Bash – Why Isn’t ‘set’ Behaving as Expected

bashenvironment-variablespath

I have observed that only the values of exported variables are appended to the my PATH variable via my Mac OS X .bash_profile. Whereas, the values of locally set variables cannot. Why can't local variables be appended to the path?

“You've misinterpreted something, but what? Post the code that puzzles you. – Gilles yesterday”

Please consider this snippet, where I set the MONGODB variable as:

set MONGODB="/usr/local/mongodb/bin"    
export PATH=${PATH}:${MONGODB}

I source .bash_profile in terminal.
I see the following echo:

PATH=~/bin:/usr/bin:/usr/local/bin:/usr/local/sbin:

Whereas, if I export MONGODB instead, and source my .bash_profile, I see the following echo:

PATH=~/bin:/usr/bin:/usr/local/bin:/usr/local/sbin:/usr/local/mongodb/bin

Perhaps, using set is improper?

Best Answer

set MONGODB="/usr/local/mongodb/bin"

This is not a variable assignment. (It is one in C shell (csh, tcsh), but not in Bourne-style shells (sh, ash, bash, ksh, zsh, …).) This is a call to the set built-in, which sets the positional parameters, i.e. $1, $2, etc. Try running this command in a terminal, then echo $1.

To assign a value to a shell variable, just write

MONGODB="/usr/local/mongodb/bin"

This creates a shell variable (also called a (named) parameter), which you can access with $MONGODB. The variable remains internal to the shell unless you've exported it with export MONGODB. If exported, the variable is also visible to all processes started by that shell, through the environment. You can condense the assignment and the export into a single line:

export MONGODB="/usr/local/mongodb/bin"

For what you're doing, there doesn't seem to be a need for MONGODB outside the script, and PATH is already exported (once a variable is exported, if you assign a new value, it is reflected in the environment). So you can write:

MONGODB="/usr/local/mongodb/bin"    
PATH=${PATH}:${MONGODB}
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