How to check if an environment variable exists and get its value

Question

I am writing a shell script  In this shell script  I am have a variable that either takes a default value  or the value of an environment variable  However  the environment variable doesn t have to be present   For instance  assume  before running the script  I perform the following operation   export DEPLOY ENV dev   How do I tell the script to search for this environment variable  and store its value in a variable inside the script  Moreover  how do I tell the script that if this environment variable does not exist  store a default variable

User · Answer

There is no difference between environment variables and variables in a script. Environment variables are just defined earlier, outside the script, before the script is called. From the script's point of view, a variable is a variable.

You can check if a variable is defined:

if [ -z "$a" ]
then
    echo "not defined"
else 
    echo "defined"
fi

and then set a default value for undefined variables or do something else.

The -z checks for a zero-length (i.e. empty) string. See man bash and look for the CONDITIONAL EXPRESSIONS section.

You can also use set -u at the beginning of your script to make it fail once it encounters an undefined variable, if you want to avoid having an undefined variable breaking things in creative ways.

User · Answer

All the answers worked  However  I had to add the variables that I needed to get to the sudoers files as follows   sudo visudo Defaults env keep      lt var1 gt    lt var2 gt         lt varn gt

User · Answer

You could just use parameter expansion        parameter -word       If parameter is unset or null  the expansion of word is substituted  Otherwise  the value of parameter is substituted    So try this   var   DEPLOY ENV -default value    There s also the   parameter-word  form  which substitutes the default value only when parameter is unset  but not when it s null    To demonstrate the difference between the two     unset DEPLOY ENV   echo     DEPLOY ENV -default value      DEPLOY ENV-default value     default value   default value    DEPLOY ENV    echo     DEPLOY ENV -default value      DEPLOY ENV-default value     default value

User · Answer

-z    DEPLOY ENV     checks whether DEPLOY ENV has length equal to zero  So you could run   if    -z    DEPLOY ENV       then   MY SCRIPT VARIABLE  Some default value because DEPLOY ENV is undefined  else   MY SCRIPT VARIABLE    DEPLOY ENV   fi    or using a short-hand version     -z    DEPLOY ENV       amp  amp  MyVar  default     MyVar    DEPLOY ENV      or even shorter use  MyVar    DEPLOY ENV -default value

User · Answer

NEW VAR    if      ENV VAR   amp  amp    ENV VAR-x      then   NEW VAR   ENV VAR  else   NEW VAR  new value  fi

User · Answer

If you don t care about the difference between an unset variable or a variable with an empty value  you can use the default-value parameter expansion   foo   DEPLOY ENV -default    If you do care about the difference  drop the colon  foo   DEPLOY ENV-default    You can also use the -v operator to explicitly test if a parameter is set   if      -v DEPLOY ENV     then     echo  DEPLOY ENV is not set  elif    -z   DEPLOY ENV      then     echo  DEPLOY ENV is set to the empty string  else     echo  DEPLOY ENV has the value   DEPLOY ENV  fi

[bash] How to check if an environment variable exists and get its value?

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