How to check if a variable is set in Bash

Question

How do I know if a variable is set in Bash   For example  how do I check if the user gave the first parameter to a function   function a         if  1 is set

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Note  I m giving a heavily Bash-focused answer because of the bash tag   Short answer  As long as you re only dealing with named variables in Bash  this function should always tell you if the variable has been set  even if it s an empty array   variable-is-set         declare -p   1   amp  gt  dev null     Why this works  In Bash  at least as far back as 3 0   if var is a declared set variable  then declare -p var outputs a declare command that would set variable var to whatever its current type and value are  and returns status code 0  success   If var is undeclared  then declare -p var outputs an error message to stderr and returns status code 1  Using  amp  gt  dev null  redirects both regular stdout and stderr output to  dev null  never to be seen  and without changing the status code  Thus the function only returns the status code   Why other methods  sometimes  fail in Bash          -n   var     This only checks if   var 0   is nonempty   In Bash   var is the same as   var 0         -n    var x      This only checks if   var 0   is set          var         0    This only checks if at least one index of  var is set       When this method fails in Bash  This only works for named variables  including      not certain special variables                           -   0   1   2       and any I may have forgotten   Since none of these are arrays  the POSIX-style   -n    var x     works for all of these special variables  But beware of wrapping it in a function since many special variables change values existence when functions are called   Shell compatibility note  If your script has arrays and you re trying to make it compatible with as many shells as possible  then consider using typeset -p instead of declare -p  I ve read that ksh only supports the former  but haven t been able to test this  I do know that Bash 3 0  and Zsh 5 5 1 each support both typeset -p and declare -p  differing only in which one is an alternative for the other  But I haven t tested differences beyond those two keywords  and I haven t tested other shells   If you need your script to be POSIX sh compatible  then you can t use arrays  Without arrays    -n    var x     works   Comparison code for different methods in Bash  This function unsets variable var  evals the passed code  runs tests to determine if var is set by the evald code  and finally shows the resulting status codes for the different tests   I m skipping test -v var    -v var    and    -v var    because they yield identical results to the POSIX standard   -n    var x      while requiring Bash 4 2   I m also skipping typeset -p because it s the same as declare -p in the shells I ve tested  Bash 3 0 thru 5 0  and Zsh 5 5 1    is-var-set-after           Set var by passed expression      unset var     eval   1         Run the tests  in increasing order of accuracy        -n   var       index 0 of  var is nonempty     nonempty          -n    var x        index 0 of  var is set  maybe empty     plus              var         0     var has at least one index set  maybe empty     count        declare -p var  amp  gt  dev null   var has been declared  any type      declared           Show test results      printf   30s   2s  2s  2s  2s n    1   nonempty  plus  count  declared     Test case code  Note that test results may be unexpected due to Bash treating non-numeric array indices as  0  if the variable hasn t been declared as an associative array  Also  associative arrays are only valid in Bash 4 0      Header  printf   30s   2s  2s  2s  2s n   test   -n    x        -p    First 5 tests  Equivalent to setting  var foo  because index 0 of an   indexed array is also the nonindexed value  and non-numerical   indices in an array not declared as associative are the same as   index 0  is-var-set-after  var foo                            0  0  0  0 is-var-set-after  var  foo                           0  0  0  0 is-var-set-after  var   0  foo                       0  0  0  0 is-var-set-after  var   x  foo                       0  0  0  0 is-var-set-after  var   y  bar  x  foo               0  0  0  0      -n   var     fails when var is empty  is-var-set-after  var                                1  0  0  0 is-var-set-after  var   0                            1  0  0  0   Indices other than 0 are not detected by    -n   var     or by      -n    var x       is-var-set-after  var   1                            1  1  0  0 is-var-set-after  var   1  foo                       1  1  0  0 is-var-set-after  declare -A var  var   x  foo       1  1  0  0   Empty arrays are only detected by  declare -p   is-var-set-after  var                                1  1  1  0 is-var-set-after  declare -a var                     1  1  1  0 is-var-set-after  declare -A var                     1  1  1  0   If  var  is unset  then it even fails the  declare -p var  test  is-var-set-after  unset var                          1  1  1  1   Test output  The test mnemonics in the header row correspond to   -n   var       -n    var x            var         0    and declare -p var  respectively                            test  -n  x    -p                       var foo   0  0  0  0                     var  foo    0  0  0  0                 var   0  foo    0  0  0  0                 var   x  foo    0  0  0  0         var   y  bar  x  foo    0  0  0  0                        var      1  0  0  0                  var   0        1  0  0  0                  var   1        1  1  0  0                 var   1  foo    1  1  0  0 declare -A var  var   x  foo    1  1  0  0                        var      1  1  1  0                declare -a var   1  1  1  0                declare -A var   1  1  1  0                     unset var   1  1  1  1   Summary        declare -p var  amp  gt  dev null is  100    reliable for testing named variables in Bash since at least 3 0      -n    var x     is reliable in POSIX compliant situations  but cannot handle arrays    Other tests exist for checking if a variable is nonempty  and for checking for declared variables in other shells  But these tests are suited for neither Bash nor POSIX scripts

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Read the  Parameter Expansion  section of the bash man page  Parameter expansion doesn t provide a general test for a variable being set  but there are several things you can do to a parameter if it isn t set   For example   function a       first arg   1-foo        rest of the function     will set first arg equal to  1 if it is assigned  otherwise it uses the value  foo   If a absolutely must take a single parameter  and no good default exists  you can exit with an error message when no parameter is given   function a           1 a must take a single argument        rest of the function      Note the use of   as a null command  which just expands the values of its arguments  We don t want to do anything with  1 in this example  just exit if it isn t set

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if       xx          then echo xx is defined  fi

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To check for non-null non-zero string variable  i e  if set  use  if   -n   1      It s the opposite of -z   I find myself using -n more than -z   You would use it like    if   -n   1     then   echo  You supplied the first parameter   else   echo  First parameter not supplied   fi

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I always find the POSIX table in the other answer slow to grok  so here s my take on it      ---------------------- ------------ ----------------------- -----------------------         if VARIABLE is          set               empty                  unset                ---------------------- ------------ ----------------------- -----------------------   -      VARIABLE-default     VARIABLE              quot  quot                     quot default quot                  VARIABLE default     VARIABLE              quot  quot                VARIABLE  quot default quot             VARIABLE default     VARIABLE              quot  quot                    exit 127                  VARIABLE default     quot default quot            quot default quot                    quot  quot                  ---------------------- ------------ ----------------------- -----------------------   -     VARIABLE -default     VARIABLE           quot default quot                 quot default quot                 VARIABLE  default     VARIABLE      VARIABLE  quot default quot       VARIABLE  quot default quot            VARIABLE  default     VARIABLE          exit 127                exit 127                 VARIABLE  default     quot default quot               quot  quot                        quot  quot                  ---------------------- ------------ ----------------------- -----------------------   Note that each group  with and without preceding colon  has the same set and unset cases  so the only thing that differs is how the empty cases are handled  With the preceding colon  the empty and unset cases are identical  so I would use those where possible  i e  use     not just    because the empty case is inconsistent   Headings   set means VARIABLE is non-empty  VARIABLE  quot something quot   empty means VARIABLE is empty null  VARIABLE  quot  quot   unset means VARIABLE does not exist  unset VARIABLE   Values    VARIABLE means the result is the original value of the variable   quot default quot  means the result was the replacement string provided   quot  quot  means the result is null  an empty string   exit 127 means the script stops executing with exit code 127    VARIABLE  quot default quot   means the result is  quot default quot  and that VARIABLE  previously empty or unset  will also be set equal to  quot default quot

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I found a  much  better code to do this if you want to check for anything in       if     1         then   echo   1 is blank  else   echo   1 is filled up  fi   Why this all  Everything in    exists in Bash  but by default it s blank  so test -z and test -n couldn t help you   Update  You can also count number of characters in a parameters    if      1    0   then   echo   1 is blank  else   echo   1 is filled up  fi

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After skimming all the answers  this also works   if    -z  SOME VAR     then read -p  Enter a value for SOME VAR    SOME VAR  fi echo  SOME VAR  SOME VAR    if you don t put SOME VAR instead of what I have  SOME VAR  it will set it to an empty value    is necessary for this to work

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I like auxiliary functions to hide the crude details of bash  In this case  doing so adds even more  hidden  crudeness     The first   negates the result  can t use -n to achieve this    the second   expands the content of varname  can t do    varname   function IsDeclared Tricky     local varname   1        -z    varname x        Because I first had bugs in this implementation  inspired by the answers of Jens and Lionel   I came up with a different solution     Ask for the properties of the variable - fails if not declared function IsDeclared       declare -p  1  amp  gt  dev null     I find it to be more straight-forward  more bashy and easier to understand remember  Test case shows it is equivalent   function main       declare -i xyz   local foo   local bar    local baz       IsDeclared Tricky xyz  echo  IsDeclared Tricky xyz        IsDeclared Tricky foo  echo  IsDeclared Tricky foo        IsDeclared Tricky bar  echo  IsDeclared Tricky bar        IsDeclared Tricky baz  echo  IsDeclared Tricky baz         IsDeclared xyz  echo  IsDeclared xyz        IsDeclared foo  echo  IsDeclared foo        IsDeclared bar  echo  IsDeclared bar        IsDeclared baz  echo  IsDeclared baz         main   The test case also shows that local var does NOT declare var  unless followed by       For quite some time I thought i declared variables this way  just to discover now that i merely expressed my intention    It s a no-op  i guess      IsDeclared Tricky xyz  1   IsDeclared Tricky foo  1   IsDeclared Tricky bar  0   IsDeclared Tricky baz  0   IsDeclared xyz  1   IsDeclared foo  1   IsDeclared bar  0   IsDeclared baz  0     BONUS  usecase  I mostly use this test to give  and return  parameters to functions in a somewhat  elegant  and safe way  almost resembling an interface        auxiliary functions function die       echo  Error   1   exit 1    function assertVariableDeclared       IsDeclared   1     die  variable not declared   1     function expectVariables       while        gt  0     do     assertVariableDeclared  1  shift   done      actual example function exampleFunction       expectVariables inputStr outputStr   outputStr   inputStr world      function bonus       local inputStr  Hello    local outputStr    remove this to trigger error   exampleFunction   echo  outputStr    bonus   If called with all requires variables declared      Hello world    else      Error  variable not declared  outputStr

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If you wish to test that a variable is bound or unbound  this works well  even after you ve turned on the nounset option   set -o noun set  if printenv variableName  gt  dev null  then       variable is bound to a value else       variable is unbound fi

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You want to exit if it s unset  This worked for me  I wanted my script to exit with an error message if a parameter wasn t set      usr bin env bash  set -o errexit    Get the value and empty validation check all in one VER    1  You must pass a version of the format 0 0 0 as the only argument     This returns with an error when it s run  peek peek      setver sh   setver sh  line 13  1  You must pass a version of the format 0 0 0 as the only argument   Check only  no exit - Empty and Unset are INVALID  Try this option if you just want to check if the value set VALID or unset empty INVALID   TSET  good val  TEMPTY    unset TUNSET  if      TSET -      then echo  VALID   else echo  INVALID  fi   VALID if      TEMPTY -      then echo  VALID   else echo  INVALID  fi   INVALID if      TUNSET -      then echo  VALID   else echo  INVALID  fi   INVALID   Or  Even short tests  -        TSET -        amp  amp  echo  VALID     echo  INVALID       TEMPTY -      amp  amp  echo  VALID     echo  INVALID       TUNSET -      amp  amp  echo  VALID     echo  INVALID    Check only  no exit - Only empty is INVALID  And this is the answer to the question  Use this if you just want to check if the value set empty VALID or unset INVALID   NOTE  the  1  in    -1   is insignificant  it can be anything  like x   TSET  good val  TEMPTY    unset TUNSET  if      TSET 1      then echo  VALID   else echo  INVALID  fi   VALID if      TEMPTY 1      then echo  VALID   else echo  INVALID  fi   VALID if      TUNSET 1      then echo  VALID   else echo  INVALID  fi   INVALID   Short tests       TSET 1        amp  amp  echo  VALID     echo  INVALID       TEMPTY 1      amp  amp  echo  VALID     echo  INVALID       TUNSET 1      amp  amp  echo  VALID     echo  INVALID    I dedicate this answer to  mklement0  comments  who challenged me to answer the question accurately   Reference http   pubs opengroup org onlinepubs 9699919799 utilities V3 chap02 html tag 18 06 02

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While most of the techniques stated here are correct  bash 4 2 supports an actual test for the presence of a variable  man bash   rather than testing the value of the variable       -v foo     echo      1  foo bar    -v foo     echo      0  foo       -v foo     echo      0   Notably  this approach will not cause an error when used to check for an unset variable in set -u   set -o nounset mode  unlike many other approaches  such as using   -z

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To check if a var is set or not  var         var     amp  amp  echo  set     echo  not set

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Declare a simple function is set which uses declare -p to test directly if the variable exists    is set         declare -p  1  gt  dev null 2 gt  amp 1      is set foo  echo    0    declare foo    is set foo  echo    1

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To clearly answer OP s question of how to determine whether a variable is set   Lionel s answer is correct  if test  quot   name x  quot   then     echo  name is set  else     echo  name is not set  fi  This question already has a lot of answers  but none of them offered bona fide boolean expressions to clearly differentiate between variables values  Here are some unambiguous expressions that I worked out   ----------------------- ------------- --------- ------------    Expression in script    name  fish    name      unset name    ----------------------- ------------- --------- ------------    test  quot  name quot             TRUE          f         f              test -n  quot  name quot          TRUE          f         f              test   -z  quot  name quot        TRUE          f         f              test    quot   name-x  quot       f             TRUE      f              test    quot   name x  quot       f             f         TRUE          ----------------------- ------------- --------- ------------   By the way  these expressions are equivalent  test  lt expression gt   lt   gt     lt expression gt    Other ambiguous expressions to be used with caution   ---------------------- ------------- --------- ------------    Expression in script   name  fish    name      unset name    ---------------------- ------------- --------- ------------    test  quot   name x  quot        TRUE          TRUE      f              test  quot   name-x  quot        TRUE          f         TRUE           test -z  quot  name quot         f             TRUE      TRUE           test    quot  name quot          f             TRUE      TRUE           test   -n  quot  name quot       f             TRUE      TRUE           test  quot  name quot            f             TRUE      TRUE          ---------------------- ------------- --------- ------------

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This is what I use every day       Check if a variable is set     param1  name of the variable   function is set            -n    1       amp  amp  test -n    eval  echo       1  x          This works well under Linux and Solaris down to bash 3 0   bash-3 00  myvar  TEST  bash-3 00  is set myvar   echo    0 bash-3 00  mavar    bash-3 00  is set myvar   echo    0 bash-3 00  unset myvar bash-3 00  is set myvar   echo    1

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My preferred way is this     var 10   if     var false  then echo  is set  else echo  NOT set  fi is set   unset -v var   if     var false  then echo  is set  else echo  NOT set  fi NOT set   So basically  if a variable is set  it becomes  a negation of the resulting false   what will be true    is set     And  if it is unset  it will become  a negation of the resulting true   as the empty result evaluates to true   so will end as being false    NOT set

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I always use this one  based on the fact that it seems easy to be understood by anybody who sees the code for the very first time   if     variable             then     echo  Variable X is empty  fi   And  if wanting to check if not empty   if       variable             then     echo  Variable X is not empty  fi   That s it

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foo      Or        foo       Or  let    foo    Or  declare -p foo

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In a shell you can use the -z operator which is True if the length of string is zero   A simple one-liner to set default MY VAR if it s not set  otherwise optionally you can display the message      -z   MY VAR      amp  amp  MY VAR  default     -z   MY VAR      amp  amp  MY VAR  default     echo  Variable already set

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case   1  in      echo  blank        echo  set  esac

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In bash you can use -v inside the       builtin       bin bash -u  if      -v SOMEVAR     then     SOMEVAR  hello  fi  echo  SOMEVAR

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Functions to check if variable is declared unset  including empty  array      The following functions test if the given name exists as a variable    The first parameter needs to be the name of the variable to be checked     See example below   var is declared              -n    1 anything        declare -p  1  amp  gt  dev null      var is unset              -z    1 anything      amp  amp    declare -p  1  amp  gt  dev null         By first testing if the variable is  un set  the call to declare can be avoided  if not necessary  If however  1 contains the name of an empty  array     the call to declare would make sure we get the right result There s never much data passed to  dev null as declare is only called if either the variable is unset or an empty array    This functions would test as showed in the following conditions    a          is not declared a          is declared a  foo     is declared a          is declared a          is declared unset a    is not declared  a          is unset a          is not unset a  foo     is not unset a          is not unset a          is not unset unset a    is unset          For more details      and a test script see my answer to the question  How do I check if a variable exists in bash            Remark  The similar usage of declare -p  as it is also shown by Peregring-lk s answer  is truly coincidental  Otherwise I would of course have credited it

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To test if a variable var is set      var x     To test if a variable is set by name       name x     To test if a positional parameter is set      N x     where N is actually an integer  This answer is almost similar to Lionel   s but explore a more minimalist take by omitting the -z  To test if a named variable is set    function is set       local v  1     echo -n  quot   v  quot      if      v x     then         echo  quot        v   quot      else         echo  quot  is unset quot      fi    To test if a positional parameter is set    function a       if     1 x     then         local arg  1         echo  quot a    arg   quot      else         echo  quot a  arg is unset quot      fi    Testing shows that extra care with white spaces and valid test expressions is not needed  set -eu  V1 a V2  V4 -gt V5  quot 1 -gt 2 quot  V6  quot   -z 1 quot  V7    exit 1    is set V1 is set V2 is set V3 is set V4 is set V5 is set V6 is set V7  a 1 a a  quot 1 -gt 2 quot  a 1 -gt 2     test sh  V1    a  V2      V3 is unset V4    -gt  V5    1 -gt 2  V6      -z 1  V7      exit 1   a  1  a  arg is unset a  1 -gt 2  a  1   Finally  notice the set -eu which protects us from common errors  such as typos in variable names  I recommand its usage  but this implies that the difference between an unset variable and a variable set with an empty string is handled correctly

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Here s how to test whether a parameter is unset  or empty   Null   or set with a value    -------------------- ---------------------- ----------------- -----------------      Expression               parameter            parameter        parameter          in script            Set and Not Null       Set But Null         Unset         -------------------- ---------------------- ----------------- -----------------      parameter -word    substitute  parameter   substitute  word   substitute  word       parameter-word     substitute  parameter   substitute null   substitute  word       parameter  word    substitute  parameter   assign  word       assign  word           parameter word     substitute  parameter   substitute null   assign  word           parameter  word    substitute  parameter   error  exit       error  exit           parameter word     substitute  parameter   substitute null   error  exit           parameter  word    substitute  word        substitute null   substitute null       parameter word     substitute  word        substitute  word   substitute null    -------------------- ---------------------- ----------------- -----------------    Source  POSIX  Parameter Expansion      In all cases shown with  substitute   the expression is replaced with the value shown  In all cases shown with  assign   parameter is assigned that value  which also replaces the expression    To show this in action    -------------------- ---------------------- ----------------- -----------------      Expression          FOO  world                FOO              unset FOO          in script            Set and Not Null       Set But Null         Unset         -------------------- ---------------------- ----------------- -----------------      FOO -hello         world                  hello             hello                 FOO-hello          world                                    hello                 FOO  hello         world                  FOO hello         FOO hello             FOO hello          world                                    FOO hello             FOO  hello         world                  error  exit       error  exit           FOO hello          world                                    error  exit           FOO  hello         hello                                                          FOO hello          hello                  hello                                -------------------- ---------------------- ----------------- -----------------

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To see if a variable is nonempty  I use  if     var     then               var  expands to a nonempty string   The opposite tests if a variable is either unset or empty   if       var     then             var  expands to the empty string  set or not    To see if a variable is set  empty or nonempty   I use  if      var x      then          var  exists  empty or nonempty  if      1 x      then           Parameter 1 exists  empty or nonempty    The opposite tests if a variable is unset   if        var x      then        var  is not set at all if        1 x      then         We were called with no arguments

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if      1  isset     then echo  It was set and not null    gt  amp 2 else echo  It was not set or it was null    gt  amp 2 fi  if      1 isset     then echo  It was set but might be null    gt  amp 2 else echo  It was was not set    gt  amp 2 fi

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To check whether a variable is set with a non-empty value  use   -n   x     as others have already indicated   Most of the time  it s a good idea to treat a variable that has an empty value in the same way as a variable that is unset  But you can distinguish the two if you need to    -n    x set         x set   expands to set if x is set and to the empty string if x is unset    To check whether a parameter has been passed  test     which is the number of parameters passed to the function  or to the script  when not in a function   see Paul s answer

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Using    -z   var     is the easiest way to know if a variable was set or not  but that option -z doesn t distinguish between an unset variable and a variable set to an empty string     set         -z   set      amp  amp  echo  Set     echo  Unset   Unset      -z   unset      amp  amp  echo  Set     echo  Unset  Unset   It s best to check it according to the type of variable  env variable  parameter or regular variable    For a env variable        env   grep  varname     wc -l  -eq 1     amp  amp  echo  Set     echo  Unset    For a parameter  for example  to check existence of parameter  5          -ge 5     amp  amp  echo  Set     echo  Unset    For a regular variable  using an auxiliary function  to do it in an elegant way    function declare var      declare -p   1   amp  gt   dev null   declare var  var name   amp  amp  echo  Set     echo  Unset    Notes             gives you the number of positional parameters    declare -p  gives you the definition of the variable passed as a parameter  If it exists  returns 0  if not  returns 1 and prints an error message     amp  gt   dev null  suppresses output from declare -p without affecting its return code

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if     1           then   echo   1 is set else   echo   1 is not set fi   Although for arguments it is normally best to test     which is the number of arguments  in my opinion   if      -gt 0    then   echo   1 is set else   echo   1 is not set fi

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On a modern version of Bash  4 2 or later I think  I don t know for sure   I would try this   if     -v SOMEVARIABLE    note the lack of a   sigil then     echo  Variable is unset  elif   -z   SOMEVARIABLE    then     echo  Variable is set to an empty string  else     echo  Variable is set to some string  fi

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The answers above do not work when Bash option set -u is enabled   Also  they are not dynamic  e g   how to test is variable with name  dummy  is defined   Try this   is var defined         if      -ne 1       then         echo  Expected exactly one argument  variable name as string  e g    my var           exit 1     fi       Tricky   Since Bash option  set -u  may be enabled  we cannot directly test if a variable       is defined with this construct      -z   var      Instead  we must use default value       substitution with this construct      -z    var -       Normally  a default value follows the       operator   -   but here we leave it blank for empty  null  string   Finally  we need to       substitute the text from  1 as  var    This is not allowed directly in Bash with this       construct      -z     1 -       We need to use indirection with eval operator        Example   1  var        Expansion for eval operator       -z     1 -     - gt       -z    var -           Code  execute      -z   var -        eval      -z     1 -         return       Pedantic      Related  In Bash  how do I test if a variable is defined in  quot -u quot  mode

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There are many ways to do this with the following being one of them   if   -z   1      This succeeds if  1 is null or unset

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For those that are looking to check for unset or empty when in a script with  set -u   if   -z    var-      then    echo  Must provide var environment variable  Exiting         exit 1 fi   The regular   -z   var    check will fail with var  unbound variable if set -u but   -z    var-     expands to empty string if var is unset without failing

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Summary  Use test -n  quot   var-  quot  to check if the variable is not empty  and hence must be defined set too   Usage  if test -n  quot   name-  quot   then   echo  quot name is set to  name quot  else   echo  quot name is not set or empty quot  fi   Use test   -z  quot   var   quot  to check if the variable is defined set  even if it s empty   Usage  if test   -z  quot   var   quot   then   echo  quot name is set to  name quot  else   echo  quot name is not set quot  fi     Note that the first use case is much more common in shell scripts and this is what you will usually want to use  Notes  This solution should work in all POSIX shells  sh  bash  zsh  ksh  dash  Some of the other answers for this question are correct but may be confusing for people unexperienced in shell scripting  so I wanted to provide a TLDR answer that will be least confusing for such people   Explanation To understand how this solution works  you need to understand the POSIX test command and POSIX shell parameter expansion  spec   so let s cover the absolute basics needed to understand the answer  The test command evaluates an expression and returns true or false  via its exit status   The operator -n returns true if the operand is a non-empty string  So for example  test -n  quot a quot  returns true  while test -n  quot  quot  returns false  Now  to check if a variable is not empty  which means it must be defined   you could use test -n  quot  var quot   However  some shell scripts have an option set  set -u  that causes any reference to undefined variables to emit an error  so if the variable var is not defined  the expression  a will cause an error  To handle this case correctly  you must use variable expansion  which will tell the shell to replace the variable with an alternative string if it s not defined  avoiding the aforementioned error  The variable expansion   var-  means  if the variable var is undefined  also called  quot unset quot    replace it with an empty string  So test -n  quot   var-  quot  will return true if  var is not empty  which is almost always what you want to check in shell scripts  The reverse check  if  var is undefined or not empty  would be test -z  quot   var-  quot   Now to the second use case  checking if the variable var is defined  whether empty or not  This is a less common use case and slightly more complex  and I would advise you to read Lionels s great answer to better understand it

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Usually  The right way  if   -z   var x     then echo  var is unset   else echo  var is set to   var    fi   where   var x  is a parameter expansion which evaluates to nothing if var is unset  and substitutes the string x otherwise   Quotes Digression  Quotes can be omitted  so we can say   var x  instead of    var x    because this syntax  amp  usage guarantees this will only expand to something that does not require quotes  since it either expands to x  which contains no word breaks so it needs no quotes   or to nothing  which results in   -z     which conveniently evaluates to the same value  true  that   -z      does as well     However  while quotes can be safely omitted  and it was not immediately obvious to all  it wasn t even apparent to the first author of this quotes explanation who is also a major Bash coder   it would sometimes be better to write the solution with quotes as   -z    var x      at the very small possible cost of an O 1  speed penalty   The first author also added this as a comment next to the code using this solution giving the URL to this answer  which now also includes the explanation for why the quotes can be safely omitted    Often  The wrong way  if   -z   var     then echo  var is blank   else echo  var is set to   var    fi   This is often wrong because it doesn t distinguish between a variable that is unset and a variable that is set to the empty string  That is to say  if var     then the above solution will output  var is blank     The distinction between unset and  set to the empty string  is essential in situations where the user has to specify an extension  or additional list of properties  and that not specifying them defaults to a non-empty value  whereas specifying the empty string should make the script use an empty extension or list of additional properties   The distinction may not be essential in every scenario though  In those cases    -z   var    will be just fine

User · Answer

You can do   function a           if     -z   1     then                 echo   1 is set          fi

[bash] How to check if a variable is set in Bash?

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