Test whether string is a valid integer

Question

I m trying to do something common enough  Parse user input in a shell script  If the user provided a valid integer  the script does one thing  and if not valid  it does something else  Trouble is  I haven t found an easy  and reasonably elegant  way of doing this - I don t want to have to pick it apart char by char   I know this must be easy but I don t know how  I could do it in a dozen languages  but not BASH   In my research I found this   Regular expression to test whether a string consists of a valid real number in base 10  And there s an answer therein that talks about regex  but so far as I know  that s a function available in C  among others   Still  it had what looked like a great answer so I tried it with grep  but grep didn t know what to do with it  I tried -P which on my box means to treat it as a PERL regexp - nada  Dash E  -E  didn t work either  And neither did -F   Just to be clear  I m trying something like this  looking for any output - from there  I ll hack up the script to take advantage of whatever I get   IOW  I was expecting that a non-conforming input returns nothing while a valid line gets repeated    snafu   echo   2    grep -E     -         0-9      0  1-9  0-9         0-9          if   -z   snafu      then    echo  Not an integer - nothing back from the grep  else    echo  Integer   fi   Would someone please illustrate how this is most easily done   Frankly  this is a short-coming of TEST  in my opinion  It should have a flag like this  if   -I  string      then    echo  String is a valid integer   else    echo  String is not a valid integer   fi

User · Accepted Answer

var     -  0-9          The   indicates the beginning of the input pattern The - is a literal  -  The   means  0 or 1 of the preceding  -   The   means  1 or more of the preceding   0-9    The   indicates the end of the input pattern   So the regex matches an optional -  for the case of negative numbers   followed by one or more decimal digits   References    http   www tldp org LDP abs html bashver3 html REGEXMATCHREF

User · Answer

You can strip non-digits and do a comparison  Here s a demo script   for num in  44   -44   44-   4-4   a4   4a    4   4 4   -4 4   09  do     match   num      digit          strip non-digits     match   match 0                 strip leading zeros     echo -en   num t match t      case  num in          match - match     echo  Integer                               echo  Not integer        esac done   This is what the test output looks like    44      44      Integer -44     44      Integer 44-     44      Not integer 4-4     44      Not integer a4      4       Not integer 4a      4       Not integer  4      4       Not integer 4 4     44      Not integer -4 4    44      Not integer 09      9       Not integer

User · Answer

Adding to the answer from Ignacio Vazquez-Abrams   This will allow for the   sign to precede the integer  and it will allow any number of zeros as decimal points   For example  this will allow  45 00000000 to be considered an integer  However   1 must be formatted to contain a decimal point   45 is not considered an integer here  but 45 0 is   if     1     -  0-9     0        then     echo  yes  this is an integer  elif     1         0-9     0        then     echo  yes  this is an integer  else     echo  no  this is not an integer  fi

User · Answer

For portability to pre-Bash 3 1  when the    test was introduced   use expr   if expr   string     -   0-9       gt  dev null then   echo  String is a valid integer   else   echo  String is not a valid integer   fi   expr STRING   REGEX searches for REGEX anchored at the start of STRING  echoing the first group  or length of match  if none  and returning success failure   This is old regex syntax  hence the excess     -   means  maybe -    0-9    means  one or more digits   and   means  end of string    Bash also supports extended globs  though I don t recall from which version onwards   shopt -s extglob case   string  of       -   0-9    0-9            echo  String is a valid integer                     echo  String is not a valid integer      esac    equivalently      string     -   0-9    0-9           -   means  - or nothing    0-9  means  digit   and    0-9   means  zero or more digits

User · Answer

or with sed      test -z   echo  2000    sed s  0-9   g   amp  amp  echo  integer     echo  no integer       integer     test -z   echo  ab12    sed s  0-9   g   amp  amp  echo  integer     echo  no integer       no integer

User · Answer

Latecomer to the party here  I m extremely surprised none of the answers mention the simplest  fastest  most portable solution  the case statement   case   variable  -    in      0-9         echo Not a number          echo Valid number    esac   The trimming of any sign before the comparison feels like a bit of a hack  but that makes the expression for the case statement so much simpler

User · Answer

Wow    there are so many good solutions here    Of all the solutions above  I agree with  nortally that using the -eq one liner is the coolest   I am running GNU bash  version 4 1 5  Debian   I have also checked this on ksh  SunSO 5 10    Here is my version of checking if  1 is an integer or not   if     1  -eq   1    2 gt  dev null then     echo   1 is an integer     else     echo  ERROR  first parameter must be an integer       echo  USAGE     exit 1 fi   This approach also accounts for negative numbers  which some of the other solutions will have a faulty negative result  and it will allow a prefix of      e g   30  which obviously is an integer   Results     int check sh 123 123 is an integer       int check sh 123  ERROR  first parameter must be an integer     int check sh -123 -123 is an integer       int check sh  30  30 is an integer       int check sh -123c ERROR  first parameter must be an integer     int check sh 123c ERROR  first parameter must be an integer     int check sh c123 ERROR  first parameter must be an integer    The solution provided by Ignacio Vazquez-Abrams was also very neat  if you like regex  after it was explained  However  it does not handle positive numbers with the   prefix  but it can easily be fixed as below       var      -    0-9

User · Answer

For laughs I roughly just quickly worked out a set of functions to do this  is string  is int  is float  is alpha string  or other  but there are more efficient  less code  ways to do this       bin bash  function strindex         x    1   2        if      x      1      then         true     else         if       x   -gt 0    then             false         else             true         fi     fi    function is int         if is empty    1    then         false         return     fi     tmp   echo    1     sed  s   0-9    g       if     tmp       1             -  tmp         1        then          echo  INT    1   tmp  tmp          true     else          echo  NOT INT    1   tmp  tmp          false     fi    function is float         if is empty    1    then         false         return     fi     if   strindex    1    -    then         false         return     fi     tmp   echo    1     sed  s   a-z      g       if     tmp             then          echo  FLOAT     1   tmp  tmp          true     else          echo  NOT FLOAT     1   tmp  tmp          false     fi    function is strict string         if is empty    1    then         false         return     fi     if       1        A-Za-z        then          echo  STRICT STRING    1            true     else          echo  NOT STRICT STRING    1            false     fi    function is string         if is empty    1      is int    1      is float    1      is strict string    1    then         false         return     fi     if     -z    1      then         true         return     fi     false   function is empty         if   -z    1         then         true     else         false     fi     Run through some tests here  I defined that  -44 is an int but 44- isn t etc      for num in  44   -44   44-   4-4   a4   4a    4   4 4   -4 4   09   hello   h3llo                 do     if is int   num   then         echo  INT    num       elif is float   num   then         echo  FLOAT    num       elif is string   num    then         echo  STRING    num       elif is strict string   num    then         echo  STRICT STRING    num      else         echo  OTHER    num      fi done   Output   INT   44 INT   -44 STRING   44- STRING   4-4 STRING   a4 STRING   4a FLOAT    4 FLOAT   4 4 FLOAT   -4 4 INT   09 STRICT STRING   hello STRING   h3llo  STRING      OTHER     OTHER      NOTE  Leading 0 s could infer something else when adding numbers such as octal so it would be better to strip them if you intend on treating  09  as an int  which I m doing   eg expr 09   0 or strip with sed

User · Answer

Here s yet another take on it  only using the test builtin command and its return code   function is int     test  quot    quot  -eq  quot    quot  2 gt   dev null       input  quot -123 quot    if is int  quot  input quot  then    echo  quot Input    input  quot     echo  quot Integer    input  quot  else    echo  quot Not an integer    input  quot  fi

User · Answer

For me  the simplest solution was to use the variable inside a      expression  as so   if   VAR  gt  0   then   echo   VAR is a positive integer   fi   Of course  this solution is only valid if a value of zero doesn t make sense for your application  That happened to be true in my case  and this is much simpler than the other solutions   As pointed out in the comments  this can make you subject to a code execution attack  The       operator evaluates VAR  as stated in the Arithmetic Evaluation section of the bash 1  man page  Therefore  you should not use this technique when the source of the contents of VAR is uncertain  nor should you use ANY other form of variable expansion  of course

User · Answer

I like the solution using the -eq test  because it s basically a one-liner   My own solution was to use parameter expansion to throw away all the numerals and see if there was anything left   I m still using 3 0  haven t used    or expr before  but glad to meet them    if      INPUT STRING   0-9            then     yes  natural number else     no  has non-numeral chars fi

[string] Test whether string is a valid integer

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