How does process substitution work with while loops? - bash

Question

I'm reading/editing a bash git integration script
This snippet is supposed to print ${SYMBOL_GIT_PUSH} or ${SYMBOL_GIT_PULL} alongside how many commits i am behind and/or ahead by.
local marks
while IFS= read -r line; do
if [[ $line =~ ^## ]]; then
[[ $line =~ ahead\ ([0-9]+) ]] && marks+=" ${BASH_REMATCH[1]}${SYMBOL_GIT_PUSH}"
[[ $line =~ behind\ ([0-9]+) ]] && marks+=" ${BASH_REMATCH[1]}${SYMBOL_GIT_PULL}"
else
marks="${SYMBOL_GIT_MODIFIED}${marks}"
break
fi
done < <(git status --porcelain --branch 2>/dev/null)
printf '%s' "$marks"
Example:
4↑ 10↓
It is working, but i am trying to understand it.
Why is there some IFS and how does it work with process substitution?
I've heard process isn't defined in sh. Is there a way to do this the /bin/sh way or at least more efficiently?
I was provided with a link that should explain what IFS does.
I switched mixed up things and managed to remove the process substitution:
local marks
git status --porcelain --branch 2>/dev/null |
while IFS= read -r line; do
if [[ $line =~ ^## ]]; then
[[ $line =~ ahead\ ([0-9]+) ]] && marks+=" ${BASH_REMATCH[1]}${SYMBOL_GIT_PUSH}"
[[ $line =~ behind\ ([0-9]+) ]] && marks+=" ${BASH_REMATCH[1]}${SYMBOL_GIT_PULL}"
else
marks="${SYMBOL_GIT_MODIFIED}${marks}"
break
fi
done
printf '%s\n' "$marks"
But now, the value of $marks isn't saved and it prints nothing.
I was provided with another link that explains why.
Will return and update on what i've found.
I used the command grouping workaround and wrapped the loop and the print statement inside curly braces:
Also, i made the /bin/sh version almost functional (the exception - show how much commits i'm ahead or behind, not hard, i'm sure i'll do something with awk or cut).
I took advantage of fact that grep returns non-0 when nothing matches.
git status --porcelain --branch 2>/dev/null | {
SYMBOL_GIT_PUSH='↑'
SYMBOL_GIT_PULL='↓'
while IFS= read -r line
do
if echo "$line" | egrep -q '^##'
then
echo "$line" | egrep -q 'ahead' && marks="$marks $SYMBOL_GIT_PUSH"
echo "$line" | egrep -q 'behind' && marks="$marks $SYMBOL_GIT_PULL"
else
marks="*$marks"
break
fi
done
printf ' %s' "$marks"
}
This was a fun learning experience! Thanks to everyone who helped. When i find the 100% solution i'll update this.

Answer 1

Here's the bashism-less git info function.
__git() {
git_eng="env LANG=C git"
ref="$($git_eng symbolic-ref --short HEAD 2>/dev/null)"
[ -n "$ref" ] && ref="$SYMBOL_GIT_BRANCH$ref" || ref="$($git_eng describe --tags --always 2>/dev/null)"
[ -n "$ref" ] || return;
git status --porcelain --branch 2>/dev/null | {
SYMBOL_GIT_PUSH='↑'
SYMBOL_GIT_PULL='↓'
while IFS= read -r line
do
if echo "$line" | grep -E -q '^##'
then
echo "$line" | grep -E -q 'ahead' &&
marks="$marks $SYMBOL_GIT_PUSH$(echo "$line" | sed 's/.*\[ahead //g' | sed 's/\].*//g')"
echo "$line" | grep -E -q 'behind' &&
marks="$marks $SYMBOL_GIT_PULL$(echo "$line" | sed 's/.*\[behind //g' | sed 's/\].*//g')"
else
marks="$SYMBOL_GIT_MODIFIED$marks"
break
fi
done
printf ' %s%s' "$ref" "$marks"
}
}
sed searches for [ahead and deletes it, as well as everything before it, then it pipes it into another sed which deletes everything past ]. This way only the number remains.