Variables

Definition of variables

a="Hello World

Reading the content of a variable

echo $a
echo ${a}

a="Hello World"
b=$a
echo $b                         # we get "Hello World"

a="Hello World"
b=a
echo $b                         # yields a
echo $$b                        # yields <pid of echo>b
echo '$'$b                      # yields "$a"
eval echo '$'$b                 # Good. we get "Hello World"

Exporting Variables

a="Hello World"
sh -c 'echo $a'                 # yields a empty line
export a
sh -c 'echo $a'                 # we get "Hello World"

Pitfalls with variables and subshells

• Variables defined in subshells are NEVER exported to the calling shell
• Pipes are executed in subshells, and in some shells while loops --> don't forget to export the variable
• bash permits to write export a=value, but this may be a syntax error in other shells
• a shellscript can be executed in the current envionment if called with source or with "."
• if a script is sourced, it must not call exit