use regular expression in if-condition in bash

Question

I wonder the general rule to use regular expression in if clause in bash    Here is an example    gg svm-grid-ch     if     gg     grid       then echo  gg  fi   svm-grid-ch     if     gg         grid       then echo  gg  fi     if     gg        grid       then echo  gg  fi     if     gg    s   grid       then echo  gg  fi          Why the last three fails to match        Hope you could give as many general rules as possible  not just for this example

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When using a glob pattern  a question mark represents a single character and an asterisk represents a sequence of zero or more characters   if     gg        grid       then echo  gg  fi   When using a regular expression  a dot represents a single character and an asterisk represents zero or more of the preceding character  So      represents zero or more of any character   a   represents zero or more  a     0-9    represents zero or more digits  Another useful one  among many  is the plus sign which represents one or more of the preceding character  So   a-z    represents one or more lowercase alpha character  in the C locale - and some others    if     gg         grid         then echo  gg  fi

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OP      Is glob pettern not only used for file names    No   glob  pattern is not only used for file names  you an use it to compare strings as well  In your examples  you can use case esac to look for strings patterns    gg svm-grid-ch     looking for the word  grid  in the string  gg  case   gg  in      grid    echo  found     esac         gg         grid      case   gg  in     grid   echo  found    esac          gg    s   grid      case   gg  in s   grid   echo  found    esac        In bash  when to use glob pattern and when to use regular expression  Thanks    Regex are more versatile and  convenient  than  glob patterns   however unless you are doing complex tasks that  globbing extended globbing  cannot provide easily  then there s no need to use regex   Regex are not supported for version of bash  lt 3 2  as dennis mentioned   but you can still use extended globbing  by setting extglob    for extended globbing  see here and some simple examples here   Update for OP  Example to find files that start with 2 characters  the dots     means 1 char  followed by  g  using regex  eg output    shopt -s dotglob   ls -1   abg degree   g    for file in    do     file       g      amp  amp  echo  file   done abg degree   g   In the above  the files are matched because their names contain 2 characters followed by  g    ie   g    The equivalent with globbing will be something like this   look at reference for meaning of   and        for file in   g   do echo  file  done abg degree   g

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Use          for regular expression check Regular Expressions Tutorial Table of Contents

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Adding this solution with grep and basic sh builtins for those interested in a more portable solution  independent of bash version  also works with plain old sh  on non-Linux platforms etc      GLOB matching gg svm-grid-ch     case   gg  in     grid   echo  gg    esac    REGEXP     if echo   gg    grep       grid    gt  dev null   then echo  gg   fi     if echo   gg    grep      grid    gt  dev null   then echo  gg   fi     if echo   gg    grep  s   grid    gt  dev null   then echo  gg   fi        Extended REGEXP if echo   gg    egrep        grid      grid  s   grid     gt  dev null   then   echo  gg fi       Some grep incarnations also support the -q  quiet  option as an alternative to redirecting to  dev null  but the redirect is again the most portable

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if     gg         grid

[regex] use regular expression in if-condition in bash

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