Linux Essentials for Developers: Shell, Filesystem & Permissions

The shell is your interface to the operating system. Bash (or Zsh on macOS) reads commands, runs programs, and chains output through pipes. Start with navigation: `pwd` shows where you are, `cd` changes directory, and `ls -la` lists files with permissions and hidden dotfiles. Use tab completion — type the first few characters and press Tab to finish filenames and commands.

Searching and filtering is where the shell saves hours. `grep -r "error" /var/log` finds lines across files; `find . -name "*.ts"` locates files by pattern; `head` and `tail` show the start or end of logs (`tail -f` follows live output). Redirect output with `>` (overwrite) and `>>` (append), and pipe between commands: `cat access.log | grep 500 | wc -l` counts server errors.

Linux organizes everything under a single root `/`. User home directories live in `/home/username`. System configuration sits in `/etc`. Logs go to `/var/log`. Installed software and libraries are under `/usr`. Temporary files use `/tmp`. Knowing this layout means you know where to look when something breaks — nginx config in `/etc/nginx`, app logs in `/var/log`, binaries in `/usr/bin`.

Create and move files with `mkdir -p path/to/dir` (creates parent folders), `cp -r src dest` for directories, `mv old new` to rename or move, and `rm -rf` to delete recursively (use carefully). View file contents with `cat`, paginated with `less`, or the first/last lines with `head`/`tail`. For large files, `less` is safer than `cat` — it does not flood your terminal.

Every file has an owner, a group, and permission bits for user/group/others: read (r), write (w), execute (x). Run `ls -l` and you see something like `-rw-r--r-- 1 deploy deploy 4096 Jul 4 app.conf` — the owner can read and write; group and others can only read. Directories need execute permission to enter them, even if you can read the listing.

Change permissions with `chmod`: `chmod 644 file` sets rw-r--r--, or use symbolic form `chmod u+x script.sh` to add execute for the owner. Change ownership with `chown user:group file` (usually requires sudo). When a web server cannot read a config or a deploy script fails with "Permission denied", check owner and mode first before assuming a code bug.

Everything running on Linux is a process. `ps aux` lists all processes; `top` or `htop` shows live CPU and memory usage. Find a process by name with `pgrep -a nginx` or kill one with `kill PID` (SIGTERM) or `kill -9 PID` (force — last resort). When an app hangs, check if the process exists and whether it is consuming CPU or stuck waiting on I/O.

Services on modern distros are managed by systemd. `systemctl status nginx` shows whether a service is running; `journalctl -u nginx -f` tails its logs. Application logs often live in `/var/log/` — read them with `tail -f /var/log/syslog` during debugging. Remote servers are accessed via SSH: `ssh user@host` using key-based auth (no password in scripts).