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Full Discussion: Need help understanding a function
Top Forums Shell Programming and Scripting Need help understanding a function Post 302883110 by neutronscott on Friday 10th of January 2014 07:58:47 PM
Old 01-10-2014
Code:
prepend() {
    [ -d "$2" ] &&
    eval $1=\"$2\$\{$1:+':'\$$1\}\" &&
    export $1
}

well [ is a command for tests. it's about the same as test.
looking at help test we see -d checks to see if $2 exists as a directory before doing anything else.

&& joins the statements. in A && B, B is only performed if A was true. similarly A || B, B is only performed if A was false. (Like short-circuit evaluation in C) Read more in the bash reference manual, section 3.2.3 Lists of Commands

A clearer version would perhaps be
Code:
prepend() {
    if [ -d "$2" ]; then
        eval $1=\"$2\$\{$1:+':'\$$1\}\" &&
        export $1
    fi
}

eval is rather bad practice, unless absolutely necessary. I personally wouldn't use this function.. So much can go wrong executing user-defined strings as code. But moving past that. It does that: executes the line again... So placing our variables into there we'll see
Code:
prepend PATH /opt/myapp/bin

become as
Code:
eval PATH="/opt/myapp/bin:$PATH"

the single quotes around the colon there isn't very helpful... But neither is this particular function.
 

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LEARN ABOUT LINUX

suspend

suspend(1)                                                         User Commands                                                        suspend(1)

NAME

       suspend - shell built-in function to halt the current shell

SYNOPSIS

   sh
       suspend

   csh
       suspend

   ksh
       suspend

DESCRIPTION

   sh
       Stops the execution of the current shell (but not if it is the login shell).

   csh
       Stop the shell in its tracks, much as if it had been sent a stop signal with ^Z. This is most often used to stop shells started by su.

   ksh
       Stops the execution of the current shell (but not if it is the login shell).

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE         |      ATTRIBUTE VALUE        |
       +-----------------------------+-----------------------------+
       |Availability                 |SUNWcsu                      |
       +-----------------------------+-----------------------------+

SEE ALSO

       csh(1), kill(1), ksh(1), sh(1), su(1M), attributes(5)

SunOS 5.10                                                          15 Apr 1994                                                         suspend(1)
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