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i would be thankfull if you could help me how to figure out, how to determine if a variable's content starts with the hash sign:

#!bin/sh    

myvar="#comment asfasfasdf"

if [ myvar = #* ] 

this does not work.

Thanks!

Jans

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I would consider the possibility that there may be spaces before the hash tag as well when testing for "comments", if that is indeed what you're doing. –  rthomson Apr 14 '11 at 14:31
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3 Answers

up vote 7 down vote accepted

One approach would be slicing off the first character of the variable's content, using "Substring Expansion":

if [[ ${x:0:1} == '#' ]]
then
    echo 'yep'
else
    echo 'nope'
fi

yep

From the Bash man page:

   ${parameter:offset}
   ${parameter:offset:length}
          Substring  Expansion.   Expands  to  up  to length characters of
          parameter starting at the character  specified  by  offset.   If
          length  is omitted, expands to the substring of parameter start-
          ing at the character specified by offset.  length and offset are
          arithmetic   expressions   (see  ARITHMETIC  EVALUATION  below).
          length must evaluate to a number greater than or equal to  zero.
          If  offset  evaluates  to  a number less than zero, the value is
          used as an offset from the end of the value  of  parameter.   If
          parameter  is  @,  the  result  is  length positional parameters
          beginning at offset.  If parameter is an array name indexed by @
          or  *,  the  result is the length members of the array beginning
          with ${parameter[offset]}.  A negative offset is taken  relative
          to  one  greater  than the maximum index of the specified array.
          Note that a negative offset must be separated from the colon  by
          at  least  one  space to avoid being confused with the :- expan-
          sion.  Substring indexing is zero-based  unless  the  positional
          parameters are used, in which case the indexing starts at 1.

EDIT:

Of course, your original approach would work just fine if you escaped the hash:

$ [[ '#snort' == \#* ]]; echo $?
0
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thanks very much! was very helpful learning! –  jan Mar 20 '11 at 0:41
    
To see this man page, I should type man what? –  duleshi Mar 31 at 4:17
    
It's the Bash man page, so "man bash" should do it. –  Insyte Mar 31 at 21:23
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POSIX-compatible version:

[ "${var%${var#?}}"x = '#x' ] && echo yes

or:

[ "${var#\#}"x != "${var}x" ] && echo yes

or:

case "$var" in
    \#*) echo yes ;;
    *) echo no ;;
esac
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I know this may be heresy, but for this kind of things I'd rather use grep or egrep rather than doing it from within the shell. It's a little more costly (I guess) but for me this solution's readability offsets that. It's a matter of personal taste though, of course.

So:

myvar="   #comment asfasfasdf"
if ! echo $myvar | egrep -q '^ *#'
then
  echo "not a comment"
else
  echo "commented out"
fi

It works with or without leading spaces. If you'd like to account for leading tabs also, use egrep -q '^[ \t]*#' instead.

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Thinking about it perhaps I just prefer egrep because I like regexes, but I consider grep/egrep to be useful in a wider arrange of situations anyway. –  Eduardo Ivanec Apr 14 '11 at 14:46
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