Drive the machine by typing small, composable commands into the shell — each a tiny program making system calls — all operating on one filesystem tree rooted at /.
On Linux you drive the machine by typing small commands into the shell. The Unix philosophy is that each command is a tiny program that does *one thing well*, and you compose them to do bigger things. Every command is really just a program asking the kernel to do the work through system calls — ls ultimately calls into the kernel to read a directory.
pwd — print working directory: the folder you are currently "in". Commands act relative to it.ls — list a directory's contents; ls -l shows the long form with permissions, owner, and size.cd path — change directory; cd .. moves up one level toward the root.cat file — print a file's contents; mkdir name makes a new folder; rm file removes one.chmod — change a file's permissions (its read/write/execute settings).Unlike Windows, Linux has no drive letters. Everything lives in a single tree starting at the root, written /. Extra disks and USB sticks are *mounted* into that one tree as folders. The standard layout is fixed by the FHS (Filesystem Hierarchy Standard), so the same folders mean the same things on every distribution.
$ pwd
/home/ada
$ ls -l
-rw-r--r-- 1 ada staff 42 Jun 30 09:00 notes.txt
drwxr-xr-x 3 ada staff 96 Jun 30 09:01 projects
$ cd projects && ls
app.py README.md/bin and /usr/bin hold commands, /etc holds system config, /home holds users' folders, /var holds changing data like logs, and /tmp holds throwaway files. Knowing the map means you can find almost anything.In ls -l, the leftmost field like -rw-r--r-- is the permission string. The first character is the type (- file, d directory). The next nine are three groups of read/write/execute for the owner, the group, and others. So rw-r--r-- means the owner can read & write, while group and others can only read.
rwx group is a 3-bit number: read=4, write=2, execute=1. So 755 means owner 4+2+1=7 (rwx), group 4+0+1=5 (r-x), others 5 (r-x). chmod 644 file gives rw-r--r--. The digits *are* the permission bits.