Shell Programming and Scripting

Hi Don,

I think there is some problem with this syntax.

test.ksh

Code:

if [ "${TEXT#*[![:alnum:] .,;:'"/\()_+=~@&*-]}" = "$TEXT" ];then echo 'no non-printables found';else echo 'non-printable found';fi

Code:

 
+ TEXT='This is a sample text with supposedly non-printable character^Y.'
./test.ksh: line 5: syntax error at line 47: `'' unmatched

Then I introduced a \ before '

Code:

if [ "${TEXT#*[![:alnum:] .,;:\'"/\()_+=~@&*-]}" = "$TEXT" ];then echo 'no non-printables found';else echo 'non-printable found';fi

then the error was -

Code:

+ TEXT='This is a sample text with supposedly non-printable character^Y.'
./test.ksh: line 5: syntax error at line 47: `{' unmatched

then I I introduced a \ before "

Code:

if [ "${TEXT#*[![:alnum:] .,;:\'\"/\()_+=~@&*-]}" = "$TEXT" ];then echo 'no non-printables found';else echo 'non-printable found';fi

error was -

Code:

 
+ TEXT='This is a sample text with supposedly non-printable character^Y.'
./test.ksh: line 5: syntax error at line 47: `)' unexpected

Then finally I introduced \ before all the shell special characters -

Code:

 
echo ${TEXT#*[![:alnum:] .,;:\'\"/\(\)\_+=~@&\*-]}

which resulted into

Code:

Y.

But I think that's wrong because it should have resulted ^ as that's the only punctuation mark not included in the list?!

-dips

I apologize for not trying this out before I posted it.

In a BRE or an ERE special RE characters lose their special meaning when inside a bracket expression, but that is not true in a shell pattern matching expression. I'm glad you were able to figure out what was needed to make it work for you. Even here, the underscore and the asterisk do not need to be escaped.

The output from:

Code:

echo ${TEXT#*[![:alnum:] .,;:\'\"/\(\)_+=~@&*-]}

is correct. The * matched (and discarded):

Code:

This is a sample text with supposedly non-printable character

and the:

Code:

[![:alnum:] .,;:\'\"/\(\)\_+=~@&\*-]

matched and discarded the ^ just leaving

Code:

Y.

in that expansion of $TEXT. The whole point of that expansion is to find a remove one character that is not in the set of characters that you are declaring to be "non-printable" with the non-matching bracket expression. The the if statement comparing the original string and the original string with a non-printable character removed compare equal if and only there are no non-printable characters in the string.

Thank you so much Don for explaining in detail! Despite that I've one more doubt (please bear with me!)

But Y is an alphabet so wouldn't [:alnum:] matches that? and a dot . is already present in the list of allowable punctuations?

-dips

The first character in the bracket expression ([![:alnum:] .,;:\'\"/\(\)_+=~@&*-]) is ! so this is a NON-matching bracket expression. This bracket expression matches any single character that is NOT alphanumeric, NOT a <space>, NOT a <period>, NOT a <comma>, NOT a <semicolon>, NOT a <colon>, NOT a <single-quote>, NOT a <double-quote>, NOT a <slash>, NOT an <open-parenthesis>, NOT a <closing-parenthesis>, NOT an <underscore>, NOT a <plus_sign>, NOT an <equal-sign>, NOT a <tilde>, NOT an <at-sign>, NOT an <ampersand>, NOT an <asterisk>, and NOT a <hyphen-dash> (in this case it matches the circumflex). So, if there is a string of characters starting with any zero or more characters followed by one character in that non-matching expression, the ${var#expression} will expand to the contents of the variable var with the string up to and including the first character that matches the non-matching expression removed.

If there aren't any characters in the variable that match the non-matching expression, there is no match for the entire expression; so the variable is expanded without removing anything. And, if ${var#expression} expands to the same thing as $var, we know that no character was found in $var that you consider to be non-printable.