Bash Ip If Then Else Statement

Bash IP IF THEN ELSE Statement

use double brackets, spaces are important:

IP=$(ifconfig eth0 | grep inet | grep -v inet6 | cut -c 21-34)
if [[ $IP = 192.168.32.1 ]]; then
  mkdir IPFolder-1
elif [[ $IP = 192.168.32.2 ]]; then
  mkdir IPFolder-2
fi

Note: on Ubuntu 11.10, I had to use cut -c 21-34, adapt as necessary.

IP address in condition statement bash profile

@GordonDavisson suggested:

if ip add show tun0 2>/dev/null
then
   export http_proxy='http://127.0.0.1:2123'
fi

but you could also write it like this:

ip add show tun0 2>/dev/null && export http_proxy='http://127.0.0.1:2123'

ip will print stuff on stdout when successful, so maybe you want to do >& /dev/null? Instead of throwing all the data away, consider storing it in a variable, then you can always add a verbose flag to your program at some later point to print the content of that variable if you need to debug it.

Check for IP validity

If you're using bash, you can do a simple regex match for the pattern, without validating the quads:

#!/usr/bin/env bash

ip=1.2.3.4

if [[ $ip =~ ^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
  echo "success"
else
  echo "fail"
fi

If you're stuck with a POSIX shell, then you can use expr to do basically the same thing, using BRE instead of ERE:

#!/bin/sh

ip=1.2.3.4

if expr "$ip" : '[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*$' >/dev/null; then
  echo "success"
else
  echo "fail"
fi

Note that expr assumes that your regex is anchored to the left-hand-side of the string, so the initial ^ is unnecessary.

If it's important to verify that each quad is less than 256, you'll obviously require more code:

#!/bin/sh

ip=${1:-1.2.3.4}

if expr "$ip" : '[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*$' >/dev/null; then
  for i in 1 2 3 4; do
    if [ $(echo "$ip" | cut -d. -f$i) -gt 255 ]; then
      echo "fail ($ip)"
      exit 1
    fi
  done
  echo "success ($ip)"
  exit 0
else
  echo "fail ($ip)"
  exit 1
fi

Or perhaps even with fewer pipes:

#!/bin/sh

ip=${1:-1.2.3.4}

if expr "$ip" : '[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*$' >/dev/null; then
  IFS=.
  set $ip
  for quad in 1 2 3 4; do
    if eval [ \$$quad -gt 255 ]; then
      echo "fail ($ip)"
      exit 1
    fi
  done
  echo "success ($ip)"
  exit 0
else
  echo "fail ($ip)"
  exit 1
fi

Or again, if your shell is bash, you could use a cumbersome regular expression for quad validation if you're not fond of arithmetic:

#!/usr/bin/env bash

ip=${1:-1.2.3.4}

re='^(0*(1?[0-9]{1,2}|2([0-4][0-9]|5[0-5]))\.){3}'
 re+='0*(1?[0-9]{1,2}|2([‌​0-4][0-9]|5[0-5]))$'

if [[ $ip =~ $re ]]; then
  echo "success"
else
  echo "fail"
fi

This could also be expressed in BRE, but that's more typing than I have in my fingers.

And lastly, if you like the idea of putting this functionality ... in a function:

#!/usr/bin/env bash

ip=${1:-1.2.3.4}

ipvalid() {
  # Set up local variables
  local ip=${1:-NO_IP_PROVIDED}
  local IFS=.; local -a a=($ip)
  # Start with a regex format test
  [[ $ip =~ ^[0-9]+(\.[0-9]+){3}$ ]] || return 1
  # Test values of quads
  local quad
  for quad in {0..3}; do
    [[ "${a[$quad]}" -gt 255 ]] && return 1
  done
  return 0
}

if ipvalid "$ip"; then
  echo "success ($ip)"
  exit 0
else
  echo "fail ($ip)"
  exit 1
fi

There are many ways you could do this. I've shown you just a few.

Bash 'if' statement in 'for' loop in an 'if' statement

The if statement needs to be inside the loop.

for i in $(seq $first $last)
do 
    ping -c 1 ${ip[0]}.${ip[1]}.${ip[2]}.$i > /dev/null
    if [ $? -eq 0 ]; then
        echo "${ip[0]}.${ip[1]}.${ip[2]}.$i is up"
    else
        echo "${ip[0]}.${ip[1]}.${ip[2]}.$i is down"
    fi
done

Checking host availability by using ping in bash scripts

Ping returns different exit codes depending on the type of error.

ping 256.256.256.256 ; echo $?
# 68

ping -c 1 127.0.0.1 ; echo $?
# 0

ping -c 1 192.168.1.5 ; echo $?
# 2

0 means host reachable

2 means unreachable

Ping Test If-else case

If you are using the standard GNU/Linux ping tool, then the manual states:

If ping does not receive any reply packets at all it will exit with code 1. If a packet count and deadline are both specified, and fewer than count packets are received by the time the deadline has arrived, it will also exit with code 1. On other error it exits with code 2. Otherwise it exits with code 0. This makes it possible to use the exit code to see if a host is alive or not.

This means you can capture the exit code from the command in the shell and switch on that. For bash:

if ping -c1 192.168.1."$i" ; then
    echo "Unit ${i} is online"
else
    echo "Unit ${i} is offline"
fi

Or as a one-liner using || and &&:

ping -c 192.168.1."$i" && echo "Unit ${i} is online" || echo "Unit ${i} is offline"

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