How can a bash script write out a binary file, without using uudecode?

Question

Hi all,

I've got a little puzzler here for the bash scripting experts... I have a bash script that needs to create a small (80 byte) binary file when it runs. The contents of the file need to be contained inside the script itself (i.e. I don't want to just package the file along with the script).

My script currently does it like this:

echo 'begin-base64 644 dummy.wav' > /tmp/dummy.uu
echo 'UklGRkgAAABXQVZFZm10IBAAAAADAAEAAHcBAADcBQAEACAAZmFjdAQAAAAAAAAAUEVBSxAAAAAB' >> /tmp/dummy.uu
echo 'AAAAQDYlTAAAAAAAAAAAZGF0YQAAAAA=' >> /tmp/dummy.uu
echo '====' >> /tmp/dummy.uu
uudecode -o /tmp/dummy.wav /tmp/dummy.uu
rm /tmp/dummy.uu

... after the above runs, I have my file /tmp/dummy.wav. But I just found out that the computer this script is to run on does not have uudecode installed (and I'm not allowed to install it), so I need to find some other way to create this file. Any ideas?

Answer 1

Some installers do something similar to this:

#!/bin/bash
tail -n +4 $0 | tar xvzf -
exit
<tgz file appended here><newline>

Answer 2

Put an exit at the end of your script, append the file to the end of the script, and use tail -c 80 to get at the contents. This will work as long as you don't need to worry about newline conversion issues.

Answer 3

If the target computer has perl available:

perl -ne 'print unpack("u",$_)' > dummy.wav <<EOD
M4DE&1D@```!7059%9FUT(!`````#``$``'<!``#<!0`$`"``9F%C=`0`````
C````4$5!2Q`````!````0#8E3```````````9&%T80``````
EOD

That's using the non-base64 format you get from just doing uuencode dummy.wav < dummy.wav on your original computer.

Failing that, you can always do this:

echo -ne '\x52\x49\x46\x46\x48\x00\x00\x00'  > dummy.wav
echo -ne '\x57\x41\x56\x45\x66\x6d\x74\x20' >> dummy.wav
echo -ne '\x10\x00\x00\x00\x03\x00\x01\x00' >> dummy.wav
echo -ne '\x00\x77\x01\x00\x00\xdc\x05\x00' >> dummy.wav
echo -ne '\x04\x00\x20\x00\x66\x61\x63\x74' >> dummy.wav
echo -ne '\x04\x00\x00\x00\x00\x00\x00\x00' >> dummy.wav
echo -ne '\x50\x45\x41\x4b\x10\x00\x00\x00' >> dummy.wav
echo -ne '\x01\x00\x00\x00\x40\x36\x25\x4c' >> dummy.wav
echo -ne '\x00\x00\x00\x00\x00\x00\x00\x00' >> dummy.wav
echo -ne '\x64\x61\x74\x61\x00\x00\x00\x00' >> dummy.wav

This little bit of bash was generated with:

$ hexdump -e '"echo -ne '\''" 8/1 "x%02x" "'\'' >> dummy.wav\n"' dummy.wav | sed 's;x;\\x;g;1s/>/ /'

Edited to add: As pointed out in a reply here, something like this is also a possibility:

xargs -d'\n' -n1 echo -ne > dummy.wav <<EOD
\x52\x49\x46\x46\x48\x00\x00\x00\x57\x41\x56\x45\x66\x6d\x74\x20
\x10\x00\x00\x00\x03\x00\x01\x00\x00\x77\x01\x00\x00\xdc\x05\x00
\x04\x00\x20\x00\x66\x61\x63\x74\x04\x00\x00\x00\x00\x00\x00\x00
\x50\x45\x41\x4b\x10\x00\x00\x00\x01\x00\x00\x00\x40\x36\x25\x4c
\x00\x00\x00\x00\x00\x00\x00\x00\x64\x61\x74\x61\x00\x00\x00\x00
EOD

(the -d argument is important to turn off xargs's own backslash processing)

You can also turn the 8/1 in my hexdump command into 80/1 and have a single long echo line.