As far as I can tell, variable assignment is the same whether it is or is not preceded by "export". What's it for?
1
-
1See this video: Export Variables - Why you need to Export BASH Variablesmfaani– mfaani2020-03-20 16:45:43 +00:00Commented Mar 20, 2020 at 16:45
Add a comment
|
3 Answers
Exported variables such as $HOME and $PATH are available to (inherited by) other programs run by the shell that exports them (and the programs run by those other programs, and so on) as environment variables. Regular (non-exported) variables are not available to other programs.
$ env | grep '^variable='
$ # No environment variable called variable
$ variable=Hello # Create local (non-exported) variable with value
$ env | grep '^variable='
$ # Still no environment variable called variable
$ export variable # Mark variable for export to child processes
$ env | grep '^variable='
variable=Hello
$
$ export other_variable=Goodbye # create and initialize exported variable
$ env | grep '^other_variable='
other_variable=Goodbye
$
For more information, see the entry for the export builtin in the GNU Bash manual, and also the sections on command execution environment and environment.
Note that non-exported variables will be available to subshells run via ( ... ) and similar notations because those subshells are direct clones of the main shell:
$ othervar=present
$ (echo $othervar; echo $variable; variable=elephant; echo $variable)
present
Hello
elephant
$ echo $variable
Hello
$
The subshell can change its own copy of any variable, exported or not, and may affect the values seen by the processes it runs, but the subshell's changes cannot affect the variable in the parent shell, of course.
Some information about subshells can be found under command grouping and command execution environment in the Bash manual.
answered Sep 14, 2011 at 5:07
Jonathan Leffler
759k145 gold badges961 silver badges1.3k bronze badges
3 Comments
fedorqui
I added some new lines and also lines just showing
$ to show more clearly that there is no output from the grep command. Of course, feel free to rollback if you think this loses readability
David Tonhofer
Note that non-exported variables will be available to subshells Yes - because they are copied for use by the subshell. And thus any modification in the subshell will have no effect as the copy is dropped on return-from-subshell: Subshells, environment variables, and scope. The same happens for environment variables: the are copied - but for any kind of subprocess, not only for subshells. So a
mysql process get value from MYSQL_PWD env var, but would have no concept of MYSQL_PWD shell var.David Tonhofer
"available to other programs" == by default available to subprocesses of the current process only. Because security. NOT to any other processes. Unless they have permission to inspect the process with said environment (e.g. debuggers running with the same user, or as root. For example, as root you can read the environment of process #1:
cat /proc/1/environ gives HOME=/TERM=linuxBOOT_IMAGE=/vmlinuz-4.19.9-300.fc29.x86_64LANG=en_GB.UTF-8 or similar.it makes the assignment visible to subprocesses.
$ foo=bar
$ bash -c 'echo $foo'
$ export foo
$ bash -c 'echo $foo'
bar
Comments
Well, it generally depends on the shell. For bash, it marks the variable as "exportable" meaning that it will show up in the environment for any child processes you run.
Non-exported variables are only visible from the current process (the shell).
From the bash man page:
export [-fn] [name[=word]] ...
export -pThe supplied names are marked for automatic export to the environment of subsequently executed commands.
If the
-foption is given, the names refer to functions. If no names are given, or if the-poption is supplied, a list of all names that are exported in this shell is printed.The
-noption causes the export property to be removed from each name.If a variable name is followed by
=word, the value of the variable is set toword.
exportreturns an exit status of 0 unless an invalid option is encountered, one of the names is not a valid shell variable name, or-fis supplied with a name that is not a function.
You can also set variables as exportable with the typeset command and automatically mark all future variable creations or modifications as such, with set -a.
