Ifs

Internal Field Separator - IFS

IFS = Internal Field Separator.

Unbelievable as it may seem, POSIX (Private) requires the treatment of IFS as a field terminator, rather than a field separator. What this means in our example is that if there's an empty field at the end of the input line, it will be discarded:

IFS=, read -ra fields <<< "a,b,"
declare -p fields
#> declare -a fields='([0]="a" [1]="b")'

Where did the empty field go? It was eaten for historical reasons ("because it's always been that way"). This behavior is not unique to bash; all conforming shells do it. A non-empty field is properly scanned:

IFS=, read -ra fields <<< "a,b,c"
declare -p fields
#> declare -a fields='([0]="a" [1]="b" [2]="c")'

So, how do we work around this nonsense? As it turns out, appending an IFS character to the end of the input string will force the scanning to work. If there was a trailing empty field, the extra IFS character "terminates" it so that it gets scanned. If there was a trailing non-empty field, the IFS <character creates a new, empty field that gets dropped.

input="a,b,"
IFS=, read -ra fields <<< "$input,"
declare -p fields
#> declare -a fields='([0]="a" [1]="b" [2]="")'

https://mywiki.wooledge.org/BashPitfalls https://mywiki.wooledge.org/IFS