env printenv set declare export

env and printenv is not shell builtin, so they can only print environment variables.

env:       run a program in a modified environment

1. --ignore-environment -i -          start with an empty environment
   

    

2. -0 -null     end each output line with NUL, not newline
   

    

    

3. --unset -u      remove variable from the environment
   

    

    

4. --chdir        change working directory
   

    

    

5. --split-string -S          allow specifying multiple parameters in a script
   

   

   

    

    

    

6. sh -c 'env --block-signal=PIPE seq inf'
   
   kill -s SIGPIPE `pgrep seq`  # can't stop seq

    

7. sh -c 'env --ignore-signal=PIPE seq inf'
   
   kill -s SIGPIPE `pidof seq`  # can't stop seq

    

8. to be continue
   
   

printenv:

1. printenv
   
   
2. printenv variable
   

    

    

declare:

1. -p     display the attributes and value of each NAME
   

    

     

2. -n       name reference
   

    

    

3. -x        make NAME export
   

    

    

4. -i        make NAME have the integer attribute
   0) value is interpreted as 0
   1) value is interpreted as integer
   2) Error
   

   In addition to modifying the value assignment behavior, variables now behave defferently in arithmetic expressions!

   

    

    

5. In bash variables may have case attributes on assignment 

   

    

    

6. -r       make NAME readonly
   +r option does not work; that is stripping a variabl of its readonly attribute is not allowed in bash
   
   
7. -a         make NAME indexed array
   -A         make NAME associative array
   

   

    

    

    

8. -g        create global variables when used in a shell function, When used in function "declare" makes NAME local, as with th "local" command, The -g option suppress this behavior
   

    

    

export:

1.  -f
   

    

   Let export the function name
   ! there is only one way to disable export a function

    export -nf function

   there are two ways to export a function

   declare -fx function  # use -f -x simultaneously
   export -f function

   

    

    

2. -n       Remove the export property from NAME