How to tell if a string is not defined in a Bash shell script

Question

If I want to check for the null string I would do    -z  mystr     but what if I want to check whether the variable has been defined at all  Or is there no distinction in Bash scripting

User · Accepted Answer

I think the answer you are after is implied (if not stated) by Vinko's answer, though it is not spelled out simply. To distinguish whether VAR is set but empty or not set, you can use:

if [ -z "${VAR+xxx}" ]; then echo VAR is not set at all; fi
if [ -z "$VAR" ] && [ "${VAR+xxx}" = "xxx" ]; then echo VAR is set but empty; fi

You probably can combine the two tests on the second line into one with:

if [ -z "$VAR" -a "${VAR+xxx}" = "xxx" ]; then echo VAR is set but empty; fi

However, if you read the documentation for Autoconf, you'll find that they do not recommend combining terms with '-a' and do recommend using separate simple tests combined with &&. I've not encountered a system where there is a problem; that doesn't mean they didn't used to exist (but they are probably extremely rare these days, even if they weren't as rare in the distant past).

You can find the details of these, and other related shell parameter expansions, the test or [ command and conditional expressions in the Bash manual.

I was recently asked by email about this answer with the question:

You use two tests, and I understand the second one well, but not the first one. More precisely I don't understand the need for variable expansion
if [ -z "${VAR+xxx}" ]; then echo VAR is not set at all; fi
Wouldn't this accomplish the same?
if [ -z "${VAR}" ]; then echo VAR is not set at all; fi

Fair question - the answer is 'No, your simpler alternative does not do the same thing'.

Suppose I write this before your test:

VAR=

Your test will say "VAR is not set at all", but mine will say (by implication because it echoes nothing) "VAR is set but its value might be empty". Try this script:

(
unset VAR
if [ -z "${VAR+xxx}" ]; then echo JL:1 VAR is not set at all; fi
if [ -z "${VAR}" ];     then echo MP:1 VAR is not set at all; fi
VAR=
if [ -z "${VAR+xxx}" ]; then echo JL:2 VAR is not set at all; fi
if [ -z "${VAR}" ];     then echo MP:2 VAR is not set at all; fi
)

The output is:

JL:1 VAR is not set at all
MP:1 VAR is not set at all
MP:2 VAR is not set at all

In the second pair of tests, the variable is set, but it is set to the empty value. This is the distinction that the ${VAR=value} and ${VAR:=value} notations make. Ditto for ${VAR-value} and ${VAR:-value}, and ${VAR+value} and ${VAR:+value}, and so on.

As Gili points out in his answer, if you run bash with the set -o nounset option, then the basic answer above fails with unbound variable. It is easily remedied:

if [ -z "${VAR+xxx}" ]; then echo VAR is not set at all; fi
if [ -z "${VAR-}" ] && [ "${VAR+xxx}" = "xxx" ]; then echo VAR is set but empty; fi

Or you could cancel the set -o nounset option with set +u (set -u being equivalent to set -o nounset).

User · Answer

https   stackoverflow com a 9824943 14731 contains a better answer  one that is more readable and works with set -o nounset enabled   It works roughly like this   if   -n    VAR-      then     echo  VAR is set and is not empty  elif      VAR DEFINED BUT EMPTY      DEFINED BUT EMPTY     then     echo  VAR is set  but empty  else     echo  VAR is not set  fi

User · Answer

not to shed this bike even further  but wanted to add  shopt -s -o nounset   is something you could add to the top of a script  which will error if variables aren t declared anywhere in the script  The message you d see is unbound variable  but as others mention it won t catch an empty string or null value  To make sure any individual value isn t empty  we can test a variable as it s expanded with   mystr     also known as dollar sign expansion  which would error with parameter null or not set

User · Answer

https   stackoverflow com a 9824943 14731 contains a better answer  one that is more readable and works with set -o nounset enabled   It works roughly like this   if   -n    VAR-      then     echo  VAR is set and is not empty  elif      VAR DEFINED BUT EMPTY      DEFINED BUT EMPTY     then     echo  VAR is set  but empty  else     echo  VAR is not set  fi

User · Answer

call set without any arguments   it outputs all the vars defined   the last ones on the list would be the ones defined in your script   so you could pipe its output to something that could figure out what things are defined and whats not

User · Answer

A summary of tests     -n   var     amp  amp  echo  var is set and not empty    -z   var     amp  amp  echo  var is unset or empty       var x      x     amp  amp  echo  var is set     may or may not be empty   -n    var x      amp  amp  echo  var is set     may or may not be empty   -z    var x      amp  amp  echo  var is unset    -z    var-x      amp  amp  echo  var is set and empty

User · Answer

The explicit way to check for a variable being defined would be     -v mystr

User · Answer

gt  if   -z  FOO    then echo  EMPTY   fi EMPTY   gt  FOO      gt  if   -z  FOO    then echo  EMPTY   fi EMPTY   gt  FOO  a    gt  if   -z  FOO    then echo  EMPTY   fi   gt     -z works for undefined variables too  To distinguish between an undefined and a defined you d use the things listed here or  with clearer explanations  here   Cleanest way is using expansion like in these examples  To get all your options check the Parameter Expansion section of the manual   Alternate word      unset FOO    if test   FOO defined   then echo  DEFINED   fi    FOO       if test   FOO defined   then echo  DEFINED   fi DEFINED   Default value      FOO       if test    FOO-default value     then echo  UNDEFINED   fi    unset FOO    if test    FOO-default value     then echo  UNDEFINED   fi UNDEFINED   Of course you d use one of these differently  putting the value you want instead of  default value  and using the expansion directly  if appropriate

User · Answer

gt  if   -z  FOO    then echo  EMPTY   fi EMPTY   gt  FOO      gt  if   -z  FOO    then echo  EMPTY   fi EMPTY   gt  FOO  a    gt  if   -z  FOO    then echo  EMPTY   fi   gt     -z works for undefined variables too  To distinguish between an undefined and a defined you d use the things listed here or  with clearer explanations  here   Cleanest way is using expansion like in these examples  To get all your options check the Parameter Expansion section of the manual   Alternate word      unset FOO    if test   FOO defined   then echo  DEFINED   fi    FOO       if test   FOO defined   then echo  DEFINED   fi DEFINED   Default value      FOO       if test    FOO-default value     then echo  UNDEFINED   fi    unset FOO    if test    FOO-default value     then echo  UNDEFINED   fi UNDEFINED   Of course you d use one of these differently  putting the value you want instead of  default value  and using the expansion directly  if appropriate

User · Answer

The Bash Reference Manual is an authoritative source of information about bash   Here s an example of testing a variable to see if it exists   if   -z   PS1     then         echo This shell is not interactive else         echo This shell is interactive fi    From section 6 3 2    Note that the whitespace after the open   and before the   is not optional     Tips for Vim users  I had a script that had several declarations as follows   export VARIABLE NAME   SOME OTHER VARIABLE path-part    But I wanted them to defer to any existing values  So I re-wrote them to look like this   if   -z   VARIABLE NAME     then         export VARIABLE NAME   SOME OTHER VARIABLE path-part  fi   I was able to automate this in vim using a quick regex   s  vexport   A-Z                n if   -z    1     then r  export  1  2 rfi r gc   This can be applied by selecting the relevant lines visually  then typing    The command bar pre-populates with    lt    gt   Paste the above command and hit enter     Tested on this version of Vim   VIM - Vi IMproved 7 3  2010 Aug 15  compiled Aug 22 2015 15 38 58  Compiled by root apple com   Windows users may want different line endings

User · Answer

call set without any arguments   it outputs all the vars defined   the last ones on the list would be the ones defined in your script   so you could pipe its output to something that could figure out what things are defined and whats not

User · Answer

The explicit way to check for a variable being defined would be     -v mystr

User · Answer

call set without any arguments   it outputs all the vars defined   the last ones on the list would be the ones defined in your script   so you could pipe its output to something that could figure out what things are defined and whats not

User · Answer

A shorter version to test undefined variable can simply be   test -z   mystr   amp  amp  echo  mystr is not defined

User · Answer

The Bash Reference Manual is an authoritative source of information about bash   Here s an example of testing a variable to see if it exists   if   -z   PS1     then         echo This shell is not interactive else         echo This shell is interactive fi    From section 6 3 2    Note that the whitespace after the open   and before the   is not optional     Tips for Vim users  I had a script that had several declarations as follows   export VARIABLE NAME   SOME OTHER VARIABLE path-part    But I wanted them to defer to any existing values  So I re-wrote them to look like this   if   -z   VARIABLE NAME     then         export VARIABLE NAME   SOME OTHER VARIABLE path-part  fi   I was able to automate this in vim using a quick regex   s  vexport   A-Z                n if   -z    1     then r  export  1  2 rfi r gc   This can be applied by selecting the relevant lines visually  then typing    The command bar pre-populates with    lt    gt   Paste the above command and hit enter     Tested on this version of Vim   VIM - Vi IMproved 7 3  2010 Aug 15  compiled Aug 22 2015 15 38 58  Compiled by root apple com   Windows users may want different line endings

User · Answer

another option  the  list array indices  expansion     unset foo   foo    echo    foo     0   foo bar   echo    foo     0   foo  bar baz    echo    foo     0 1   the only time this expands to the empty string is when foo is unset  so you can check it with the string conditional     unset foo         foo         echo    1   foo          foo         echo    0   foo bar         foo         echo    0   foo  bar baz          foo         echo    0   should be available in any bash version    3 0

User · Answer

Advanced bash scripting guide  10 2  Parameter Substitution      var blahblah   if var is defined   blahblah  is substituted for the expression  else null is substituted   var-blahblah   if var is defined  it is itself substituted  else  blahblah  is substituted   var blahblah   if var is defined  it is substituted  else the function exists with  blahblah  as an error message      to base your program logic on whether the variable  mystr is defined or not  you can do the following   isdefined 0   mystr  export isdefined 1    now  if isdefined 0 then the variable was undefined  if isdefined 1 the variable was defined  This way of checking variables is better than the above answer because it is more elegant  readable  and if your bash shell was configured to error on the use of undefined variables  set -u   the script will terminate prematurely     Other useful stuff   to have a default value of 7 assigned to  mystr if it was undefined  and leave it intact otherwise   mystr   mystr- 7    to print an error message and exit the function if the variable is undefined       mystr  not defined    Beware here that I used     so as not to have the contents of  mystr executed as a command in case it is defined

User · Answer

call set without any arguments   it outputs all the vars defined   the last ones on the list would be the ones defined in your script   so you could pipe its output to something that could figure out what things are defined and whats not

User · Answer

gt  if   -z  FOO    then echo  EMPTY   fi EMPTY   gt  FOO      gt  if   -z  FOO    then echo  EMPTY   fi EMPTY   gt  FOO  a    gt  if   -z  FOO    then echo  EMPTY   fi   gt     -z works for undefined variables too  To distinguish between an undefined and a defined you d use the things listed here or  with clearer explanations  here   Cleanest way is using expansion like in these examples  To get all your options check the Parameter Expansion section of the manual   Alternate word      unset FOO    if test   FOO defined   then echo  DEFINED   fi    FOO       if test   FOO defined   then echo  DEFINED   fi DEFINED   Default value      FOO       if test    FOO-default value     then echo  UNDEFINED   fi    unset FOO    if test    FOO-default value     then echo  UNDEFINED   fi UNDEFINED   Of course you d use one of these differently  putting the value you want instead of  default value  and using the expansion directly  if appropriate

User · Answer

not to shed this bike even further  but wanted to add  shopt -s -o nounset   is something you could add to the top of a script  which will error if variables aren t declared anywhere in the script  The message you d see is unbound variable  but as others mention it won t catch an empty string or null value  To make sure any individual value isn t empty  we can test a variable as it s expanded with   mystr     also known as dollar sign expansion  which would error with parameter null or not set

User · Answer

Here is what I think is a much clearer way to check if a variable is defined   var defined         local var name  1     set   grep     var name    1 gt  dev null     return        Use it as follows   if var defined foo  then     echo  foo is defined  else     echo  foo is not defined  fi

User · Answer

another option  the  list array indices  expansion     unset foo   foo    echo    foo     0   foo bar   echo    foo     0   foo  bar baz    echo    foo     0 1   the only time this expands to the empty string is when foo is unset  so you can check it with the string conditional     unset foo         foo         echo    1   foo          foo         echo    0   foo bar         foo         echo    0   foo  bar baz          foo         echo    0   should be available in any bash version    3 0

User · Answer

Here is what I think is a much clearer way to check if a variable is defined   var defined         local var name  1     set   grep     var name    1 gt  dev null     return        Use it as follows   if var defined foo  then     echo  foo is defined  else     echo  foo is not defined  fi

User · Answer

Advanced bash scripting guide  10 2  Parameter Substitution      var blahblah   if var is defined   blahblah  is substituted for the expression  else null is substituted   var-blahblah   if var is defined  it is itself substituted  else  blahblah  is substituted   var blahblah   if var is defined  it is substituted  else the function exists with  blahblah  as an error message      to base your program logic on whether the variable  mystr is defined or not  you can do the following   isdefined 0   mystr  export isdefined 1    now  if isdefined 0 then the variable was undefined  if isdefined 1 the variable was defined  This way of checking variables is better than the above answer because it is more elegant  readable  and if your bash shell was configured to error on the use of undefined variables  set -u   the script will terminate prematurely     Other useful stuff   to have a default value of 7 assigned to  mystr if it was undefined  and leave it intact otherwise   mystr   mystr- 7    to print an error message and exit the function if the variable is undefined       mystr  not defined    Beware here that I used     so as not to have the contents of  mystr executed as a command in case it is defined

User · Answer

A shorter version to test undefined variable can simply be   test -z   mystr   amp  amp  echo  mystr is not defined

User · Answer

gt  if   -z  FOO    then echo  EMPTY   fi EMPTY   gt  FOO      gt  if   -z  FOO    then echo  EMPTY   fi EMPTY   gt  FOO  a    gt  if   -z  FOO    then echo  EMPTY   fi   gt     -z works for undefined variables too  To distinguish between an undefined and a defined you d use the things listed here or  with clearer explanations  here   Cleanest way is using expansion like in these examples  To get all your options check the Parameter Expansion section of the manual   Alternate word      unset FOO    if test   FOO defined   then echo  DEFINED   fi    FOO       if test   FOO defined   then echo  DEFINED   fi DEFINED   Default value      FOO       if test    FOO-default value     then echo  UNDEFINED   fi    unset FOO    if test    FOO-default value     then echo  UNDEFINED   fi UNDEFINED   Of course you d use one of these differently  putting the value you want instead of  default value  and using the expansion directly  if appropriate

User · Answer

A summary of tests     -n   var     amp  amp  echo  var is set and not empty    -z   var     amp  amp  echo  var is unset or empty       var x      x     amp  amp  echo  var is set     may or may not be empty   -n    var x      amp  amp  echo  var is set     may or may not be empty   -z    var x      amp  amp  echo  var is unset    -z    var-x      amp  amp  echo  var is set and empty

[bash] How to tell if a string is not defined in a Bash shell script

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