Correctly Escape echo -ne

Question

I'm outputting an echo -ne command to a file in my set-up script for a usb mouse HID.

Here is the script, which is run as root through /etc/rc.local at bootup:

#accidentally had a newline here <---  
#!/bin/bash
cd /sys/kernel/config/usb_gadget/
mkdir -p isticktoit
cd isticktoit
echo 0x1d6b > idVendor # Linux Foundation
echo 0x0104 > idProduct # Multifunction Composite Gadget
echo 0x0100 > bcdDevice # v1.0.0
echo 0x0200 > bcdUSB # USB2
mkdir -p strings/0x409
echo "fedcba9876543210" > strings/0x409/serialnumber
echo "JW" > strings/0x409/manufacturer
echo "DoItForTheWenHua" > strings/0x409/product
mkdir -p configs/c.1/strings/0x409
echo "Config 1" > configs/c.1/strings/0x409/configuration
echo 0x80 > configs/c.1/bmAttributes
echo 250 > configs/c.1/MaxPower
# Add functions here
mkdir -p functions/hid.usb0
echo 1 > functions/hid.usb0/protocol
echo 1 > functions/hid.usb0/subclass
echo 3 > functions/hid.usb0/report_length
# NEXT LINE CAUSES ISSUES 
echo -ne \\x05\\x01\\x09\\x02\\xa1\\x01\\x09\\x01\\xa1\\x00\\x05\\x09\\x19\\x01\\x29\\x03\\x15\\x00\\x25\\x01\\x95\02\\x81\\x06\\xc0\\xc0 > functions/hid.usb0/report_desc
ln -s functions/hid.usb0 configs/c.1/
# End functions
ls /sys/class/udc > UDC

The expected result of the echo -ne is a sequence of chars being written to file functions/hid.usb0/report_desc starting with 0x05, 0x01, 0x09, etc. Instead I got

$ cat /sys/kernel/config/usb_gadget/isticktoit/functions/hid.usb0/report_desc
-ne \x05\x01\x09\x02\xa1\x01\x09\x01\xa1\x00\x05\x09\x19\x01\x29\x03\x15\x00\x25\x01\x95\02\x81\x06\xc0\xc0

When I tried to test this, I couldn't reproduce it. Running the command in bash (works):

pi@raspberrypi:~ $ echo -ne "\\x05\\x01\\x09\\x02\\xa1\\x01\\x09\\x01\\xa1\\x00\\x05\\x09\\x19\\x01\\x29\\x03\\x15\\x00\\x25\\x01\\x95\\x03\\x75\\x01\\x81\\x02\\x95\\x01\\x75\\x05\\x81\\x03\\x05\\x01\\x09\\x30\\x09\\x31\\x15\\x81\\x25\\x7f\\x75\\x08\\x95\\x02\\x81\\x06\\xc0\\xc0"
        ▒       ▒       )%▒u▒▒u▒        0       1▒▒▒▒▒

Running a test script (works):

pi@raspberrypi:~ $ cat test
        ▒       ▒       )%▒u▒▒u▒        0       1▒▒▒▒▒
pi@raspberrypi:~ $ cat test.sh
#!/bin/bash

echo -ne "\\x05\\x01\\x09\\x02\\xa1\\x01\\x09\\x01\\xa1\\x00\\x05\\x09\\x19\\x01\\x29\\x03\\x15\\x00\\x25\\x01\\x95\\x03\\x75\\x01\\x81\\x02\\x95\\x01\\x75\\x05\\x81\\x03\\x05\\x01\\x09\\x30\\x09\\x31\\x15\\x81\\x25\\x7f\\x75\\x08\\x95\\x02\\x81\\x06\\xc0\\xc0" > test
pi@raspberrypi:~ $ ./test.sh
pi@raspberrypi:~ $ cat test
        ▒       ▒       )%▒u▒▒u▒        0       1▒▒▒▒▒P

Bash redirect (works):

pi@raspberrypi:~ $ echo -ne "\\x05\\x01\\x09\\x02\\xa1\\x01\\x09\\x01\\xa1\\x00\\x05\\x09\\x19\\x01\\x29\\x03\\x15\\x00\\x25\\x01\\x95\\x03\\x75\\x01\\x81\\x02\\x95\\x01\\x75\\x05\\x81\\x03\\x05\\x01\\x09\\x30\\x09\\x31\\x15\\x81\\x25\\x7f\\x75\\x08\\x95\\x02\\x81\\x06\\xc0\\xc0" > test
pi@raspberrypi:~ $ cat test
        ▒       ▒  )%▒u▒▒u▒        0       1▒▒▒▒▒

So, what did I do wrong here?

Answer: I didn't put the shebang on the right line, it caused another shell to interpret my script.

Answer 1

The behaviour of echo is notoriously unportable. Your script probably runs with some other shell when it doesn't work. (You're not running sh scriptname, are you?) See, e.g. how echo differs in Bash and Dash:

$ bash -c 'echo -ne \\x41\\x42\\x43\\x0a'
ABC

$ dash -c 'echo -ne \\x41\\x42\\x43\\x0a'
-ne \x41\x42\x43\x0a

Usually, it's better to use printf instead, but that alone won't help, as not all printfs support hex character escapes (\x00), only octals (\000) so you'd need to change the content too. In any case, in Bash printf \\x41\\x42\\x43\\x0a would do the same as Bash's echo -en \\x41\\x42\\x43\\x0a but slightly more portably.

You'll need to check what shell runs the script.

See also: Why is printf better than echo?