Bash / KSH: Define Delimiter (IFS) While Using read Command

in Categories , , last updated April 4, 2014

How do I can set IFS (internal field separator) while using read command in bash loops?

The IFS variable is used in as the input field separator. If you set IFS to | (i.e. IFS=| ), | will be treated as delimiters between words/fields when splitting a line of input. In the read command, IFS is used to split the line of input so that each variable gets a single field of the input. The default value is . You can print it with the following command:

cat -etv <<<"$IFS"

Sample outputs:

 ^I$
$

In this example, read first and last name using read command and set IFS to a white space:

IFS=' ' read -p 'Enter your first and last name : ' first last 
echo "Hello, $first $last"

Sample outputs:

Enter your first and last name : Vivek Gite
Hello, Vivek Gite

In this example set IFS to | and read data:

IFS='|' read domain ip4 ip6 <<< 'google.com|74.125.236.65|2404:6800:4007:801::1008'
echo "$domain has $ip4 IPv4 and $ipv6 IPv6 address."

Sample outputs:

google.com has 74.125.236.65 IPv4 and  IPv6 address.

while loop example with IFS and read command

Create a text file (named foo.txt) as follows:
$ cat foo.txt
Sample outputs:

google.com|74.125.236.65|2404:6800:4007:801::1008
i.theos.in|58.27.86.81|2600:807:320:305::3f6e:f648
cyberciti.biz|75.126.153.206|2600:807:320:305::3f6e:f649

Create a bash shell script as follows:

#!/bin/bash
_input="foo.txt"
# set IFS (internal field separator) to |
# read file using while loop
while IFS='|' read -r domain ip4 ip6 
do
   echo "$domain has $ip4 IPv4 and $ip6 IPv6 address."
done < "$_input"

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

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7 comment

  1. Thx Vivek, i use custom IFS sometimes when i do bash scripts, but i’ve never had the idea to include it directly inside the while loop !
    That easy, quick, efficient and class, just what i like.

    Thx for the tips.

    Pierre B.

  2. Hi Vivek,

    I did try to apply this trick to one of my script, but it seems that specifying the IFS this way broke something, when i put it back as it was before : it works again.

    Here is the function where i do use the custom IFS :

    Log_Chk() {
  # Usage: $0 called with the "log name" as $@ (as a list)
  #+ Note :only the "esmlog" is checked by this function, as it the only relevant log for hardware components status
  DOMAIN="ESMLOG"
  PrintDomainStart ${DOMAIN}
  OLD_IFS=$IFS # backup the default IFS

while read line ; do    # Set IFS to its new value, defined by the "cdv" value
    IFS=";"
    omconfig preferences cdvformat delimiter=semicolon &>/dev/null # Set the "semicolon" as the "cdv"
    read Status Data_or_Date Description <</dev/null
    elif [[ "${Status}" = 'Non-Critical' ]] ; then
      PrintWarning "${Description:0:22}... @ ${Data_or_Date#[[:upper:]][[:alpha:]][[:alpha:]][[:space:]]}" "${Status}" "1" && RETCODE="${NonCritical}" 2>/dev/null
    elif [[ "${Status}" = 'Ok' ]] ; then
      PrintOk "${Description:0:22}... @ ${Data_or_Date#[[:upper:]][[:alpha:]][[:alpha:]][[:space:]]}" "${Status}" "1" # Do nothing !
    fi
  done << EOF
	  $(omreport system esmlog -fmt cdv |tail -10 |grep -E "^(Ok|Non-Critical|Critical)")
EOF
IFS=$OLD_IFS # Back to default IFS

  PrintDomainEnd
  return $RETCODE
}

    1. Ok, i finally change that piece of *** i posted for something cleaner, and then it works.
      Just in case i post it there :

      Log_Chk() {
  # Usage: $0 called with the "log name" as $@ (as a list)
  #+ Note :only the "esmlog" is checked by this function, as it the only relevant log for hardware components status
  if [[ "${ChassisModel}" =~ 'R710' ]] ; then
    DOMAIN="ESMLOG"
    log="esmlog"
  else
    DOMAIN="ALERTLOG"
    log="alertlog"
  fi
  PrintDomainStart ${DOMAIN}
 
  while IFS=";" read Status Data_or_Date Description ; do
    omconfig preferences cdvformat delimiter=semicolon &>/dev/null # Set the "semicolon" as the "cdv"
    if [[ "${Status}" = 'Critical' ]] ; then
      PrintFailure "${Description:0:24}... @ ${Data_or_Date#[[:upper:]][[:alpha:]][[:alpha:]][[:space:]]}" "${Status}" "1" && declare -r RETCODE="${Critical}" 2>/dev/null
    elif [[ "${Status}" = 'Non-Critical' ]] ; then
      PrintWarning "${Description:0:24}... @ ${Data_or_Date#[[:upper:]][[:alpha:]][[:alpha:]][[:space:]]}" "${Status}" "1" && RETCODE="${NonCritical}" 2>/dev/null
    elif [[ "${Status}" = 'Ok' ]] ; then
      PrintOk "${Description:0:24}... @ ${Data_or_Date#[[:upper:]][[:alpha:]][[:alpha:]][[:space:]]}" "${Status}" "1"
    fi
  done <<<"$(omreport system "${log}" -fmt cdv |tail -10 |grep -E "^(Ok|Non-Critical|Critical)" |tac)"
 
  PrintDomainEnd
  return $RETCODE
}

  3. Just a side note:
    with cat, both -e and -t specifies -v implicitly.
    So you’re actually specifying -v three times.

    cat -etv = cat -et

  4. IFS='|' read domain ip4 ip6 <<< 'google.com|74.125.236.65|2404:6800:4007:801::1008'
echo "$domain has $ip4 IPv4 and $ipv6 IPv6 address."

    There is a typo in this example, ip6 will not give an output since you define $ipv6

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