var=a
 
if [ "$var" -eq "$var" ] 2>/dev/null; then
  echo number
else
  echo not a number
fi

2 means the standard error output. If $var is a number, [ "$var" -eq "$var" ] will not output and error message. And an error message will occur when $var is not a number.

This brings out something interesting that I didn’t know before.

    if somenonsense 2>/dev/null; then
        echo no error message
    else
        echo some error mesage
    fi

The output is “some error message”.
This is easy to understand. As “somenonsense” is not a command or a program, it will output an error message

But…

var1=1
var2=2
 
if [ "$var1" -eq "$var2" ] 2>/dev/null; then
  echo no error message
else
  echo some error message
fi

The output is “not equal”. Why? I don’t know. It seems illogical as this time [ "$var1" -eq "$var2" ] will not output any error message. If you know why please let me know.

This also works:

echo '/etc/python3/dummy' | sed 's|/etc/python3/|/etc/python2/|'

output:

/etc/python2/dummy

In this example above I used | to replace the normally used separator /

printf "line 1
line2
line 3 containing a variable: ${PWD}
line 4 contains a quote mark ' and \"
">temp.txt
 
cat temp.txt

Notice double quote ” needs to be escaped.

2. Use Here Doc:

cat > temp.txt <<EOF
line 1
line2
line 3 containing a variable: ${PWD}
line 4 contains a quote mark ' and "
EOF
cat temp.txt

Notice double quote ” doesn’t need to be escaped.

Also notice “EOF” can actually be anything. This also works

cat > temp.txt <<SEPARATOR
line 1
line2
line 3 containing a variable: ${PWD}
line 4 contains a quote mark ' and "
SEPARATOR
cat temp.txt

Notice double quote ” doesn’t need to be escaped.

http://en.wikipedia.org/wiki/Here_document#Unix-Shells

emails="email1@gmail.com;email2@gmail.com;email3@gmail.com"
emails=${emails//;/ } #replace ; with space
arr_emails=($emails) # put them into an array
for item in ${arr_emails[@]}; do
  echo $item
done

In fact this works also. You don’t really need arrays

emails="email1@gmail.com;email2@gmail.com;email3@gmail.com"
emails=${emails//;/ } #replace ; with space
for item in $emails; do
  echo $item
done

But with using arrays, you can do many other operations http://www.thegeekstuff.com/2010/06/bash-array-tutorial/

For the bash or sh version that “emails=${emails//;/ } ” doesn’t work, use tr command:

emails="email1@gmail.com;email2@gmail.com;email3@gmail.com"
emails=$(echo $emails | tr ";" " ")
for item in $emails; do
  echo $item
done

This also works:

emails="email1@gmail.com;email2@gmail.com;email3@gmail.com"
emails=$(echo $emails | tr ";" "\n")
for item in $emails; do
  echo $item
done

echo 'iweigh297lbs' | sed 's/.*[^0-9]\([0-9]\+\).*/\1/'

The expression has to match the whole line.
Greed: The expression will match the largest possible from left to right.
This won’t work.

echo 'iweigh297lbs' | sed 's/.*\([0-9]\+\).*/\1/'

Output 7

This won’t work

echo 'iweigh297lbs' | sed 's/.*[^0-9]\([0-9]*\).*/\1/'

This works

echo 'iweigh297lbs' | sed 's/.*[^0-9]\([0-9][0-9]*\).*/\1/'

+ has to be escaped. * should not be escaped.

Last time I wrote a tutorial on how to build an PPTP server. Now a new way to connect emerged: Shadowsocks! Shadowsocks is already a hot topic among some Chinese-speaking communities but still largely unknown to the expats I think. Here we go: the first tutorial on building a shadowsocks server in English!

After finishing this tutorial you should be able to visit blocked websites on your Windows computer using your own Shadowsocks system.

Shadowsocks is a creation by clowwindy.

Step one: First get a VPS

Get a Linux VPS. A VPS is shortened for Virtual Private Server. It’s not a whole machine (too costly!) but one portion of a machine. A 256 Ram VPS is about 60-90 USD a year and that’s all you have to pay. And if you manage to sell your shadowsocks accounts, you can even make profits!

For China users, vr.org offers the best Hong Kong VPS servers.

A decent VPS provider is linode.com which provides Japan and US servers fast for China.

A cheaper choice is budgetvm.com

You need to get a VPS with linux centos, Debian or Ubuntu installed. After purchasing you will get an IP for your VPS, and a SSH (never mind those terms. Few knows what they actually are) account.

Step two: install Shadowsocks server application on your VPS

Use Putty (software name) to log on to your remote VPS using the IP and SSH account provided. And you get a DOS like black Window with a command line like this:

Type at the command line:

wget https://breakwall.net/promo/install_ss.sh

And press enter
Then type at the command line:

bash install_ss.sh

to run the script.
First the script will ask you to type a password. Then it will ask you whether to install nodejs. Press ‘y’ for yes.
After the script finishes, then all is set on the server side! So easy:-)
Step Three: run the Windows client program
Click this link https://breakwall.net/soft/yingwa.zip to download the Shadowsocks Windows client. Unzip the file and run yingwa.exe, you will get this screen:

Now fill the server with your linux VPS server IP. Fill your password you have set on the server in step two. For port number type 999 .

And click connect. Now open IE or chrome and enjoy your work.

Both shadowsocks server and client are open source projects hosted at github.com