find walks a folder and everything below it, printing the paths that match. The first argument is where to start, then come the tests.
$ find . -name "*.txt"
./notes.txt
./projects/duck/todo.txt
$ find /var/log -name "*.log" -mtime -1
/var/log/syslog.log-name matches the file name (quote any wildcard so the shell leaves it for find), and -iname ignores case. -type f keeps only files and -type d only folders. -size +100M finds files over 100 megabytes, and -mtime -7 finds files changed in the last seven days.
To search inside files rather than by name, use grep -r: grep -r "password" /etc checks every file under /etc. Add -l to list just the names of the files that match.
which says where a program lives: which python3 might print /usr/bin/python3. If it prints nothing, the program isn't on your path (more about the path in the Processes and scripts course).
Searching from / reads the whole disk, so it's slow and prints a lot of Permission denied errors for folders you can't read. Start as close to where the file probably is, and add 2> /dev/null to hide the errors.
Find every file under the current folder named notes.txt.
| find dir -name "*.txt" | Files by name (-iname ignores case). |
|---|---|
| -type f, -type d | Only files; only folders. |
| -size +100M, -mtime -7 | Bigger than 100 MB; changed in the last 7 days. |
| grep -r word dir | Search inside every file; -l lists names. |
| which cmd | Where a program lives. |