How to compare strings in Bash

Question

How do I compare a variable to a string  and do something if they match

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a  abc  b  def     Equality Comparison if     a       b     then     echo  Strings match  else     echo  Strings don t match  fi    Lexicographic  greater than  less than  comparison  if     a    lt    b     then     echo   a is lexicographically smaller then  b  elif     a    gt    b     then     echo   b is lexicographically smaller than  a  else     echo  Strings are equal  fi   Notes    Spaces between if and   and   are important  gt  and  lt  are redirection operators so escape it with   gt  and   lt  respectively for strings

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I have to disagree one of the comments in one point     quot  x quot      quot valid quot     amp  amp  echo  quot valid quot     echo  quot invalid quot   No  that is not a crazy oneliner It s just it looks like one to  hmm  the uninitiated    It uses common patterns as a language  in a way  And after you learned the language  Actually  it s nice to read It is a simple logical expression  with one special part  lazy evaluation of the logic operators     quot  x quot      quot valid quot     amp  amp  echo  quot valid quot     echo  quot invalid quot   Each part is a logical expression  the first may be true or false  the other two are always true       quot  x quot      quot valid quot      amp  amp  echo  quot valid quot       echo  quot invalid quot   Now  when it is evaluated  the first is checked  If it is false  than the second operand of the logic and  amp  amp  after it is not relevant  The first is not true  so it can not be the first and the second be true  anyway  Now  in this case is the the first side of the logic  or    false  but it could be true if the other side - the third part - is true  So the third part will be evaluated - mainly writing the message as a side effect   It has the result 0 for true  which we do not use here  The other cases are similar  but simpler - and - I promise  are - can be - easy to read   I don t have one  but I think being a UNIX veteran with grey beard helps a lot with this

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I would probably use regexp matches if the input has only a few valid entries  E g  only the  start  and  stop  are valid actions    if       ACTION          start stop       then   echo  valid action  fi   Note that I lowercase the variable  ACTION by using the double comma s  Also note that this won t work on too aged bash versions out there

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Using variables in if statements  if     x     valid     then   echo  x has the value  valid   fi   If you want to do something when they don t match  replace   with     You can read more about string operations and arithmetic operations in their respective documentation   Why do we use quotes around  x   You want the quotes around  x  because if it is empty  your Bash script encounters a syntax error as seen below   if      valid     then     Non-standard use of    operator  Note that Bash allows    to be used for equality with    but this is not standard   Use either the first case wherein the quotes around  x are optional   if      x      valid      then   or use the second case   if     x     valid     then

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you can also use use case esac  case   string  in    pattern    echo  found    esac

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I did it in this way that is compatible with Bash and Dash  sh    testOutput  my test  pattern  my   case  testOutput in     pattern        echo  if there is a match      exit 1                 echo there is no coincidence    esac

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Bash 4  examples  Note  not using quotes will cause issues when words contain spaces  etc  Always quote in Bash  IMO   Here are some examples in Bash 4    Example 1  check for  yes  in string  case insensitive        if       str          yes       then   Example 2  check for  yes  in string  case insensitive        if       echo   str    tr    upper       lower           yes       then   Example 3  check for  yes  in string  case sensitive         if       str        yes       then   Example 4  check for  yes  in string  case sensitive         if       str       yes      then   Example 5  exact match  case sensitive         if       str       yes      then   Example 6  exact match  case insensitive         if       str         yes      then   Example 7  exact match        if     a      b     then   Enjoy

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Or  if you don t need else clause       x      valid     amp  amp  echo  x has the value  valid

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The following script reads from a file named  testonthis  line by line and then compares each line with a simple string  a string with special characters and a regular expression  If it doesn t match  then the script will print the line  otherwise not   Space in Bash is so much important  So the following will work       LINE      table name       But the following won t      LINE      table name      So please use as is   cat testonthis   while read LINE do if     LINE      table name     amp  amp      LINE      --------------------------------     amp  amp       LINE         space        amp  amp       LINE     SQL      then echo  LINE fi done

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To compare strings with wildcards use  if      stringA       stringB      then     Do something here else     Do Something here fi

[string] How to compare strings in Bash

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