bash subshells vs bash -c

Question

Bash subshells (cmd1;cmd2) whill run in a sub bash process, while it can access the variable not exported, how this can be? Meanwhile, bash -c structure can not access unexported variable but it also run in a new shell,so what's the difference?

x=1
(echo $x)
bash -c 'echo $x'

Answer 1

Both subshell and bash -c contexts can access exported variables. To export a variable, one declares the variable as exported (eg, declare -x VAR ) or specifically exports it ( export VAR ). Your example, it seems, does not export x .

More of the parent context is inherited by subshells (eg, (... ) ), than are by specific command executions (eg, bash -c... ).

For example, the following items are inherited by subshells:

• Current working directory; eg, as set with cd , pushd , and/or popd
• Directory stack ( DIRSTACK ); eg, as affected by cd , pushd , popd
• File creation mask; eg, as set with umask
• File handles/descriptors; eg, initially and as affected by exec redirections
• Settings; eg, as affected by set , shopt , and alias
• Shell variables and functions; eg, as set with declare and =
• Exported variables and functions; eg, as declared with declare -x or exported with export
• Signal traps for ERR ; eg, as set with trap... ERR
• Certain special shell parameters; eg, PPID

For example, the following items are inherited by command executions (w. bash -c ):

• Current working directory; eg, as set with cd , pushd , and/or popd
• File creation mask; eg, as set with umask
• File handles/descriptors; eg, initially and as affected by exec redirections
• Exported variables and functions; eg, as declared with declare -x or exported with export

For additional detail, check out the Bash manpage, starting with the COMMAND EXECUTION ENVIRONMENT section.

Note: Each command in a pipeline executes in dedicated processes/subshells. With lastpipe enabled, the final pipeline command may run within the outer shell process.

Note: Other than when a command list is fully backgrounded with a trailing & , command lists are executed within the current shell process. When backgrounded, command lists are executed in a dedicated process/subshell.

Answer 2

1. Assume that I have written following line inside $HOME/.bashrc file using cygwin in Windows.

    export testing="testing$$"

   After this I do Windows + R => bash.exe

    $ echo $testing testing7853

2. I open "cmd.exe"

   Inside that command prompt, I type

   C:> bash.exe -c "echo [$testing]" [] C:>

   Reason:

   When we execute bash.exe it'll execute ~/.bashrc and other related dependent default profile files. If we execute bash.exe -c "related command" then .bashrc is not executed.

   We are informing bash to execute related command. If you need that environment variable available inside bash.exe -c execute:

    C:\Windows\System32\rundll32.exe sysdm.cpl,EditEnvironmentVariables

   Set your required environment variable. For example I set certs environment variable.

    C:>bash.exe -c "echo [$certs]" [C:\apache-tomcat\certs]

   When we execute bash.exe it is a process waiting for the user input to exit from that process.

   When we execute bash.exe -c , a process is created. It will execute required commands and exit from that process.

    C:\Windows\System32\tasklist.exe | grep bash

   The above command will display related output of running process.