Defining a variable with or without export

Question

What is export for   What is the difference between   export name value   and  name value

User · Answer

export will make the variable available to all shells forked from the current shell

User · Answer

export makes the variable available to sub-processes   That is   export name value   means that the variable name is available to any process you run from that shell process  If you want a process to make use of this variable  use export  and run the process from that shell   name value   means the variable scope is restricted to the shell  and is not available to any other process  You would use this for  say  loop variables  temporary variables etc   It s important to note that exporting a variable doesn t make it available to parent processes  That is  specifying and exporting a variable in a spawned process doesn t make it available in the process that launched it

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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  amp   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

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Others have answered that export makes the variable available to subshells  and that is correct but merely a side effect   When you export a variable  it puts that variable in the environment of the current shell  ie the shell calls putenv 3  or setenv 3    The environment of a process is inherited across exec  making the variable visible in subshells   Edit  with 5 year s perspective   this is a silly answer   The purpose of  export  is to make variables  be in the environment of subsequently executed commands   whether those commands be subshells or subprocesses   A naive implementation would be to simply put the variable in the environment of the shell  but this would make it impossible to implement export -p

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To illustrate what the other answers are saying     foo  Hello  World    echo  foo Hello  World   bar  Goodbye    export foo   bash bash-3 2  echo  foo Hello  World bash-3 2  echo  bar  bash-3 2

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Although not explicitly mentioned in the discussion  it is NOT necessary to use export when spawning a subshell from inside bash since all the variables are copied into the child process

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The accepted answer implies this  but I d like to make explicit the connection to shell builtins      As mentioned already  export will make a variable available to both the shell and children  If export is not used  the variable will only be available in the shell  and only shell builtins can access it    That is    tango 3 env   grep tango   prints nothing  since env is a child process set   grep tango   prints tango 3 -  type set  shows  set  is a shell builtin

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By default  variables created within a script are only available to the current shell  child processes  sub-shells  will not have access to values that have been set or modified  Allowing child processes to see the values  requires use of the export command

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Just to show the difference between an exported variable being in the environment  env  and a non-exported variable not being in the environment   If I do this     MYNAME Fred   export OURNAME Jim   then only  OURNAME appears in the env  The variable  MYNAME is not in the env     env   grep NAME OURNAME Jim   but the variable  MYNAME does exist in the shell    echo  MYNAME Fred

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Here s yet another example   VARTEST  value of VARTEST    export VARTEST  value of VARTEST   sudo env   grep -i vartest  sudo echo   SUDO USER    SUDO UID    SUDO GID     VARTEST    sudo bash -c  echo   SUDO USER    SUDO UID    SUDO GID     VARTEST        Only by using export VARTEST the value of VARTEST is available in sudo bash -c          For further examples see    http   mywiki wooledge org SubShell bash-hackers org wiki doku php scripting processtree

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As you might already know  UNIX allows processes to have a set of environment variables  which are key value pairs  both key and value being strings  Operating system is responsible for keeping these pairs for each process separately   Program can access its environment variables through this UNIX API    char  getenv const char  name   int setenv const char  name  const char  value  int override   int unsetenv const char  name     Processes also inherit environment variables from parent processes  Operating system is responsible for creating a copy of all  envars  at the moment the child process is created   Bash  among other shells  is capable of setting its environment variables on user request  This is what export exists for   export is a Bash command to set environment variable for Bash  All variables set with this command would be inherited by all processes that this Bash would create   More on Environment in Bash  Another kind of variable in Bash is internal variable  Since Bash is not just interactive shell  it is in fact a script interpreter  as any other interpreter  e g  Python  it is capable of keeping its own set of variables  It should be mentioned that Bash  unlike Python  supports only string variables   Notation for defining Bash variables is name value  These variables stay inside Bash and have nothing to do with environment variables kept by operating system   More on Shell Parameters  including variables   Also worth noting that  according to Bash reference manual      The environment for any simple command or function may be augmented   temporarily by prefixing it with parameter assignments  as described   in Shell Parameters  These assignment statements affect only the   environment seen by that command      To sum things up    export is used to set environment variable in operating system  This variable will be available to all child processes created by current Bash process ever after  Bash variable notation  name value  is used to set local variables available only to current process of bash Bash variable notation prefixing another command creates environment variable only for scope of that command

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Two of the creators of UNIX  Brian Kernighan and Rob Pike  explain this in their book  The UNIX Programming Environment   Google for the title and you ll easily find a pdf version   They address shell variables in section 3 6  and focus on the use of the export command at the end of that section      When you want to make the value of a variable accessible in sub-shells  the shell s export command should be used   You might think about why there is no way to export the value of a variable from a sub-shell to its parent

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It should be noted that you can export a variable and later change the value  The variable s changed value will be available to child processes   Once export has been set for a variable you must do export -n  lt var gt  to remove the property     K 1   export K   K 2   bash -c  echo   K-unset   2   export -n K   bash -c  echo   K-unset   unset

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It has been said that it s not necessary to export in bash when spawning subshells  while others said the exact opposite  It is important to note the difference between subshells  those that are created by             or loops  and subprocesses  processes that are invoked by name  for example a literal bash appearing in your script      Subshells will have access to all variables from the parent  regardless of their exported state   Subprocesses will only see the exported variables     What is common in these two constructs is that neither can pass variables back to the parent shell     noexport noexport  export export export   echo subshell   noexport  export  subshell subshell   bash -c  echo subprocess   noexport  export  subprocess subprocess   echo parent   subshell  subprocess subshell  noexport export subprocess  export parent    There is one more source of confusion  some think that  forked  subprocesses are the ones that don t see non-exported variables  Usually fork  s are immediately followed by exec  s  and that s why it would seem that the fork   is the thing to look for  while in fact it s the exec    You can run commands without fork  ing first with the exec command  and processes started by this method will also have no access to unexported variables     noexport noexport  export export export  exec bash -c  echo execd process   noexport  export  execd execd   echo parent   execd execd process  export   Note that we don t see the parent  line this time  because we have replaced the parent shell with the exec command  so there s nothing left to execute that command

[linux] Defining a variable with or without export

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