Linux Bash strict mode understanding IFS

Links: - 104 Linux Index
- Linux - Bash strict mode

Understanding IFS¶

The IFS variable - which stands for Internal Field Separator - controls what Bash calls word splitting.
- When set to a string, each character in the string is considered by Bash to separate words.

IFS=$' '
items="a b c"
for x in $items; do
    echo "$x"
done

# Output:
# a
# b
# c

IFS=$'\n'
for y in $items; do
    echo "$y"
done

# Output:
# a b c

For the first loop, IFS is a space, meaning that words are separated by a space character.
For the second loop, words are separated by a newline, which means bash considers the whole value of items as a single word.
If IFS is more than one character, splitting will be done on any of those characters.
Changing IFS

names=(
  "Aaron Maxwell"
  "Wayne Gretzky"
  "David Beckham"
  "Anderson da Silva"
)

echo "With default IFS value..."
for name in ${names[@]}; do
  echo "$name"
done

echo ""
echo "With strict-mode IFS value..."
IFS=$'\n\t'
for name in ${names[@]}; do
  echo "$name"
done

# Output:
# With default IFS value...
# Aaron
# Maxwell
# Wayne
# Gretzky
# David
# Beckham
# Anderson
# da
# Silva
#
# With strict-mode IFS value...
# Aaron Maxwell
# Wayne Gretzky
# David Beckham
# Anderson da Silva

Why change the default IFS value?

If you invoke myscript.sh notes todo-list 'My Resume.doc', then with the default IFS value, the third argument will be mis-parsed as two separate files - named "My" and "Resume.doc". When actually it's a file that has a space in it, named "My Resume.doc".
Setting IFS to $'\n\t' means that word splitting will happen only on newlines and tab characters. This very often produces useful splitting behaviour.
By default, bash sets this to $' \n\t' - space, newline, tab - which is too eager. This means words are split on the basis of empty space rather than being split on newline or tab.

Last updated: 2022-12-18