Wildcards and find

Wildcards: patterns for file NAMES

The payoff of this lesson is acting on groups of files in one command — back up every .png, delete every .tmp — and, with find, locating a file you know exists somewhere in a big project without clicking through directories.

grep searches inside files. To match file names, the shell gives you wildcards (also called globs):

• * matches any run of characters: *.txt means every name ending in .txt
• ? matches exactly one character: photo?.png matches photo1.png but not photo10.png
• [ab] matches one character from the set: report[12].txt

Key mental model: the shell expands the pattern before the command runs. When you type ls *.txt, ls never sees the star. The shell rewrites it to ls draft.txt notes.txt first. Wildcards work with any command: cp *.png backup/, rm *.tmp, grep todo *.md.

find: search the whole tree

Wildcards only look in the current directory. find walks a directory and all its subdirectories:

find . -name "*.txt"
find . -type d

• The first argument is where to start (. = here, from lesson 2-1).
• -name "*.txt" filters by name. The quotes matter: they stop the shell from expanding the star early, so find itself gets the pattern.
• -type d finds directories only, -type f files only.

find prints results in whatever order it walks the tree, so in the example we pipe into sort for a predictable order (pipes get the full treatment in unit 5).