #commands that uses 3 ..... exec 3>&- #we don't need 3 any more
I've seen some people using this as a way to discard say stderr, using something like: command 2>&-. Though it might work, I'm not sure if you can expect all the applications to behave correctly with a closed stderr.
In doubt, I prefer to use 2>/dev/null for this.
An Example
This example comes from this post (ffe4c2e382034ed9) on the comp.unix.shell group:
{
{
cmd1 3>&- |
cmd2 2>&3 3>&-
} 2>&1 >&4 4>&- |
cmd3 3>&- 4>&-
} 3>&2 4>&1
The redirection are processed from left to right, but as the file
descriptors are inherited we will also have to work from the outer to
the inner contexts. We will assume that we run this command in a
terminal. Let's start with the outer { } 3>&2 4>&1.
--- +-------------+ --- +-------------+ ( 0 ) ---->| /dev/pts/5 | ( 3 ) ---->| /dev/pts/5 | --- +-------------+ --- +-------------+ --- +-------------+ --- +-------------+ ( 1 ) ---->| /dev/pts/5 | ( 4 ) ---->| /dev/pts/5 | --- +-------------+ --- +-------------+ --- +-------------+ ( 2 ) ---->| /dev/pts/5 | --- +-------------+
We only made 2 copies of stderr and stdout. 3>&1 4>&1 would have
produce the same result here because we run the command in a terminal
and thus 1 and 2 goes to the terminal. As an exercise you can start
with 1 pointing to file.stdout and 2 pointing to file.stderr, you will
see why these redirections are very nice.
Let's continue with the right part of the second pipe: | cmd3 3>&- 4>&-
--- +-------------+ ( 0 ) ---->| 2nd pipe | --- +-------------+ --- +-------------+ ( 1 ) ---->| /dev/pts/5 | --- +-------------+ --- +-------------+ ( 2 ) ---->| /dev/pts/5 | --- +-------------+
It inherits the previous file descriptors, close 3 and 4 and setup a pipe
for read. Now for the left part of the second pipe {…} 2>&1 >&4 4>&- |
--- +-------------+ --- +-------------+ ( 0 ) ---->| /dev/pts/5 | ( 3 ) ---->| /dev/pts/5 | --- +-------------+ --- +-------------+ --- +-------------+ ( 1 ) ---->| /dev/pts/5 | --- +-------------+ --- +-------------+ ( 2 ) ---->| 2nd pipe | --- +-------------+
First, The file descriptor 1 is connected to the pipe (|), then 2 is
made a copy of 1 and thus is made a fd to the pipe (2>&1), then 1 is
made a copy of 4 (>&4), then 4 is closed. These are the file
descriptors of the inner {}, let's go inside and have a look at the
right part of the first pipe: | cmd2 2>&3 3>&-
--- +-------------+ ( 0 ) ---->| 1st pipe | --- +-------------+ --- +-------------+ ( 1 ) ---->| /dev/pts/5 | --- +-------------+ --- +-------------+ ( 2 ) ---->| /dev/pts/5 | --- +-------------+
It inherits the previous file descriptors, connect 0 to the 1st pipe, the filedescriptor 2 is made a copy of 3 and 3 is closed. Finally the for the left part of the pipe:
--- +-------------+ ( 0 ) ---->| /dev/pts/5 | --- +-------------+ --- +-------------+ ( 1 ) ---->| 1st pipe | --- +-------------+ --- +-------------+ ( 2 ) ---->| 2nd pipe | --- +-------------+
It also inhertits the file descriptor of the left part of the 2nd
pipe, filedescriptor 1 is connected to the first pipe, 3 is closed.
The purpose of all this becomes clear if we take only the commands:
cmd2
--- +-------------+
-->( 0 ) ---->| 1st pipe |
/ --- +-------------+
/
/ --- +-------------+
cmd 1 / ( 1 ) ---->| /dev/pts/5 |
/ --- +-------------+
/
--- +-------------+ / --- +-------------+
( 0 ) ---->| /dev/pts/5 | / ( 2 ) ---->| /dev/pts/5 |
--- +-------------+ / --- +-------------+
/
--- +-------------+ / cmd3
( 1 ) ---->| 1st pipe | /
--- +-------------+ --- +-------------+
------------>( 0 ) ---->| 2nd pipe |
--- +-------------+ / --- +-------------+
( 2 ) ---->| 2nd pipe |/
--- +-------------+ --- +-------------+
( 1 ) ---->| /dev/pts/5 |
--- +-------------+
--- +-------------+
( 2 ) ---->| /dev/pts/5 |
--- +-------------+
As said previously, as an exercise you can start with 1 open on a file
and 2 open on an other file to see how the stdin from cmd2 and cmd3 goes
to the original stdin and how the stderr goes to the original stderr.
Syntax
I used to have trouble to choose between 0&<3 3&>1 3>&1 ->2 -<&0 &-<0
0<&- etc… (I think probably because the syntax represent more the result ie the redirection than
what is done ie opening closing duplicating filedescriptors).
If it's also your case, then maybe the following “rules” will help you, a redirection is always like:
lhs op rhs
lhsis always a file descriptor i.e. a number:- Either one we want to open, duplicate, move or one we want to close. If the op is
<then there is an implicit 0, if it's>or>>there is an implicit 1.
opis either<,>,>>,>|, or<>:<if the file decriptor inlhswill be read,>if it will be written,>>if data will be appended to the file,>|to overwrite an existing file or<>if it will be both read and written.
rhsis the thing that the filedescriptor will describe:- It can be either the name of a file, the place where another descriptor goes (
&1), or the special nowhere,&-, which will close the filedescriptor.
You might not like this description, and find it a bit incomplete or
inexact, but I think it really helps to easily find that say &->0 is
incorrect.
Conclusion
I hope this tutorial worked for you.
I lied, I did not explain 1>&3-, go check the manual 
Thanks to Stéphane Chazelas from whom I stole both the intro and the example….
The intro is inspired by this introduction, you'll find a nice exercise there too:
The last example comes from this post:
See also
- Internal: Redirection syntax overview
