What is shell expansion (globbing) and what are the basic wildcards?

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Accepted Answer

Globbing is the shell expanding wildcard characters like * and ? into matching filenames before the command runs — so rm *.tmp is really rm handed the full list of .tmp files.

* The glob wildcard set the shell expands before the command ever runs. *
The crucial mental model: the command never sees the wildcard. The shell expands the pattern against existing filenames first, then passes the resulting names as arguments. By the time ls *.pdf runs, ls is invoked as ls report.pdf notes.pdf — it has no idea a * was ever involved.

Pattern
Matches
Example

*
Any run of characters (incl. none)
*.txt → all .txt files

?
Exactly one character
file?.txt → file1.txt, fileA.txt

[abc]
One character from the set
file[123].txt

[!abc] / [^abc]
One character not in the set
file[!0-9].txt

[a-z]
One character in the range
file[a-c].txt

ls *.pdf            # every PDF here
rm *.tmp            # delete all .tmp files
cp /etc/*.conf ~/backup/   # copy every .conf in /etc

Two consequences of "shell expands first" that bite people:

No matches → the literal pattern is passed through. If no file matches *.xyz, the command receives the raw text *.xyz (in default bash), often producing a confusing "No such file" error.
To stop expansion, quote it. find . -name "*.txt" quotes the pattern so the shell leaves it alone and find does the matching itself. Unquoted, the shell would expand it prematurely against the current directory.

Go deeper:

glob(7) — Linux manual page — *, ?, [...], ranges and negation, including the "no match → literal pattern" rule.

Pattern	Matches	Example
`*`	Any run of characters (incl. none)	`*.txt` → all `.txt` files
`?`	Exactly one character	`file?.txt` → `file1.txt`, `fileA.txt`
`[abc]`	One character from the set	`file[123].txt`
`[!abc]` / `[^abc]`	One character not in the set	`file[!0-9].txt`
`[a-z]`	One character in the range	`file[a-c].txt`