Bash Scripting Examples

Bash C Style For Loop Example and Syntax

How do I use the bash C style for loop under UNIX or Linux operating systems?

The bash C-style for loop share a common heritage with the C programming language. It is characterized by a three-parameter loop control expression; consisting of an initializer (EXP1), a loop-test or condition (EXP2), and a counting expression (EXP3). The syntax is as follows:

for (( EXP1; EXP2; EXP3 ))

do

        shell-command-1

        shell-command-2

done

Bash C-Style Example

#!/bin/bash

# Display message 5 times

for ((i = 0 ; i < 5 ; i++)); do

  echo "Welcome $i times."

done

Sample outputs:

Welcome 0 times.

Welcome 1 times.

Welcome 2 times.

Welcome 3 times.

Welcome 4 times.

Read An Array Using C Style For Loop

Bash provides one-dimensional array variables. An array is created automatically using the following compound assignments syntax:

array=( item1 item2 item3 ... itemN)

You can read an array using for loop as follows:

#!/bin/bash

# define an array called fruits

fruits=("Apple" "Mango" "Pineapple" "Banana" "Orange" "Papaya" "Watermelon")

len=${#fruits[*]}         # get total elements in an array

# print it

for (( i=0; i<${len}; i++ ));

do

        echo "${fruits[$i]}"

done

Here is another practical example that generates lighttpd web server configuration file to log real IP address of the visitors:

#!/bin/bash

_frontend_proxy_lan_ips=("10.10.29.72" "10.10.29.71" "10.10.29.70" "10.10.29.69" "10.10.29.68")

t="/tmp/lighttpd.backend.conf.$$"

at=${#_frontend_proxy_lan_ips[*]}         # get total elements in an array

s=""

echo '### Log real client ips on all backends ###' >"$t"

echo 'server.modules += ( "mod_extforward" )'   >>"$t"

echo 'extforward.headers = ("X-Forwarded-For")' >>"$t"

echo 'extforward.forwarder = (' >>"$t"

# For loop

for (( i=0; i<${at}; i++ ));

do

        [ $i -lt $(( $at - 1 )) ] && s="," || s=""      # remove , for last item in an array

        echo "      \"${_frontend_proxy_lan_ips[$i]}\" => \"trust\"${s} " >>"$t"

done

echo ')' >>"$t"

# Copy it

cp -f "$t" /etc/lighttpd/

# remove temp file

[ -f "$t" ] && rm -f "$t"

Sample outputs (sample config file generated by above script):

### Log real client ips on all backends ###

server.modules += ( "mod_extforward" )

extforward.headers = ("X-Forwarded-For")

extforward.forwarder = (

      "10.10.29.72" => "trust",

      "10.10.29.71" => "trust",

      "10.10.29.70" => "trust",

      "10.10.29.69" => "trust",

      "10.10.29.68" => "trust"

)