First, you need to protect the pattern from expansion by the shell. The easiest way to do that is to put single quotes around it. Single quotes prevent expansion of anything between them (including backslashes); the only thing you can't do then is have single quotes in the pattern.
grep -- 'foo*' *.txt
(also note the
-- end-of-option-marker to stop some
grep implementations including GNU
grep from treating a file called
-foo-.txt for instance (that would be expanded by the shell from
*.txt) to be taken as an option (even though it follows a non-option argument here)).
If you do need a single quote, you can write it as
'\'' (end string literal, literal quote, open string literal).
grep -- 'foo*'\''bar' *.txt
Second, grep supports at least¹ two syntaxes for patterns. The old, default syntax (basic regular expressions) doesn't support the alternation (
|) operator, though some versions have it as an extension, but written with a backslash.
grep -- 'foo\|bar' *.txt
The portable way is to use the newer syntax, extended regular expressions. You need to pass the
-E option to
grep to select it (formerly that was done with the
egrep separate command²)
grep -E -- 'foo|bar' *.txt
Another possibility when you're just looking for any of several patterns (as opposed to building a complex pattern using disjunction) is to pass multiple patterns to
grep. You can do this by preceding each pattern with the
grep -e foo -e bar -- *.txt
Or put patterns on several lines:
grep -- 'foo
Or store those patterns in a file, one per line and run
grep -f that-file -- *.txt
Note that if
*.txt expands to a single file,
grep won't prefix matching lines with its name like it does when there are more than one file. To work around that, with some
grep implementations like GNU
grep, you can use the
-H option, or with any implementation, you can pass
/dev/null as an extra argument.
grep implementations support even more like perl-compatible ones with
-P, or augmented ones with
-K for ksh wildcards...
egrep has been deprecated by POSIX and is sometimes no longer found on some systems, on some other systems like Solaris when the POSIX or GNU utilities have not been installed, then
egrep is your only option as its
/bin/grep supports none of
\| or multi-line patterns