An example script to show how to fetch from all default remotes and do any fast-forwards of local branches from upstream possible

git-fetch-and-fast-forward-all-branches.sh

#!/bin/sh

set -e

CURRENT_BRANCH="$(git symbolic-ref HEAD)"
CURRENT_BRANCH_UPSTREAM="$(git rev-parse --symbolic-full-name @{u} 2> /dev/null)"

# Update the remote-tracking branches for every remote for which
# remote.<name>.skipDefaultUpdate is not true:
git remote update default

git for-each-ref --shell --format="REF=%(refname); UPSTREAM=%(upstream)" \
    refs/heads | while read entry
do
    eval "$entry"

    echo "Considering $REF ..."

    # Deal with the current branch last:
    if [ "$REF" = "$CURRENT_BRANCH" ]
    then
        echo "  Skipping the current branch for now."
        continue
    fi

    if [ x"$UPSTREAM" = x ]
    then
        echo "  No upstream branch is set for $REF"
        continue
    fi

    REF_HASH="$(git rev-parse --verify $REF)"
    UPSTREAM_HASH="$(git rev-parse --verify $UPSTREAM)"
    MERGE_BASES="$(git merge-base $REF $UPSTREAM)"

    if [ x"$REF_HASH" = x"$UPSTREAM_HASH" ]
    then
        echo "  The branch is already at the upstream version"
        continue
    fi

    # Now test if the branch can be fast-forwarded:
    if [ x"$MERGE_BASES" = x"$REF_HASH" ]
    then
        echo "  Fast-forwarding $REF from $UPSTREAM"
        echo "  $REF_HASH -> $UPSTREAM_HASH"
        git update-ref "$REF" "$UPSTREAM_HASH"
    else
        echo "  Cannot fast-forward $REF with $UPSTREAM"
    fi

done

if [ x"$CURRENT_BRANCH_UPSTREAM" = x ]
then
    echo "No upstream branch was found for the current branch ($CURRENT_BRANCH)"
else
    echo "Now deal with the current branch:"
    echo "While on $CURRENT_BRANCH, merging from $CURRENT_BRANCH_UPSTREAM"
    git merge $CURRENT_BRANCH_UPSTREAM
fi

Fish version:

# https://gist.github.com/mhl/936480

set -l CURRENT_BRANCH (git symbolic-ref HEAD)
set -l CURRENT_BRANCH_UPSTREAM (git rev-parse --symbolic-full-name @{u} 2> /dev/null)

# Update the remote-tracking branches for every remote for which
# remote.<name>.skipDefaultUpdate is not true:
git remote update default
   
for entry in (git for-each-ref --shell --format="%(refname) %(upstream)" refs/heads)
   set -l REF (string trim --chars "'" -- (string split ' ' -- $entry)[1])
   set -l UPSTREAM (string trim --chars "'" -- (string split ' ' -- $entry))[2]
      
   if test "$REF" = "$CURRENT_BRANCH"
      # echo "$REF -> $UPSTREAM: is the current branch => skipping for now"
      continue
   end
   if test -z "$UPSTREAM"
      # echo "$REF -> $UPSTREAM: upstream is absent => skipping for now"
      continue
   end
      
   set -l REF_HASH (git rev-parse --verify $REF)
   set -l UPSTREAM_HASH (git rev-parse --verify $UPSTREAM)
   set -l MERGE_BASES (git merge-base $REF $UPSTREAM)

   if test "$REF_HASH" = "$UPSTREAM_HASH"
      # echo "$REF -> $UPSTREAM: REF_HASH($REF_HASH) is the same as UPSTREAM_HASH => skipping..."
      continue
   end
      
   if test "$MERGE_BASES" = "$REF_HASH"
      echo -e (set_color -o brwhite)Fast-forwarding $REF(set_color normal) -\> $UPSTREAM
      git update-ref "$REF" "$UPSTREAM_HASH"
   else
      echo -e (set_color -o brwhite)Cannot fast-forward: $REF(set_color normal) -\> $UPSTREAM
   end      
end
if test -z "$CURRENT_BRANCH_UPSTREAM"
   # echo "No upstream branch was found for the current branch ($CURRENT_BRANCH)"
else
   echo -e (set_color -o brwhite)Merging $CURRENT_BRANCH(set_color normal) from $CURRENT_BRANCH_UPSTREAM
   git merge $CURRENT_BRANCH_UPSTREAM
end