What command shows the symbols in an object file?

readelf -s or nm lists an object's symbols — names, types, and whether each is defined (T/D/B) or undefined (U).

$ readelf -s main.o
Symbol table '.symtab' contains 9 entries:
   Num: Size Type    Bind   Ndx Name
     4:    8 OBJECT  GLOBAL   3 buf
     5:   83 FUNC    GLOBAL   1 main
     8:    0 NOTYPE  GLOBAL UND swap

$ nm main.o
0000000000000000 D buf
0000000000000000 T main
                 U swap

nm symbol types:

• T - text section (code)
• D - initialized data section
• B - BSS (uninitialized data)
• U - undefined (external reference)
• Lowercase = local, Uppercase = global

For shared libraries, also use:

# Dynamic symbols
$ objdump -T libc.so.6 | grep malloc
# Dynamic section
$ readelf -d executable

Go deeper:

• nm(1) — symbol type codes — the full table behind T/D/B/U and the local-vs-global case rule.