Check existence of input argument in a Bash shell script

Question

I need to check the existence of an input argument  I have the following script  if     1  -gt  -1      then echo hi fi   I get       integer expression expected   How do I check the input argument1 first to see if it exists

User · Accepted Answer

It is  if      -eq 0     then     echo  quot No arguments supplied quot  fi  The    variable will tell you the number of input arguments the script was passed  Or you can check if an argument is an empty string or not like  if   -z  quot  1 quot      then     echo  quot No argument supplied quot  fi  The -z switch will test if the expansion of  quot  1 quot  is a null string or not  If it is a null string then the body is executed

User · Answer

Try       bin bash  if        -eq   0       then      echo  No arguments supplied   else      echo  Hello world   fi

User · Answer

It is better to demonstrate this way  if       -eq 0      then     echo  some message      exit 1 fi   You normally need to exit if you have too few arguments

User · Answer

As a small reminder  the numeric test operators in Bash only work on integers  -eq  -lt  -ge  etc    I like to ensure my  vars are ints by  var     var   0      before I test them  just to defend against the     integer arg required  error

User · Answer

In some cases you need to check whether the user passed an argument to the script and if not  fall back to a default value  Like in the script below   scale   2 -1  emulator   1 -scale  scale   Here if the user hasn t passed scale as a 2nd parameter  I launch Android emulator with -scale 1 by default    varname -word  is an expansion operator  There are other expansion operators as well      varname  word  which sets the undefined varname instead of returning the word value    varname  message  which either returns varname if it s defined and is not null or prints the message and aborts the script  like the first example     varname  word  which returns word only if varname is defined and is not null  returns null otherwise

User · Answer

one liner bash function validation  myFunction              1  forgot to supply an argument       if     1  -gt  -1     then         echo hi     fi      add function name and usage  myFunction              1  forgot to supply an argument   FUNCNAME 0     Usage     FUNCNAME 0   some integer       if     1  -gt  -1     then         echo hi     fi      add validation to check if integer  to add additional validation  for example to check to see if the argument passed is an integer  modify the validation one liner to call a validation function       1  forgot to supply an argument   FUNCNAME 0     Usage     FUNCNAME 0   some integer    amp  amp  validateIntegers  1    die  Must supply an integer     then  construct a validation  function that validates the argument  returning 0 on success  1 on failure and a die function that aborts script on failure  validateIntegers          if        1       0-9        then         return 1   failure     fi     return 0  success     die     echo      1 gt  amp 2   exit 1      Even simpler - just use set -u  set -u makes sure that every referenced variable is set when its used  so just set it and forget it  myFunction         set -u     if     1  -gt  -1     then         echo hi     fi

User · Answer

I often use this snippet for simple scripts      bin bash  if   -z   1     then     echo -e   nPlease call   0  lt argument gt   to run this command  n      exit 1 fi

User · Answer

Only because there s a more base point to point out I ll add that you can simply test your string is null   if     1     then   echo yes else   echo no fi   Likewise if you re expecting arg count just test your last   if     3     then   echo has args correct or not else   echo fixme fi   and so on with any arg or var

User · Answer

In my case the only working solution to check if last arguments exists is  if     quot  7 quot             then   echo  quot error quot    exit fi

User · Answer

Another way to detect if arguments were passed to the script            amp  amp  echo No arguments supplied    Note that    expr    causes the expression to be evaluated as per rules of Shell Arithmetic   In order to exit in the absence of any arguments  one can say            amp  amp  echo No arguments supplied   amp  amp  exit 1     Another  analogous  way to say the above would be   let       echo No arguments supplied  let         echo No arguments supplied  exit 1       Exit if no arguments    help let says      let  let arg  arg         Evaluate arithmetic expressions            Exit Status    If the last ARG evaluates to 0  let returns 1  let returns 0 otherwise

User · Answer

If you d like to check if the argument exists  you can check if the   of arguments is greater than or equal to your target argument number  The following script demonstrates how this works test sh    usr bin env bash  if      -ge 3   then   echo script has at least 3 arguments fi  produces the following output     test sh       test sh 1       test sh 1 2       test sh 1 2 3 script has at least 3 arguments     test sh 1 2 3 4 script has at least 3 arguments

[bash] Check existence of input argument in a Bash shell script

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