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 help you to understand 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, with Bash running in a Linux terminal emulator, you'll see:
# lsof +f g -ap $BASHPID -d 0,1,2
COMMAND PID USER FD TYPE FILE-FLAG DEVICE SIZE/OFF NODE NAME
bash 12135 root 0u CHR RW,LG 136,13 0t0 16 /dev/pts/5
bash 12135 root 1u CHR RW,LG 136,13 0t0 16 /dev/pts/5
bash 12135 root 2u CHR RW,LG 136,13 0t0 16 /dev/pts/5
This /dev/pts/5 is a pseudo terminal used to emulate a real terminal. 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 alters the file descriptors belonging to the command echo. 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, i.e., 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 used by applications to print errors.
What will command 3> file do? It will open a new file descriptor pointing to file. The command will then start 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 Redirection "n\< file"¶
When you run a commandusing command < file, it changes the file descriptor 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 console.
As with >, < can be used to open a new file descriptor for reading, command 3<file. Later we will see how this can be useful.
Pipes |¶
What does this | do? Among other things, it connects the standard output of the command on the left to the standard input of the command on the right. That is, it creates a special file, a pipe, which is opened as a write destinaton for the left command, and as a read source for the right command.
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 called duplicating? Because after 2>&1, we have 2 file descriptors pointing to the same file. Take care not to call this "File Descriptor Aliasing"; if we redirect stdout after 2>&1 to a file B, file descriptor 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 two file descriptors s and t like:
--- +-----------------------+
a descriptor ( s ) ---->| /some/file |
--- +-----------------------+
--- +-----------------------+
a descriptor ( t ) ---->| /another/file |
--- +-----------------------+
Using a t>&s (where t and s are numbers) it means:
Copy whatever file descriptor
scontains into file descriptort
So you got a copy of this descriptor:
--- +-----------------------+
a descriptor ( s ) ---->| /some/file |
--- +-----------------------+
--- +-----------------------+
a descriptor ( t ) ---->| /some/file |
--- +-----------------------+
Internally each of these is represented by a file descriptor opened by the operating system's fopen calls, and is likely just a pointer to the file which has been opened for reading (stdin or file descriptor 0) or writing (stdout /stderr).
Note that the file reading or writing positions are also duplicated. If you have already read a line of s, then after t>&s if you read a line from t, you will get the second line of the file.
Similarly for output file descriptors, writing a line to file descriptor s will append a line to a file as will writing a line to file descriptor t.
| 
The syntax is somewhat confusing in that you would think that the arrow would point in the direction of the copy, but it's reversed. So it's target>&source effectively. | | --- |
So, as a simple example (albeit slightly contrived), is the following:
exec 3>&1 # Copy 1 into 3
exec 1> logfile # Make 1 opened to write to logfile
lotsa_stdout # Outputs to fd 1, which writes to logfile
exec 1>&3 # Copy 3 back into 1
echo Done # Output to original stdout
Order Of Redirection, i.e., "> file 2>&1" vs. "2>&1 >file"¶
While it doesn't matter where the redirections appears on the command line, their order does matter. They are set up 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 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 changes stdout:
--- +-----------------------+
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 writes 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 output has already been redirected, with the additional side effect that, because we used >, "file" gets 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 have to do with redirection? exec also allow us to manipulate the file descriptors. If you don't specify a program, the redirection after exec modifies the file descriptors of the current shell.
For example, all the commands after exec 2>file will have file descriptors 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 ran myscript 2>file.
exec can be used, if, for instance, you want to log the errors 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 printing each line, to do a pause, waiting for the user to press a key:
while read -r line;do echo "$line"; read -p "Press any key" -n 1;done < file
And, surprise this doesn't work. Why? because the shell descriptor of the while loop looks like:
--- +-----------------------+
standard input ( 0 ) ---->| file |
--- +-----------------------+
--- +-----------------------+
standard output ( 1 ) ---->| /dev/pts/5 |
--- +-----------------------+
--- +-----------------------+
standard error ( 2 ) ---->| /dev/pts/5 |
--- +-----------------------+
and our read inherits these descriptors, and our command (read -p "Press any key" -n 1) inherits them, and thus reads from file and not from our terminal.
A quick look at help read tells us that we can specify a file descriptor from which read should read. Cool. Now let's use exec to get another descriptor:
exec 3<file
while read -u 3 line;do echo "$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 |
--- +-----------------------+
--- +-----------------------+
new descriptor ( 3 ) ---->| file |
--- +-----------------------+
and it works.
Closing The File Descriptors¶
Closing a file through 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 is perhaps a good idea to close the file descriptors you open. For instance, if you open a file descriptor with exec 3>file, all the commands afterwards will inherit it. It's probably better 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 applications to behave correctly with a closed stderr.
When in doubt, I use 2>/dev/null.
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 redirections 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 produced the same result here because we ran the command in a terminal and thus 1 and 2 go 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, closes 3 and 4 and sets up a pipe for reading. 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 an 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 {}. Lcet'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, connects 0 to the 1st pipe, the file descriptor 2 is made a copy of 3, and 3 is closed. Finally, for the left part of the pipe:
--- +-------------+
( 0 ) ---->| /dev/pts/5 |
--- +-------------+
--- +-------------+
( 1 ) ---->| 1st pipe |
--- +-------------+
--- +-------------+
( 2 ) ---->| 2nd pipe |
--- +-------------+
It also inherits the file descriptor of the left part of the 2nd pipe, file descriptor 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 another 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 choosing between 0&<3 3&>1 3>&1 ->2 -<&0 &-<0 0<&- etc... (I think probably because the syntax is more representative of the result, i.e., the redirection, than what is done, i.e., opening, closing, or duplicating file descriptors).
If this fits your situation, then maybe the following "rules" will help you, a redirection is always like the following:
lhs op rhs
lhsis always a file description, i.e., a number:- Either we want to open, duplicate, move or we want to close. If the op is
<then there is an implicit 0, if it's>or>>, there is an implicit 1.
- Either we want to open, duplicate, move or we want to close. If the op is
opis<,>,>>,>|, or<>:<if the file decriptor inlhswill be read,>if it will be written,>>if data is to be appended to the file,>|to overwrite an existing file or<>if it will be both read and written.
rhsis the thing that the file descriptor will describe:- It can be the name of a file, the place where another descriptor goes (
&1), or,&-, which will close the file descriptor.
- It can be the name of a file, the place where another descriptor goes (
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.
A note on style¶
The shell is pretty loose about what it considers a valid redirect. While opinions probably differ, this author has some (strong) recommendations:
- Always keep redirections "tightly grouped" -- that is, do not include whitespace anywhere within the redirection syntax except within quotes if required on the RHS (e.g. a filename that contains a space). Since shells fundamentally use whitespace to delimit fields in general, it is visually much clearer for each redirection to be separated by whitespace, but grouped in chunks that contain no unnecessary whitespace.
- Do always put a space between each redirection, and between the argument list and the first redirect.
- Always place redirections together at the very end of a command after all arguments. Never precede a command with a redirect. Never put a redirect in the middle of the arguments.
- Never use the Csh
&>fooand>&fooshorthand redirects. Use the long form>foo 2>&1. (see: obsolete)
# Good! This is clearly a simple commmand with two arguments and 4 redirections
cmd arg1 arg2 <myFile 3<&1 2>/dev/null >&2
# Good!
{ cmd1 <<<'my input'; cmd2; } >someFile
# Bad. Is the "1" a file descriptor or an argument to cmd? (answer: it's the FD). Is the space after the herestring part of the input data? (answer: No).
# The redirects are also not delimited in any obvious way.
cmd 2>& 1 <<< stuff
# Hideously Bad. It's difficult to tell where the redirects are and whether they're even valid redirects.
# This is in fact one command with one argument, an assignment, and three redirects.
foo=bar<baz bork<<< blarg>bleh
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: