While working on Moose, I accidentally deleted the wrong file in git, though code in that file was very small & easy, so I rewrote it and added it to git. But if it would have been a bigger file, then it might not be easy for me to rewrite it, so recovering that file using git is a better option. Here is what I did:

$ git add file1
$ git commit -m "added file1"
$ git rm file1    # I was not supposed to do this
$ git add file2 file3
$ git commit -m "added file2 and file3"
$ git log -p
commit 51f30ce9ce4cbf6573861427f2229c010e6f03a5
Author: Upasana Shukla <me@upasana.me>
Date:   Wed Jul 31 18:13:03 2013 +0530

    added file2 and file3

diff --git a/file1 b/file1
deleted file mode 100644
index 433eb17..0000000
--- a/file1
+++ /dev/null
@@ -1 +0,0 @@
-this is file1
diff --git a/file2 b/file2
new file mode 100644
index 0000000..f138820
--- /dev/null
+++ b/file2
@@ -0,0 +1 @@
+this is file2
diff --git a/file3 b/file3
new file mode 100644
index 0000000..a309e46
--- /dev/null
+++ b/file3
@@ -0,0 +1 @@
+this is file3

commit 6df7df21ca590a751e1e7ac6a423c426ada1cfac
Author: Upasana Shukla <me@upasana.me>
Date:   Wed Jul 31 18:11:22 2013 +0530

    added file1

diff --git a/file1 b/file1
new file mode 100644
index 0000000..433eb17
--- /dev/null
+++ b/file1
@@ -0,0 +1 @@
+this is file1

How to recover file1?

Now I deleted file1, but I need it, so I did this to recover it:

• I copied file2 & file3 to somewhere else, other than git repository
  
$ cp file1 file3 ~/

• Checked out HEAD^.
  HEAD points to what I have checked out right now i.e. commit with SHA 51f30ce9ce4cbf6573861427f2229c010e6f03a5
  ^ points to commit before HEAD. In this case, ^ is the commit in which I had file1 i.e. commit with SHA 6df7df21ca590a751e1e7ac6a423c426ada1cfac
  
$ git checkout HEAD^
or I can directly checkout the commit with SHA 6df7df21ca590a751e1e7ac6a423c426ada1cfac, like this:
  
$ git checkout 6df7df21ca590a751e1e7ac6a423c426ada1cfac


Output of git log -p after executing above commands:

$ git log -p
commit 6df7df21ca590a751e1e7ac6a423c426ada1cfac
Author: Upasana Shukla <me@upasana.me>
Date:   Wed Jul 31 18:11:22 2013 +0530

    added file1

diff --git a/file1 b/file1
new file mode 100644
index 0000000..433eb17
--- /dev/null
+++ b/file1
@@ -0,0 +1 @@
+this is file1

• Now I want file2 and file3 in my repository, so I did this to add those two files:
  
$ mv ~/file2 ~/file3 . #here, '.' is the directory of my git repository
$ git add file2 file3
$ git commit "added file2 & file3"


End result:

$ git log -p
commit 646c1b61731c66ca09d63f0a87a2a68785476474
Author: Upasana Shukla <me@upasana.me>
Date:   Wed Jul 31 18:24:19 2013 +0530

    added file2 & file3

diff --git a/file2 b/file2
new file mode 100644
index 0000000..f138820
--- /dev/null
+++ b/file2
@@ -0,0 +1 @@
+this is file2
diff --git a/file3 b/file3
new file mode 100644
index 0000000..a309e46
--- /dev/null
+++ b/file3
@@ -0,0 +1 @@
+this is file3

commit 6df7df21ca590a751e1e7ac6a423c426ada1cfac
Author: Upasana Shukla <me@upasana.me>
Date:   Wed Jul 31 18:11:22 2013 +0530

    added file1

diff --git a/file1 b/file1
new file mode 100644
index 0000000..433eb17
--- /dev/null
+++ b/file1
@@ -0,0 +1 @@
+this is file1

Credits:

Thanks to Jesse, for a super nice explanation.