Size() vs ls -la vs du -h which one is correct size?

Question

I was compiling a custom kernel, and I wanted to test the size of the image file. These are the results:

ls -la | grep vmlinux
-rwxr-xr-x   1 root   root   8         


        

Answer 1

They are all correct, they just show different sizes.

• ls shows size of the file (when you open and read it, that's how many bytes you will get)
• du shows actual disk usage which can be smaller than the file size due to holes
• size shows the size of the runtime image of an object/executable which is not directly related to the size of the file (bss uses no bytes in the file no matter how large, the file may contain debugging information that is not part of the runtime image, etc.)

If you want to know how much RAM/ROM an executable will take excluding dynamic memory allocation, size gives you the information you need.

Answer 2

Two definition need to be understood

1 runtime vs storetime (this is why size differs)

2 file depth vs directory (this is why du differs)

Look at the below example:

[root@localhost test]# ls -l
total 36
-rw-r--r-- 1 root root   712 May 12 19:50 a.c
-rw-r--r-- 1 root root  3561 May 12 19:42 a.h
-rwxr-xr-x 1 root root 71624 May 12 19:50 a.out
-rw-r--r-- 1 root root  1403 May  8 00:15 b.c
-rw-r--r-- 1 root root  1403 May  8 00:15 c.c
[root@localhost test]# du -abch --max-depth=1
1.4K    ./b.c
1.4K    ./c.c
3.5K    ./a.h
712     ./a.c
70K     ./a.out
81K     .
81K     total
[root@localhost test]# ls -l
total 36
-rw-r--r-- 1 root root   712 May 12 19:50 a.c
-rw-r--r-- 1 root root  3561 May 12 19:42 a.h
-rwxr-xr-x 1 root root 71624 May 12 19:50 a.out
-rw-r--r-- 1 root root  1403 May  8 00:15 b.c
-rw-r--r-- 1 root root  1403 May  8 00:15 c.c
[root@localhost test]# size a.out
   text    data     bss     dec     hex filename
   3655     640      16    4311    10d7 a.out

If using size not on executable, OS will report an error.