tee results in a null file

Question

I try to change Apache configuration with this:

sudo awk '/<Directory \/var\/www\/>/,/AllowOverride None/{sub("None", "All",$0)}{print}' /etc/apache2/apache2.conf | sudo tee /etc/apache2/apache2.conf > /dev/null

After this /etc/apache2/apache2.conf is empty. If I change the destination file to ~/apache2.conf for example, the output is correct.

Why?

What does `awk '//,/AllowOverride None/{sub("None", "All",$0)}{print}' /etc/apache2/apache2.conf` output? You do not need sudo before `awk`. — Pilot6, Aug 20 '15 at 12:29
@Pilot6 It outputs the configuration file with the correct modification. — MikkoP, Aug 20 '15 at 12:30
You are referencing the same file twice in the pipeline, `tee` will overwrite the file before `awk` gets to it. Either use a temporary file or use `sponge` from `moreutils`. If you are using a recent version of GNU awk, it has an [-i inplace argument](http://stackoverflow.com/questions/16529716/awk-save-modifications-inplace) — Thor, Aug 20 '15 at 12:35

score 4 · Accepted Answer · edited May 23 '17 at 12:39

4

You are referencing the same file twice in the pipeline, tee will overwrite the file before awk gets to it. Either use a temporary file or use sponge from moreutils.

Recent versions of GNU awk, 4.1.1, have a -i inplace argument which simulates editing file in-place:

edited May 23 '17 at 12:39

Community

1

answered Aug 20 '15 at 16:50

Thor

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Another (uglier) way to do this, in case someone can't use `sponge` for any reason is to redirect the processed file to `tee` using a command substitution: `<<<"$(sudo awk '//,/AllowOverride None/{sub("None", "All",$0)}{print}' /etc/apache2/apache2.conf) sudo tee /etc/apache2/apache2.conf > /dev/null"`. This way the file is read before getting truncated by `tee` (although a trailing newline in the original file will be "eat" by to the command substitution). – kos Aug 20 '15 at 19:03

score 1 · Answer 2 · edited Apr 13 '17 at 12:23

That your awk/tee adventure does not work reliably, which has already been said here. ;)

You could try another way:

_drum-roll

Use the power of `perl`!

^{again drum-roll}

sudo perl -i.bak -0777pe 's/(<Directory \/var\/www\/>([^<].*\n)*.*AllowOverride\s)None/$1All/' /etc/apache2/apache2.conf

-i.bak

in-place edit and create a backup /etc/apache2/apache2.conf.bak
-0777

slurps the whole file at once

Example

Input file

cat foo

<Directory /var/www/>
    foo bar
    AllowOverride None
</Directory>


<Directory /var/www1/>
        AllowOverride None
</Directory>

The command

perl -i.bak -0777pe 's/(<Directory \/var\/www\/>([^<].*\n)*.*AllowOverride\s)None/$1All/' foo

The content in the file after starting the command

cat foo

<Directory /var/www/>
    foo bar
    AllowOverride All
</Directory>


<Directory /var/www1/>
        AllowOverride None
</Directory>

tee results in a null file

2 Answers2

Use the power of perl!

Example

Use the power of `perl`!