Part II · Your system

23.2 ELF and "command not found"

alsocommand not foundelfno such file or directoryexec format errorbad interpretershebangcannot execute binary filefile command

TL;DR

"command not found": no file of that name in any folder of your PATH. Install the package, or call the program by its full path.
"No such file or directory" for a file that exists: what's missing is its interpreter, the #! line of a script, or the loader a binary was built for.
"Exec format error": the binary is built for another CPU (ARM vs x86).

file ./mytool          # architecture, linking, interpreter
head -1 deploy.sh      # the #! line of a script

On your machine

alice@laptopexample

$ atfq 23.2
  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

A script saved with Windows line endings asks for /bin/bash\r: "bad interpreter". Fix it with dos2unix or sed -i 's/\r$//' deploy.sh.
"Permission denied" on your own file: it lacks the x bit (chmod +x), or sits on a filesystem mounted noexec.
Programs built for musl (Alpine) don't run on glibc systems, nor the other way round, unless they are statically linked.
To find the package that provides a command: apt-file search bin/NAME (Debian, Ubuntu), dnf provides '*/bin/NAME' (Fedora), pacman -F NAME (Arch).

Deep dive 2 questions

What is ELF?

The file format of Linux programs and libraries. Its header names the CPU architecture; then come the code and the data. A dynamically linked program also names its interpreter, the loader that the kernel starts first to load the libraries: /lib64/ld-linux-x86-64.so.2 for x86-64 with glibc. readelf -l program shows it as "Requesting program interpreter".

What does the kernel do with a script?

When a file starts with #!, the kernel runs the program named on that line and gives it the script. #!/usr/bin/env python3 asks env to find python3 through PATH, so the same script works where Python is installed in different places.

Commands: file readelf
See also: 23.3 PATH and command resolution
23.1 Dynamic linking and ldd
15.1 rwx, octal and umask