Debugging Hidden Whitespace in Makefiles with `od -c`

21 August 2025

Today I learned how od -c can save you hours when tracking down a deceptively simple whitespace issue in a Makefile. The culprit? A hidden trailing space that was invisible in my text editor but wreaking havoc on variable assignments. That one invisible trailing space...

The Problem

I was working on a Makefile with a volume name variable that looked correct:

VOL := $(shell basename "$(PWD)" | tr -d '[:space:]')_usr # space-stripped volume name

But when I ran make snapshot-usr, I saw:

>> Creating snapshot .usr_snapshot.tar.gz from volume 'sig_usr '

Notice the trailing space after sig_usr? That space shouldn't be there, and it was causing issues downstream.

The Debugging Journey

Step 1: Verify the Shell Command Works

First, I tested the shell command directly:

$ basename "$PWD" | tr -d '\n' | od -c
s   i   g

The command correctly produced sig with no trailing whitespace. So the issue wasn't with the shell logic.

Step 2: Check the Full Command

$ echo "$(basename "$PWD" | tr -d '\n')_usr" | od -c
s   i   g   _   u   s   r  \n

This also looked correct - just sig_usr followed by a newline from echo. The trailing space had to be coming from somewhere else.

Step 3: The make -n Revelation

Using make -n to see what commands would actually run:

$ make -n snapshot-usr | grep "echo.*volume"
echo ">> Creating snapshot .usr_snapshot.tar.gz from volume 'sig_usr '"

There it was! The trailing space was definitely in the VOL variable. But where?

Step 4: The od -c Breakthrough

The key insight came from using od -c (octal dump with character format) on the Makefile itself:

$ sed -n '6p' Makefile | od -c
0000000    V   O   L                           ?   =       $   (   s   h
0000020    e   l   l       b   a   s   e   n   a   m   e       "   $   $
0000040    P   W   D   "       |       t   r       -   d       '   \   n
0000060    '   )   _   u   s   r       #       s   p   a   c    e   -   s
0000100    t   r   i   p   p   e   d       v   o   l   u   m    e       n
0000120    a   m   e  \n

Look closely at the output around _usr. Can you see it?

_   u   s   r       #

There's a space character between r and #... that trailing space was being included in the VOL variable definition.

The Fix

Simple fix, just remove the trailing space:

# Before (with trailing space)
VOL ?= $(shell basename "$$PWD" | tr -d '\n')_usr # comment

# After (space removed)
VOL ?= $(shell basename "$$PWD" | tr -d '\n')_usr

od

Unlike the xxd or hexdump commands, the od command is included in the POSIX specification, so you'll find it installed in many systems without relying on third-party package managers.

The next time you see unexpected behavior in a Makefile, remember: sometimes the bug is hiding in plain sight, invisible to the naked eye! 🔍 And knowing that sweet sweet command-line fu can save you HOURS of debugging.

This entry was tagged as makefile debugging od whitespace bash linux