When looking for txt files, I run this command:
find . -name "*.txt" -print
This gives me a list of all the text files beneath current directory.
However, find . -name *.txt -print gives me the following error:
find: paths must precede expression: mein.txt
Is this the generally expected behavior? What difference do the quotation marks make?
Within a token that is not quoted, it is your shell that will perform expansion, not the command that you are running.
This means that, when you enter find . -name "*.txt" -print, then
find receives the literal *.txt as one of its parameters, and uses
this pattern as the argument to its -name option, which will match
the names of files found against it before applying -print.
On the other hand, when you enter find . -name *.txt -print, the
shell passes the expanded version of *.txt to find. Several cases
are possible:
There are no files matching *.txt in the current directory: find
receives a literal *.txt (assuming default bash settings);
there is exactly one file matching *.txt in the current
directory; let's say it is a.txt: find receives this file name,
and matches all files named a.txt found starting at the current
directory;
several files match *.txt in the current directory (this appears
to be your case): -name receives the first one as its parameter,
and the others are further path parameters to find, which complains
about not being given all paths before the expression.
This is the expected behavior.
Let's assume the following file hierarchy:
.
├── a.txt
├── b.txt
├── c.txt
└── foo
├── a.txt
├── b.txt
└── c.txt
The actual parameters that find receives in eah case can be observed
by replacing the call to find with printf '%s\n', which will print
each expanded argument on its own line:
$ printf '%s\n' . -name "*.txt" -print
.
-name
*.txt
-print
$ printf '%s\n' . -name *.txt -print
.
-name
a.txt
b.txt
c.txt
-print
As you can see, the second invokation that you posted is equivalent,
given the existing files, to find . -name a.txt b.txt c.txt -print.
This is expected behavior - it's is known as "shell globbing". Without the quotation marks, the *.txt is expanded by the shell to pass the entire list of files that match that expression to the find command. By surrounding it in quotes, you tell the shell to pass the literal string *.txt to find. You can achieve the same protection by using other escaping methods, such as find . -name \*.txt or find . -name '*.txt'.
