Is there a way to squash a number of commits non-interactively?

I'm trying to squash a range of commits - HEAD to HEAD~3. Is there a quick way to do this, or do I need to use rebase --interactive?

Make sure your working tree is clean, then

git reset --soft HEAD~3
git commit -m 'new commit message'

I personally like wilhelmtell's solution:

git reset --soft HEAD~3
git commit -m 'new commit message'

However, I made an alias with some error checking so that you can do this:

g.squash 3 'my commit message'

I recommend setting up aliases that actually run scripts so that it is easier to (a) code up your scripts and (b) do more complex work with error checking. Below is a script that does the work of squashing. I put that in a scripts folder in my HOME path.

Script for squashing (squash.sh)

#!/bin/bash
#

#get number of commits to squash
squashCount=$1

#get the commit message
shift
commitMsg=$@

#regular expression to verify that squash number is an integer
regex='^[0-9]+$'

echo "---------------------------------"
echo "Will squash $squashCount commits"
echo "Commit message will be '$commitMsg'"

echo "...validating input"
if ! [[ $squashCount =~ $regex ]]
then
	echo "Squash count must be an integer."
elif [ -z "$commitMsg" ]
then
	echo "Invalid commit message.  Make sure string is not empty"
else
	echo "...input looks good"
	echo "...proceeding to squash"
	git reset --soft HEAD~$squashCount
	git commit -m "$commitMsg"
	echo "...done"
fi

echo
exit 0

Then to hook up that squash.sh script to an alias, I add the following to my .zprofile:

export PATH="$PATH:$HOME/scripts" # Add scripts folder to PATH
...
alias g.squash='function _gSquash(){ sh squash.sh $1 $2; };_gSquash'
...

Note: You can make your alias anything you want. I have my a lot of my git shortcuts as g.<myCommand>

To add to the answer by wilhelmtell I find it convenient to soft reset to HEAD~2 and then amending the commit of HEAD~3:

git reset --soft HEAD~2
git commit --all --amend --no-edit    

This will merge all commits to the HEAD~3 commit and use its commit message. Be sure to start from a clean working tree.

I used:

EDITOR="sed -i '2,/^$/s/^pick\b/s/'" git rebase -i <ref>

Worked quite fine. Just don't try to have a commit log with a line that starts with "pick" :)

Use the following command to squash the last 4 commits within the last commit:

git squash 4

With the alias:

squash = !"f() { NL=$1; GIT_EDITOR=\"sed -i '2,$NL s/pick/squash/;/# This is the 2nd commit message:/,$ {d}'\"; git rebase -i HEAD~$NL; }; f"
sq = !git squash $1
sqpsf = !git squash $1 && git psf 

From https://github.com/brauliobo/gitconfig/blob/master/configs/.gitconfig

Here is a one liner to squash the last 2 commits. In this example, the message of second last commit will be retained. You may change the message as you wish.

git commit -am "$(git log -1 --skip=1 --pretty=%B | xargs && git reset --soft HEAD~2)"

This command will be very useful if you create an alias for this command and use the alias instead.

To squash everything since the branch was forked from master:

git reset --soft $(git merge-base --fork-point master) \
  && git commit --verbose --reedit-message=HEAD --reset-author

You can get pretty close with

git rebase --onto HEAD4 HEAD master

This assumes you're on master with a linear history. It's not quite a squash because it discards the intermediate commits. You'd need to amend the new HEAD to modify the commit message.