ifs

The Internal Field Separator (IFS) is a special shell variable that defines how Bash—and other POSIX-compatible shells—split strings into fields. While environment variables provide static, system-wide settings, shell variables like IFS are dynamic and scoped to your shell session. Properly managing IFS enhances portability by ensuring your scripts behave consistently across different systems.

By default, IFS includes space, tab, and newline. This default behavior impacts many common tasks, such as iterating over lists of filenames or parsing command output.

Table of Default IFS Separators

Default IFS in Action

Consider a simple Bash script that iterates over a space-separated list:

#!/usr/bin/env bash
elements="alpha beta gamma"
for element in ${elements}; do
  echo "${element} is now separated from the elements list"
done

Running this script yields:

alpha is now separated from the elements list
beta is now separated from the elements list
gamma is now separated from the elements list

Because elements splits on spaces by default, each word becomes its own iteration.

Customizing IFS with ANSI-C Quoting

To explicitly redefine IFS to include space, tab, and newline, use ANSI-C quoting:

IFS=$' \t\n'

Changing the Field Separator

If your data uses a different delimiter—such as a colon (:)—override IFS before expanding variables. Without overriding, the string remains intact:

#!/usr/bin/env bash
elements="one:two:three"
for element in ${elements}; do
  echo "${element} is now separated from the elements list"
done

Output:

one:two:three is now separated from the elements list

Now set IFS to a colon:

#!/usr/bin/env bash
IFS=":"
elements="one:two:three"
for element in ${elements}; do
  echo "${element} is now separated from the elements list"
done

Output:

one is now separated from the elements list
two is now separated from the elements list
three is now separated from the elements list

IFS and Literal Strings

IFS only applies when expanding variables. Looping over a quoted literal string bypasses field splitting:

#!/usr/bin/env bash
IFS=":"
for element in "one:two:three"; do
  echo "${element} is a value"
done

Output:

one:two:three is a value

Because there’s no variable expansion, IFS has no effect.

Interactive IFS Changes

You can also reassign IFS directly in your interactive shell session:

$ IFS=","
$ val="apple,banana,cherry"
$ set -- ${val}
$ echo $1
apple
$ echo $2
banana
$ echo $3
cherry

Here, set -- ${val} replaces the shell’s positional parameters with the split values of val.

Restoring IFS and Handling Empty Strings

1. Restore IFS to default

   IFS=$' \t\n'
   

   This command works in Bash, Zsh, and KornShell. Older shells might require specialized syntax.

2. Set IFS to an empty string

   IFS=''
   

   When IFS is empty, no field splitting occurs—every expansion remains as a single, unsplit string.

References

• Bash Manual: Special Parameters
• POSIX Shell Syntax
• Advanced Bash-Scripting Guide