Defining a variable with or without export

Question

What is export for?

What is the difference between:

export name=value

and

name=value

Answer 1

export NAME=value for settings and variables that have meaning to a subprocess.

NAME=value for temporary or loop variables private to the current shell process.

In more detail, export marks the variable name in the environment that copies to a subprocesses and their subprocesses upon creation. No name or value is ever copied back from the subprocess.

• A common error is to place a space around the equal sign:

  $ export FOO = "bar"  
  bash: export: `=': not a valid identifier
  

• Only the exported variable (B) is seen by the subprocess:

  $ A="Alice"; export B="Bob"; echo "echo A is \$A. B is \$B" | bash
  A is . B is Bob
  

• Changes in the subprocess do not change the main shell:

  $ export B="Bob"; echo 'B="Banana"' | bash; echo $B
  Bob
  

• Variables marked for export have values copied when the subprocess is created:

  $ export B="Bob"; echo '(sleep 30; echo "Subprocess 1 has B=$B")' | bash &
  [1] 3306
  $ B="Banana"; echo '(sleep 30; echo "Subprocess 2 has B=$B")' | bash 
  Subprocess 1 has B=Bob
  Subprocess 2 has B=Banana
  [1]+  Done         echo '(sleep 30; echo "Subprocess 1 has B=$B")' | bash
  

• Only exported variables become part of the environment (man environ):

   $ ALICE="Alice"; export BOB="Bob"; env | grep "ALICE\|BOB"
   BOB=Bob
  

So, now it should be as clear as is the summer's sun! Thanks to Brain Agnew, alexp, and William Prusell.