The file command can be used to display a file type. Let's create an empty file.

touch foo.txt

The file command will show that foo.txt is an empty file.

~]# file foo.txt
foo.txt: empty

Let's create a directory.

mkdir bar

The file command will show that bar is a directory.

~]# file bar
bar: directory

Let's append text to foo.txt.

echo "Hello World" > foo.txt

Now the file command should return the following.

~]# file foo.txt
foo.txt: ASCII text

XML files should return XML <version> document text.

~]$ file example.xml
example.xml: XML 1.0 document text

Let's say you have a certificate file, foo.cer, that contains something like this.

-----BEGIN CERTIFICATE-----
MIIGhTCCBW2gAwIBAgITFwAS0Zj4+uylATknJgAAABLRmDANBgkqhkiG9w0BAQsF
ADBMMRQwEgYKCZImiZPyLGQBGRYEY29ycDEYMBYGCgmSJomT8ixkARkWCHRocml2
ysR4VfQLr+A3zbM59CQjewP40y7oFgrpNuj8Hp1AXud3nsakEYFaGcc=
-----END CERTIFICATE-----

Something like this should be returned.

foo.cer: PEM certificate

Let's say you have a binary file foo.file. Attempting to view the content of foo.file will probably return mumbo jumbo, something like this.

~]# cat foo.file
h++-://c_+.+h_i+e++.c-+/ce_+e+_-+

The file command should identify foo.file as a data file.

~]# file foo.file
foo.file: data