The git show command can show various types of objects in various formats:

• commits
• trees
• tags
• blobs

In addition, git show is used to show the state of repository contents as of a given commit, or even as it is in the index at the moment.

$ git show v4.12
tag v4.12
Tagger: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun Jul 2 16:07:11 2017 -0700

Linux 4.12
-----BEGIN PGP SIGNATURE-----

iQEcBAABAgAGBQJZWXyfAAoJEHm+PkMAQRiGUSgH/36/pHSuUXjiwbeExWGl80tU
1sq62VBO2CUZdjgqxESdapRzmQZpiYJNfl5lo7p0UgW5GcSdxtB/9DXLJaIFu3z/
xB0i4OmZc6GX6hxY/Ve2ZbxjzMRyQYluzjbtT+mKrzkw6HWdxz19OAKpdDFqYoga
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=HBF4
-----END PGP SIGNATURE-----

commit 6f7da290413ba713f0cdd9ff1a2a9bb129ef4f6c
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun Jul 2 16:07:02 2017 -0700

    Linux 4.12

diff --git a/Makefile b/Makefile
index 6d8a984..283c623 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 VERSION = 4
 PATCHLEVEL = 12
 SUBLEVEL = 0
-EXTRAVERSION = -rc7
+EXTRAVERSION =
 NAME = Fearless Coyote
 
 # *DOCUMENTATION*
$

$ git show --stat v4.12
tag v4.12
Tagger: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun Jul 2 16:07:11 2017 -0700

Linux 4.12
-----BEGIN PGP SIGNATURE-----

iQEcBAABAgAGBQJZWXyfAAoJEHm+PkMAQRiGUSgH/36/pHSuUXjiwbeExWGl80tU
1sq62VBO2CUZdjgqxESdapRzmQZpiYJNfl5lo7p0UgW5GcSdxtB/9DXLJaIFu3z/
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Pmx8E/dU9wcXu5ou3gWwqxa5ODEjrgmDLD1g0+tFJVhHFszrhSmgqIteK478fPAs
cLfOo1AyzCQwqxhk34ppxmKheq6RRQzIynfd/bdmi/fWVwU9YlaTHixuc8pVViM=
=HBF4
-----END PGP SIGNATURE-----

commit 6f7da290413ba713f0cdd9ff1a2a9bb129ef4f6c
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun Jul 2 16:07:02 2017 -0700

    Linux 4.12

 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
$

$ git show --oneline
15d8ffc96464 (HEAD) Merge tag 'mmc-v4.14' of git://git.kernel.org/pub/scm/linux/kernel/git/ulfh/mmc
$

$ git show <commit>:<filename>

Regardless of your current working directory, show relative to the root of the repository:

$ git show HEAD:Makefile
$ git show master:Makefile
$ git show topic:Makefile
$ git show v4.19:Makefile
$ git show 2241b8bcf2b5f1b01ebb1cbd1231bbbb72230064:Makefile

You need to use the ./ syntax:

$ cd scripts/
$ git show HEAD:./Makefile
$ git show v4.19:./Makefile

If you leave off the commit part, you're referring to the state of the file in the index:

$ git show :Makefile
$ git show :./Makefile