# Linux command quick reference

Use these examples in your own disposable workspace. Replace sample paths only after checking your current directory and the intended target.

## Files and directories

| Task | Command | Check |
|---|---|---|
| Show current directory | `pwd` | Run before using relative paths. |
| List files | `ls -lah` | `-a` includes hidden files; `-h` formats sizes. |
| Change directory | `cd "$HOME/project"` | Run `pwd` after changing location. |
| Create directories | `mkdir -p project/{rtl,tb,sim,logs}` | `-p` also creates missing parents. |
| Create an empty file | `touch rtl/dut.sv` | Existing file timestamps are updated. |
| Copy a file | `cp -i rtl/dut.sv rtl/dut_backup.sv` | `-i` asks before overwriting. |
| Move or rename | `mv -i old.sv rtl/dut.sv` | Verify source and destination order. |
| Remove a file | `rm -i unwanted.txt` | Resolve and list the target first. |
| Identify system | `cat /etc/os-release`; `uname -r` | Record release and kernel separately. |
| Find command | `type tool`; `command -v tool` | A missing command returns nonzero. |
| Inspect path | `pwd`; `realpath path`; `stat path`; `file path` | Resolve symlinks before acting. |
| Find files | `find . -type f -print` | Limit the starting tree. |

## Logs, processes, and engineering evidence

| Task | Command | Check |
|---|---|---|
| Read logs | `less run.log`; `tail -n 30 run.log` | Preserve the original log. |
| Search text | `grep -E 'ERROR|FAIL' run.log` | No match returns status `1`. |
| Count records | `wc -l run.log`; `sort names.txt | uniq -c` | `uniq` groups adjacent lines only. |
| Separate errors | `tool >out.log 2>err.log` | `>` overwrites its target. |
| Inspect identity | `id`; `groups`; `stat -c '%a %U %G' file` | File and directory permissions differ. |
| Inspect process | `ps -p PID -o pid,ppid,stat,cmd`; `pgrep -a name` | Confirm owner and PID before signals. |
| Inspect capacity | `df -h path`; `du -sh directory`; `free -h` | Quotas may impose a separate limit. |
| Inspect package | `rpm -qf /usr/bin/tool`; `dnf info package` | Check repository and signature provenance. |
| Read service state | `systemctl status unit --no-pager`; `journalctl -u unit -n 30` | Do not restart a shared service. |
| Inspect network | `ip address`; `ip route`; `ss -ltn` | Do not publish private addresses in reports. |
| Edit source in a terminal | `vim rtl/dut.sv` or `nano rtl/dut.sv` | Save deliberately and review changes with `diff` or version control. |
| Verify artifact | `sha256sum archive.tgz`; `tar -tzf archive.tgz` | Inspect member paths before extraction. |

Exit status is available as `$?` immediately after a command. Quoting `"$path"` prevents spaces and wildcard characters in a filename from changing the arguments passed to a command. `sudo` is not required for the Linux practice tasks.