Part II · Your system

23.1 Dynamic linking and ldd

alsolddshared libraryshared libraries.sold.soldconfigld_library_patherror while loading shared librariesdynamic linking

TL;DR

Most programs don't contain the libraries they use: at start-up, the dynamic loader finds the .so files they need and loads them.
ldd program lists those libraries and where each was found. "not found" marks the one that's missing.
The loader searches LD_LIBRARY_PATH first, then the folders ldconfig knows (from /etc/ld.so.conf.d/), then the standard ones.

ldd /usr/bin/curl
ldconfig -p | grep libssl

On your machine

alice@laptopexample

$ atfq 23.1
  PATH      /home/alice/.local/bin
            /usr/local/bin (doesn't exist)
            /usr/bin
            . ⚠ the current folder, wherever you are
  Repeated  /usr/bin (listed more than once: harmless, the first one wins)
  Libraries LD_LIBRARY_PATH=/opt/oracle/lib
  Runs      programs built for x86-64, glibc

An example: Alice's laptop, one of the test machines. In your terminal, this block shows your own machine, read live.

Pitfalls

"error while loading shared libraries: libfoo.so.1": install the package that provides it, rather than copying or linking another version.
After putting a library in a new folder, list that folder in a file in /etc/ld.so.conf.d/, then run sudo ldconfig.
Don't run ldd on a program you don't trust: it may run the program's code. objdump -p program | grep NEEDED only reads it.
LD_LIBRARY_PATH set for everything breaks other programs: set it only for the command that needs it.

Deep dive 2 questions

Why link dynamically?

One copy of a library serves every program, on disk and in RAM, and a security fix in the library fixes all of them at once. Static linking puts everything in the program: easier to copy to another machine, but bigger, and every program must be rebuilt to get a fix. Go programs are often linked that way.

What is the number in libfoo.so.1?

The major version, part of the library's "soname". A program asks for libfoo.so.1; the file is often libfoo.so.1.2.3, and libfoo.so.1 is a link to it. A new major version (.so.2) means an incompatible change, and it can be installed next to the old one.

Commands: ldd ldconfig objdump
Paths: /etc/ld.so.conf.d/
See also: 23.2 ELF and "command not found"
21.4 syscalls and strace