Shell = Check if variable begins with #

Question

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

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
As you accepted a answer which involves a `bash` solution, maybe you could edit your question, as it implies you're wishing for a `sh`/`dash` solution. — Charles Roberto Canato, Oct 01 '17 at 02:37
I recommend using an absolute path in the `#!` line as a relative path in that line is apparently taken relative to the current directory of the process executing the script rather than relative to where the script is located. — kasperd, Dec 01 '18 at 19:37

score 21 · Accepted Answer · edited Nov 30 '18 at 08:06

Your original approach would work just fine if you escaped the hash:

$ [[ '#snort' == \#* ]]; echo $?
0

Another 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.

To see this man page, I should type man what? – duleshi Mar 31 '14 at 04:17 — duleshi, Mar 31 '14 at 04:17
It's the Bash man page, so "`man bash`" should do it. – Insyte Mar 31 '14 at 21:23 — Insyte, Mar 31 '14 at 21:23
That's bashism, but tag is about "shell" not "bash" :(. – pevik May 02 '18 at 13:05 — pevik, May 02 '18 at 13:05

Kambus · Answer 2 · 2011-04-14T13:27:24.243

17

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

edited Apr 14 '11 at 13:27

answered Mar 27 '11 at 15:12

Kambus

271
1
4

2

+1 for the posix answer. In some rudimentary timing tests, the `case` method seems to be the fastest of the three on a fairly old 32-bit x86 computer. All three are **far** more efficient than a simple example which pipes to a forked stdin processor like: `echo $var | cut -c1`. My analysis was quite basic based on 10,000,000 iterations on just one computer/os - YMMV. I just used that slower computer to exaggerate any differences and make them more visible - at least on that one platform. – Juan Mar 25 '18 at 22:22
BTW, for those 10,000,000 iterations, the builtin methods described in this answer ran in about a minute (on aforementioned slow box). The pipe to `cut` flavor took 5+ hours. – Juan Mar 26 '18 at 21:36
What if you want to find things starting with " – Justin Nov 06 '19 at 21:38
@Juan - It is surprising than `case` runs faster than parameter expansions. Any clues? – midnite Apr 17 '23 at 09:57

Eduardo Ivanec · Answer 3 · 2011-04-14T14:43:49.583

6

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.

edited Apr 14 '11 at 14:43

answered Apr 14 '11 at 14:36

Eduardo Ivanec

14,881
1
37
43

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

score 0 · Answer 4 · edited Jan 19 '17 at 22:19

0

Here is another way...

# assign to var the value of argument actual invocation
var=${1-"#default string"}

if [[ "$var" == "#"* ]]
then
  echo "$var starts with a #"
fi

Just copy paste the content to a file, grant execution permissions, and watch how it just works ;).

Hope it helps!

Greetings.

edited Jan 19 '17 at 22:19

Isaac Ressler

3
3

answered May 07 '15 at 20:24

Victor

101
2

3

That's bashism, but tag is about "shell" not "bash" :(. – pevik May 02 '18 at 13:05

Shell = Check if variable begins with #

4 Answers4