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Some Bash Tricks (II)

Posted on Edited on In Disqus:
Following the last Bash tricks article, this episode introduces two more

Printing colored logs in bash

Mainly using ascii color characters; the colored LOG effect can basically be achieved:

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#!/bin/bash

colorize(){
    GREEN="\033[0;32m"
    CYAN="\033[0;36m"
    RED="\033[0;31m"
    GRAY="\033[0;37m"
    BLUE="\033[0;34m"
    YELLOW="\033[0;33m"
    NORMAL="\033[m"
    color=\$${1:-NORMAL}
    # activate color passed as argument
    echo -ne "`eval echo ${color}`"
    # read stdin (pipe) and print from it:
    echo "${*:2}"
    # Note: if instead of reading from the pipe, you wanted to print
    # the additional parameters of the function, you could do:
    # shift; echo $*
    # back to normal (no color)
    echo -ne "${NORMAL}"
}

success() {
    colorize GREEN $@
}

failed() {
    colorize RED $@
}


# check whether port 22 is open
type=TCP
server=127.0.0.1
port=22
nc -z -w5 $server $port
result=$?

# print different colors for different results
if [  "$result" != 0 ]; then
    failed "$type port $server:$port is closed"
else
    success "$type port $server:$port is open"
fi

Checking Whether Specific TCP and UDP Ports Are Open

TCP port check

  1. Using telnet
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~ ❤️  telnet 127.0.0.1 6380
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused
~ ❤️  telnet 127.0.0.1 6379
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
^]
telnet> q
Connection closed.
  1. Using netcat
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~ ❤️  nc -z -w5 127.0.0.1 6380
~ ❤️  echo $?
1
~ ❤️  nc -z -w5 127.0.0.1 6379
~ ❤️  echo $?
0

UDP port detection

Please note: the UDP protocol differs from TCP — even if an application listens on the port, a successful probe is not guaranteed.

Using netcat:

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 ~ ❤️ nc -vz -w5 -u 127.0.0.1 519
localhosti [127.0.0.1] 519 (?) : Connection refused
~ ❤️  echo $?
1

 ~ ❤️  nc -vz -w5 -u 127.0.0.1 514
localhost [127.0.0.1] 514 (syslog) open
~ ❤️  echo $?
0

For UDP port detection, I generally only use it as a reference when confirming iptables configuration; this method is not 100% reliable. Please test carefully and mind the applicable scenarios.

Combining the Two Check Methods

I wrote the two probing methods into one function. If you don’t want to use the failed and success functions above, just replace them with echo.

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# Test is port open
# Usage:
#   test_port TCP 1.2.3.4 22
#   test_port UDP 1.2.3.4 514
test_port() {
    [ $# != 3 ] && return

    type=$1
    server=$2
    port=$3
    if [ $type == "TCP" ]; then
        nc -z -w5 $server $port
        result=$?
    else
        nc -vz -w5 -u $server $port >> /dev/null 2>&1
        result=$?
    fi


    if [  "$result" != 0 ]; then
        failed "$type port $server:$port is closed"
    else
        success "$type port $server:$port is open"
    fi
}
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