Category Archives: Memo

Useful bash functions

Get bash file absolute path

  while [ -h "$path" ] ; do path="$(readlink "$path")"; done
  echo "$(cd "$(dirname "$path")"; echo -n "$(pwd)/$(basename "$path")")";


log() {
  if [[ -n "$VERBOSE" ]]; then echo -e "[email protected]"; else test 1; fi

error() {
  echo "[email protected]" >&2
  exit 1
warning() {
  echo "[email protected]" >&2

function check_status {
  if [ $? -ne 0 ];then
    error ${@:-"Encountered an error, aborting!"}

Loop find

find . -type f -iname "*.txt" -print0 | while IFS= read -r -d $'\0' line; do
  echo "$line"
  ls -l "$line"

Process Substitution <(LIST) >(LIST)

Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files. It takes the form of


The process list is run with its input or output connected to a FIFO or some file in /dev/fd. The name of this file is passed as an argument to the current command as the result of the expansion. If the >(list) form is used, writing to the file will provide input for list. If the <(list) form is used, the file passed as an argument should be read to obtain the output of list. Note that no space may appear between the < or > and the left parenthesis, otherwise the construct would be interpreted as a redirection.

For example:
$ cat < <(ls)
$ (echo "YES")> >(read str; echo "1:${str}:first";)> >(read sstr; echo "2:$sstr:two")> >(read ssstr; echo "3:$ssstr:three")

See also:

Bash Reference Manual
Bash Hackers Wiki

Invisible characters mess js and css up

Some invisible characters may cause program fault, and it’s hard to locate.

For example UTF-8 BOM in the middle of css file may interrupt the css parser, you can find it by
find . -name '*.js' -type f -print0 | xargs -0 grep -r $'\xEF\xBB\xBF'

UTF-8 Line Separator may cause the debugger misunderstand the right line, you can find it by
find . -name '*.js' -type f -print0 | xargs -0 grep -r $'\xe2\x80\xa8'

And you also can cat to display all non-printing characters
BSD cat
cat -evt file
GNU cat
cat -A file

Or sed (better for multi-byte characters)
sed -n "l" file

Fix rtl8192cu monitor mode on raspberry pi

The rtlwifi(for Wifi chips such as those based on Realtek’s 8192cu) is the upstream driver and rtl8192cu is the out-of-stream driver, and the rtlwifi driver seems to be rather unstable on the Raspberry PI (It works fine on x86 machines), so it doesn’t generally get included in most distributions, in the commit 6d4d3a978afbc332af02e548bd0e8ced16dff296 non-mainline source for rtl8192cu wireless driver
is added and rtlwifi is disabled for stabilization. The result is you can not use monitor mode any more.

$ sudo iwconfig wlan0 mode monitor
Error for wireless request "Set Mode" (8B06) :
SET failed on device wlan0 ; Invalid argument.

The only way to solve it is to build your own kernel… (T▽T)

Follow the raspberry manual to build kernel, notice you should change some files before compiling.

Enable the rtlwifi

Uncomment this line in drivers/net/wireless/Makefile

#obj-$(CONFIG_RTLWIFI) += rtlwifi/

Also uncomment this line in drivers/net/wireless/Kconfig
#source “drivers/net/wireless/rtlwifi/Kconfig”

Fixing the regulatory domain (CRDA)

The rtlwifi driver ignores the regulatory set by the CRDA service. Cause of that the card will only have channels 1 to 11 and runs at maximum 20 dBm. This is the world standard of CRDA.
The easiest way is to modified the world standard definition in the driver.
Changing this line

#define RTL819x_2GHZ_CH01_11 \
REG_RULE(2412-10, 2462+10, 40, 0, 20, 0)


#define RTL819x_2GHZ_CH01_11 \
REG_RULE(2412-10, 2484+10, 40, 0, 33, 0)

The REG_RULE function is defined as follow:
REG_RULE(min_freq, max_freq, kHz, max_dbm_with_antenna, max_dbm, flags)
So the value I changed will make them to use all 14 channels available worldwide and to use a maximum dBm of 33 (what is about 2000 mW). Of course you can change it to any value reasonable.

Then you can compile the kernel, after that, you have to add the 8192cu to blacklist avoiding the supplied driver. modify or add /etc/modprobe.d/8192cu.conf

blacklist 8192cu

Ok, that’s all I fixed.
I do not find any wrong using the old rtlwifi driver so far, but if you wanna recover to the original driver, just comment the blacklist in the 8192cu.conf.

搭设 OpenConnect VPN for IOS

OpenConnect server, also known as ocserv, is a VPN server that communicates over SSL. By design, its goal is to become a secure, lightweight, and fast VPN server. OpenConnect server uses the OpenConnect SSL VPN protocol. At the time of writing, it also has experimental compatibility with clients that use the AnyConnect SSL VPN protocol.
Why AnyConnect? Although any connect protocol is simple for GFW to discover, it has been used for many large companies having relation of GDP. So right now it’s more safe than pptp openvpn and some other VPN protocols.

Update On Jan 2018


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How to create an accesspoint using a RealTek 8192cu Usb Wifi Dongle In RPI1 B+

8192cu is now supplied in default kernel, but it is not working when running as an access point. So I need to recompile the driver.

1.get the kernel’s headers

You have to get the kernel’s header files, the common way is

sudo apt-get install linux-headers-...

But in RPI, the easiest way is rpi-source.

sudo wget -O /usr/bin/rpi-source && sudo chmod +x /usr/bin/rpi-source && /usr/bin/rpi-source -q --tag-update

The kernel is compiled by gcc-4.8.3+, so if you have the gcc below 4.8.3, you have to install it.
first add

deb jessie main contrib non-free rpi

in /etc/apt/sources.list, and then

sudo apt-get install -t jessie gcc-4.8 g++-4.8

maybe you want to manage multi gcc versions

sudo update-alternatives --remove-all gcc
sudo update-alternatives --remove-all g++
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.6 20
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.8 50
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.6 20
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.8 50

also you need curses for make menuconfig

sudo apt-get install libncurses5-dev

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