比较变量的子字符串与UNIX中的另一个字符串

Question

I am a beginner in Unix. My apologies if the question sounds too lame.

I will be passing an argument to a variable fullstring=$1 . The input for this variable might be something similar to these options, for example tester.txt or tester.sh or testapiece.zip or testing.bad or *.* Basically it is something like the string containing the filename wildcard pattern along with file type.

Here I need to cut just the file type from the string that is passed in the argument ie basically i need to cut the string from "." till the end and I need to compare it with multiple strings in the IF clause.

Code structure outline will be similar to the below structure:

IF substring of variable(just the filetype in the variable)  is not equals to any of the set of 4 predefined file type strings (txt,zip,csv,*)
THEN 
     ECHO "file is not in the required file type"
     EXIT
ELSE
     ECHO "file is in required file type"
FI

It will be helpful if some one can help me with the condition to compare in IF clause. Thanks in advance.

Note: we can pass literal * as the file type, it should be compared literally to the *

Answer 1

It works like this:

str="a.txt"
if [ "${str##*.}" = "txt" ] ; then
    do this
else
    do that
fi

${str##*.} is a so called parameter expansion. Find more info about that here: http://www.informit.com/articles/article.aspx?p=99035&seqNum=3

Answer 2

Expanding on hek2mgl's ksh solution to include testing for the 4 different extensions:

str="a.txt"
if [[ "${str##*.}" = @(txt|zip|csv|\*) ]]; then
    do this
else
    do that
fi

The '@' is a multi-pattern matching construct. In this case it's asking for an exact match of strings 'txt', 'zip', 'csv' or the asterisk as a character '*'.

The asterisk has to be escaped otherwise it's treated as a wildcard character, eg:

if [[ "${str##*.}" = @(*txt*) ]] ...

will match ' txt ', 'abc txt ', ' txt def', 'abc txt def'