#!/bin/bash
FILENAME="/etc/passwd"
while IFS=: read -r username password userid groupid comment homedir cmdshell
do
  echo "$username, $userid, $comment $homedir"
done < $FILENAME

In unix password file, user information is stored line by line, each line consisting of information for a user separated by colon (:) character. In this example while reading the file line by line, the line is also split into fields using colon character as delimiter which is indicated by the value given for IFS.

Sample input

mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash
pulse:x:497:495:PulseAudio System Daemon:/var/run/pulse:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
tomcat:x:91:91:Apache Tomcat:/usr/share/tomcat6:/sbin/nologin
webalizer:x:67:67:Webalizer:/var/www/usage:/sbin/nologin

Sample Output

mysql, 27, MySQL Server /var/lib/mysql
pulse, 497, PulseAudio System Daemon /var/run/pulse
sshd, 74, Privilege-separated SSH /var/empty/sshd
tomcat, 91, Apache Tomcat /usr/share/tomcat6
webalizer, 67, Webalizer /var/www/usage

To read line by line and have the entire line assigned to variable, following is a modified version of the example. Note that we have only one variable by name line mentioned here.

#!/bin/bash
FILENAME="/etc/passwd"
while IFS= read -r line
do
  echo "$line"
done < $FILENAME

Sample Input

mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash
pulse:x:497:495:PulseAudio System Daemon:/var/run/pulse:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
tomcat:x:91:91:Apache Tomcat:/usr/share/tomcat6:/sbin/nologin
webalizer:x:67:67:Webalizer:/var/www/usage:/sbin/nologin

Sample Output

mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash
pulse:x:497:495:PulseAudio System Daemon:/var/run/pulse:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
tomcat:x:91:91:Apache Tomcat:/usr/share/tomcat6:/sbin/nologin
webalizer:x:67:67:Webalizer:/var/www/usage:/sbin/nologin

readarray -t arr <file

Or with a loop:

arr=()
while IFS= read -r line; do
   arr+=("$line")
done <file

while IFS= read -r line; do
   echo "$line"
done <file

If file may not include a newline at the end, then:

while IFS= read -r line || [ -n "$line" ]; do
   echo "$line"
done <file

var='line 1
line 2
line3'
readarray -t arr <<< "$var"

or with a loop:

arr=()
while IFS= read -r line; do
   arr+=("$line")
done <<< "$var"

var='line 1
line 2
line3'
while IFS= read -r line; do
   echo "-$line-"
done <<< "$var"

or

readarray -t arr <<< "$var"
for i in "${arr[@]}";do
    echo "-$i-"
done

while IFS= read -r line;do
    echo "**$line**"
done < <(ping google.com)

or with a pipe:

ping google.com |
while IFS= read -r line;do
    echo "**$line**"
done

Let's assume that the field separator is : (colon) in the file file.

while IFS= read -d : -r field || [ -n "$field" ]; do
   echo "$field"
done <file

For a content:

first : se
con
d:
    Thi rd:
    Fourth

The output is:

**first **
** se
con
d**
**
    Thi rd**
**
    Fourth
**

Let's assume that the field separator is :

var='line: 1
line: 2
line3'
while IFS= read -d : -r field || [ -n "$field" ]; do
   echo "-$field-"
done <<< "$var"

Output:

-line-
- 1
line-
- 2
line3
-

Let's assume that the field separator is :

arr=()
while IFS= read -d : -r field || [ -n "$field" ]; do
   arr+=("$field")
done <file

Let's assume that the field separator is :

var='1:2:3:4:
newline'
arr=()
while IFS= read -d : -r field || [ -n "$field" ]; do
   arr+=("$field")
done <<< "$var"
echo "${arr[4]}"

Output:

newline

Let's assume that the field separator is :

while IFS= read -d : -r field || [ -n "$field" ];do
    echo "**$field**"
done < <(ping google.com)

Or with a pipe:

ping google.com | while IFS= read -d : -r field || [ -n "$field" ];do
    echo "**$field**"
done