Linux I/O Redirection
Updated June 01, 2005
Created October 11, 2001

• The following command saves stdout and stderr to the files "out.txt" and "err.txt", respectively.




[root@server /root]# ./cmd 1>out.txt 2>err.txt



• The following command appends stdout and stderr to the files "out.txt" and "err.txt", respectively.




[root@server /root]# ./cmd 1>>out.txt 2>>err.txt



• The following command functions similar to the above two commands, but also copies stdout and stderr to the files "stdout.txt" and "stderr.txt", respectively.




[root@server /root]# (((./cmd | tee stdout.txt) 3>&1 1>&2 2>&3\ |tee stderr.txt) 3>&1 1>&2 2>&3) 1>out.txt 2>err.txt


Note: Lines that end in a backslash are continued on the next line. Any such lines should be keyed in as one complete line. The lines are too long to fit on the web page without formatting them this way.


• Cool Tar Command




[root@server /root]# (cd /var/ftp/pub/rh71prof/disk1.iso.dir \ && tar -cvf - .) | (cd /var/ftp/pub/rh71prof/i386 && tar -xvf -)


Note: Lines that end in a backslash are continued on the next line. Any such lines should be keyed in as one complete line. The lines are too long to fit on the web page without formatting them this way.


• Cool Find Command - looks in each file for searchstring




[root@server /root]# find . -type f -exec grep -i \ searchstring \{\} --with-filename \;


Note: Lines that end in a backslash are continued on the next line. Any such lines should be keyed in as one complete line. The lines are too long to fit on the web page without formatting them this way.


• Sample Test Script




#!/bin/sh #You can use this sample script for testing. The echo # statements explain how this script works. echo "This is Standard Out" >&1 echo "This is Standard Error" >&2



• For loop demonstrating stderr and stdout




echo Standard Out >stdout.txt echo Standard Error >stderr.txt for X in bzImage modules modules_install; do make $X; done 1>>stdout.txt 2>>stderr.txt



• Redirection in Linux

0 = stdin
1 = stdout
2 = stderr


• Using the tee command to save stdout to tee.txt. stdout is still displayed on the screen.




[root@server /root]# cmd | tee tee.txt



• Using the tee command to append stdout to tee.txt. stdout is still displayed on the screen.




[root@server /root]# cmd | tee -a tee.txt



• Using the script command to capture both stderr and stdout




[root@server stdout]# script Script started, file is typescript [root@server stdout]# ./cmd This is Standard Out This is Standard Error [root@server stdout]# exit exit Script done, file is typescript [root@server stdout]# cat typescript Script started on Thu Oct 11 11:47:36 2001 [root@server stdout]# ./cmd This is Standard Out This is Standard Error [root@server stdout]# exit exit Script done on Thu Oct 11 11:47:39 2001 [root@server stdout]#



• Notes about pipe "|"
• Lets consider a channel one of stdout or stderr.
• The pipe can only carry one channel, namely stdout.
• When you have a command line which utilizes the pipe command, stderr gets sent to the display while stdout gets sent through the pipe on to the other awaiting commands.
• Swapping stdout and stderr will allow stdout to be sent to the display while stderr can be sent through the pipe.
• Pipe by itself will only take stdout. If you swap stdout and stderr before you get to the pipe then the pipe will take stdout (which is now stderr).
• stdout and stderr can be combined and sent through the pipe; however, you have now combined both stderr and stdout and there is no way to separate them.
• using a pipe in a subshell "()" causes stdout to go through the pipe and causes stderr to pass through the subshell back to the display. This provides a way to grab stdout and stderr. Basically you could get both stdout and stderr out of a subshell.



• Order of redirecting
• Method 1




[root@server /root]# cmd 2>&1 1>outfile.txt


#The above command causes the original stderr to go to stdout,
# and causes the original stdout to go to outfile.txt.
#Notice at the time that fd (file descriptor) 2 is redefined, fd 1 still pointed to stdout; therefore, fd 2 will continue to point to stdout independently of what later happens to fd 1. Basically fd 2 copies the item (or address) that fd 1 is pointing to.
(Corrected an incorrect statement - Thanks to Morten for bringing it to my attention.)


• Method 2
#The following command causes both the original stdout and stderr to go to outfile.txt.



[root@server /root]# cmd 1>outfile.txt 2>&1


#Notice that 1=outfile.txt when 2 is redefined, so 2 goes to outfile.txt too (the two
# channels are combined).

Capturing stderr with tee
Swapping stderr and stdout

• The full command

[root@server /root]# (((./cmd | tee stdout.txt) 3>&1 1>&2 2>&3 \
| tee stderr.txt) 3>&1 1>&2 2>&3) 1>out.txt 2>err.txt

• The walk through

1. Contents of the ./cmd script




#!/bin/sh #You can use this sample script for testing. The echo # statements explain how this script works. echo "This is Standard Out" >&1 echo "This is Standard Error" >&2



2. Results from running the ./cmd script




[root@server /root]# ./cmd This is Standard Out This is Standard Error [root@server /root]#


Although you see both lines printed on the screen, behind the scenes one actually went to stdout and the other went to stderr. If you were to do a pipe, only stdout goes through the pipe. Normally this is the desired effect.


3. Capturing stdout


The following will capture a copy of stdout and save it to a file called "stdout.txt"



[root@server /root]# ./cmd | tee stdout.txt


stdout goes through the pipe and tee is able to save a copy of it to the file "stdout.txt"; however, we just lost control of stderr. stderr will not go through the pipe, instead it goes directly to our display.


4. Gaining control of stderr and stdout.


Lets gain control again of stderr and stdout. We do this by surrounding our command with a set of parenthesis.



[root@server /root]# (./cmd | tee stdout.txt)



5. Swapping stdout and stderr.


Now that we have captured stdout, we wish to capture stderr using tee as well. The pipe will only accept stdout, so we must swap stderr and stdout to do this.

Note: The switch is using the standard variable switch method -- 3 variables (buckets) are required to swap 2 variables with each other. (you have 2 variables and you need to switch the contents - you must bring in a 3rd temporary variable to hold the contents of one value so you can properly swap them).



[root@server /root]# (./cmd | tee stdout.txt) 3>&1 1>&2 2>&3



6. Capturing stderr


Now that we have swapped our stdout and stderr, lets hook up tee once again. tee will now capture stderr (tee believes that it is really stdout because stdout is the only thing that can come through the pipe).



[root@server /root]# (./cmd | tee stdout.txt) 3>&1 1>&2 2>&3 \ | tee stderr.txt



7. Gaining control of stderr and stdout, for the 2nd time.


Tee grabs stderr, but once again the channel that doesn't go through the pipe gets sent to the display and we loose it. Lets capture both our stderr and stdout, once again, by using parenthesis.



[root@server /root]# ((./cmd | tee stdout.txt) 3>&1 1>&2 2>&3 \ | tee stderr.txt)



8. Swapping stdout and stderr back to their normal state.


Now we have, once again, captured both stderr and stdout for our use. Currently they are reversed. Lets switch them back for proper use in our pipeline. Again, lets use the standard variable switch to swap them around.



[root@server /root]# ((./cmd | tee stdout.txt) 3>&1 1>&2 2>&3 \ | tee stderr.txt) 3>&1 1>&2 2>&3



9. Gaining control of stderr and stdout, for the 3rd time.


At this point we have swapped stdout and stderr back to their normal positions; however, if we will be manipulating stdout and stderr any further, we should complete this command with either a pipe or another set of parenthesis.

Since we want to be as complete as possible in this example we will use parenthesis. Using parenthesis will gain control over both stdout and stderr. Using a pipe will only gain control over stdout.

Note: If we use a pipe or parenthesis the next process that hooks up to this command will see stderr and stdout in their proper place. If we don't add the last set of parenthesis, or go through a pipe, the order will remain messed up.



[root@server /root]# (((./cmd | tee stdout.txt) 3>&1 1>&2 2>&3 \ | tee stderr.txt) 3>&1 1>&2 2>&3)



10. Redirecting stdout and stderr to separate files


Now lets do something productive with stdout and stderr so that we can really prove that everything went back to their proper place. Lets tell our command to redirect stdout to "out" and stderr to "err".



[root@server /root]# (((./cmd | tee stdout.txt) 3>&1 1>&2 2>&3 \ | tee stderr.txt) 3>&1 1>&2 2>&3) 1>out.txt 2>err.txt


Note: This last step is optional, normally you would insert your other required commands here, commands that would more than likely operate on stdout.

Please note that the results for "out" and "err" are the same as when you run the following command. This proves that we restored stdout and stderr back to their normal usable posisitions. The above command; however, gives us the capability to copy out stdout and stderr using tee and still be able to use stdout and stderr like we always have.



[root@server /root]# ./cmd 1>out.txt 2>err.txt

Capturing stderr in one file and stderr and stdout combined in another file

Here is our testing command we will use to generate both stderr 
and stdout on proper channels:
root@server:~> (echo out >&1; echo err >&2)
out
err
And here is the command to do the work:
root@server:~> (((echo out >&1; echo err >&2) 3>&2 2>&1 1>&3 | \
tee stderr.txt ) 3>&2 2>&1 1>&3 ) > combined.txt 2>&1
root@server:~> cat stderr.txt
err
root@server:~> cat combined.txt
out
err
root@server:~>

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