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In the Terminal preferences, I'm able to configure the Terminal to "Open shells with:" -> "Command (complete path):" /opt/local/bin/bash.

What options is the terminal passing when invoking this command? Terminal appears to run each shell as a login shell, so I'm assuming --login is being passed?

Is there any Apple documentation that gives an overview of exactly what is being run when you open a terminal window?

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No arguments are passed. The terminal runs login -pfq $USER $PROGRAM, which in turn runs the specified program. login signals to the program that it is a login shell by prepending a - to argv[0] (the program name).

For the default shell, Terminal actually does something different. It invokes login -pfl $USER /bin/bash -c exec -la bash /bin/bash, where the -l option to login tells it not to run the command as login shell, while the -l option to exec tells it to run its argument as a login shell (by adding the dash). The reason while this is done is that login also sets the current directory to the user's home directory when it runs a program as a login shell, while exec doesn't. This allows the Terminal to open shells in the same current directory as the current tab, without login setting it back to the home directory.

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  • When I change the terminal preferences to /opt/local/bin/bash, how does login -pfq $USER $PROGRAM change? Just so I understand your first paragraph, do you mean Terminal runs login -pfq cwilson /opt/local/bin/-bash? – cdwilson Jan 3 '14 at 2:18
  • No, the terminal runs login -pfq cwilson /opt/local/bin/bash. Then login executes bash, but passes -bash in argv[0]. – LaC Jan 15 '14 at 8:16
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The command:

/opt/local/bin/bash

essentially instructs Terminal.app to load an interactive bash shell (the path for your example is likely the macports version). Initially when you open Terminal it starts a login shell session (in bash). Login shells can read files such as ~/.profile, ~/.bash_profile that setup environment variables for your session, so what is being run is mostly indicative of what is configured within those files.

In general, by default when opening a bash shell from Terminal these things would be run:

Processes

bash
login

Open Files and Ports

/bin/bash
/usr/lib/dyld
/private/var/db/dyld/dyld_shared_cache_x86_64
/dev/ttys000

More Information:

Official Bash Website

Apple Man Page: bash(1)

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