How do you analyze an ELF file using readelf?

Run readelf with the flag for what you want: -a (everything), -s (symbol table), -S (section headers), -r (relocations).

# Print ALL information
$ readelf -a file.o
# Print SYMBOL table
$ readelf -s file.o
# Print SECTION headers
$ readelf -S file.o
# Print RELOCATION info
$ readelf -r file.o

Example symbol table output:

Num: Size Type    Bind   Ndx Name
  9:    8 OBJECT  GLOBAL   3 buf
 10:   83 FUNC    GLOBAL   1 main
 13:    0 NOTYPE  GLOBAL UND swap

Key fields:

• Type: OBJECT (data), FUNC (code), NOTYPE (undefined)
• Bind: GLOBAL (visible outside), LOCAL (file-private)
• Ndx: Section index (1=.text, 3=.data, UND=undefined)

UND = Undefined: The symbol is referenced but defined elsewhere - the linker must find it.

Go deeper:

• readelf(1) man page — every flag (-a, -s, -S, -r) and what each dump means.
• elf(5) — the ELF format man page — how to read the symbol-table fields readelf prints.