Find Out If a Variable Is Set or Not（转载）

Bash Shell: Find Out If a Variable Is Set or Not

by NIXCRAFT on SEPTEMBER 25, 2011 · 2 COMMENTS· last updated at SEPTEMBER 25, 2011

http://www.cyberciti.biz/faq/linux-unix-howto-check-if-bash-variable-defined-not/

也可以参考：

http://stackoverflow.com/questions/228544/how-to-tell-if-a-string-is-not-defined-in-a-bash-shell-script

How do I check if a bash shell variable called $input is defined or not under BSD / Apple OS X / Unix / Linux like operating systems?

Method #1: Bash Variable Existence Check

The syntax is as follows to determine if $input is defined or not:

 ${Variable?Error \$Variable is not defined} 

OR

 ${Variable:?Error \$Variable is not defined} 

In this example your script will stop executing if the variable $input is not defined:

 input="Foo bar"echo ${input?Error \$input is not defined.}unset inputecho ${input?Error \$input is not defined.} 

Sample outputs:

Foo barbash: input: Error $input is not defined.

In this example, make sure $input is defined and is not empty, enter:

 [[  $input && ${input-x} ]]input="Foo"[[  $input && ${input-x} ]] && echo "Found" || echo "Not found"unset input[[  $input && ${input-x} ]] && echo "Found" || echo "Not found" 

Here is an example that make sure $_php_map_extension is defined:

         # read config data        loadConfigData "${_t_domain_php_conf}" $LINENO "${FUNCNAME[0]}" # make sure it is defined and not empty if [[ $_php_map_extension && ${_php_map_extension-_} ]]        then                at=${#_php_map_extension[*]}                                            # get total elements in an array                s=""                echo '## Map extension to .php? ##'                echo 'fastcgi.map-extensions = ('                for (( i=0; i<${at}; i++ ));                do                        [ $i -lt $(( $at - 1 )) ] && s="," || s=""                      # remove , for last item in an array                                            echo "      \".${_php_map_extension[i]}\" => \".php\"${s}"                done                echo ')'elseecho "Skiping php map extension as \$_php_map_extension is not defined in /usr/local/etc/nixcraft/conf/php.conf."        fi 

Sample outputs:

Skiping php map extension as $_php_map_extension is not defined in /usr/local/etc/nixcraft/conf/php.conf

OR

## Map extension to .php? ##fastcgi.map-extensions = (      ".html" => ".php",      ".htm" => ".php",      ".phtml" => ".php",      ".php3" => ".php",      ".php4" => ".php")

Method #2: isvarset() function

The above examples are useful for a sanity checking. Finally, you can use the following code:

 isvarset(){ local v="$1" [[ ! ${!v} && ${!v-unset} ]] && echo "Variable not found." || echo "Variable found."} # find out if $vech defined or notvech="Bus" && isvarset vechvech="" && isvarset vechunset vech && isvarset vech 

Method 3: Using the Length of STRING

The -z option to test command returns TRUE of the Length of STRING is zero. You can use the following syntax:

 ### set or not???input="Foo"[ -z "${input+x}" ] && echo "\$input is not set" || echo "\$input found and is set to \"$input\"." ### Not set at ALLunset input[ -z "${input+x}" ] && echo "\$input is not set" || echo "\$input found and is set to \"$input\"." ### 'set but empty' or not?input=""[ -z "$input" -a "${input+x}" = "x" ] && echo "\$input variable is set with empty value." || echo "\$input found and is set to "\$input\"" 

The above syntax will tell if a variable is defined or not defined or defined with a empty value in a bash shell script.

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