When you start working with Linux, it's good to have a rough view with regard to Linux philosophy.
Disclaimer: This is my own highly opinionated view.
1) Everything is a file design
- Files are files, devices drivers are files, directories, system configuration, kernel parameters, and... even processes are all represented as files on the filesystem.
- Everything, whether a plain-text file, block, character special device driver, or kernel state behaves a lot like a file.
# check how shell was started, using the PID of the current process, and leveraging everything is a file design
ls -l /proc/$$/cmdline
# check the file descriptors of it
ls -l /proc/$$/fd
2) Each command does one thing and one thing very well.
- Finding things is very easy, being a d (directory) c (character special file), b (block special file), l (symbolic link), p (named pipe), s (socket), and f (normal file).
find <location> -name <file_name> -type <file_type>- Measuring things is very easy. For example:
# Calculate size of each of the system top-level dirs
du --max-depth=1 -hx /
3) Everything running is a process
- In a basic form, Linux processes can be visualized as a running instance of a program.
- Every process is started by a parent, except the init process (on most distributions, systemd is the parent of all other processes directly or indirectly) which is started by the Linux kernel and has a PID of 1.
# check which process has PID 1
ps -p 1
# list all files opened by a process
lsof -p 1
- Every process has a file descriptor associated with it, a process can open all sorts of files like /dev files, UNIX Sockets, Network sockets, Library files /lib /lib64, so the latter becomes:
# check file descriptors
ls -l /proc/1/fd
4) Pipelines allow composition
- Anything that can be read or written. For any process, we have three standard file things: what it can read (stdin) and where it can write (stdout and stderr).
# find PID of python process and kill it
ps aux | grep python | grep -v grep | awk '{print $2}' | xargs kill5) Shell comes in multiple flavors