New to Shell Scripting: Need help interpreting example function

Shell Programming and Scripting
1

In this example function below, I cannot figure out what certain parts mean.

if ! echo $PATH

what is "if !"?

(^|:)$1($|:)

What is

^|:

and

$|:

?

pathmunge () {
        if ! echo $PATH | /bin/egrep -q "(^|:)$1($|:)" ; then
           if [ "$2" = "after" ] ; then
              PATH=$PATH:$1
           else
              PATH=$1:$PATH
           fi
        fi
}

# Path manipulation
if [ `id -u` = 0 ]; then
        pathmunge /sbin
        pathmunge /usr/sbin
        pathmunge /usr/local/sbin
fi

pathmunge /usr/X11R6/bin after

unset pathmunge
2

As glad as we are to help, please consider reading man pages, e.g. man bash/ksh/your shell , to help yourself; those are immense sources of knowledge...!

1) man bash :

so: if ! means if NOT

2) man regex :

so (^|:)$1($|:) means $1 enclosed in : or at the begin-of-line ( ^ ) or at the end-of-line ( $ )...

3

Actually, think it more like this:

! (echo $PATH | /bin/egrep -q "(^|:)$1($|:)")

The highlighted parenthesis represents how you should see that line of code. They are not intended to be included.
echo $PATH output becomes the input of egrep , which is asked to find a pattern in that output of echo.
It is looking for the following pattern:

Start of string or character : + argument string + end of string or character :

By placing a ! in front of the whole expression, it turns it into if not found

Look how pathmunge is used below, as an example:

pathmunge /usr/X11R6/bin after

In English terms:
If /usr/X11R6/bin does not exist append to the path (at the end). That's the after argument.

4

Attention, if ! ( ... ) would run ... in a sub-shell. This is extra fork-overhead, and can have some other impact, like a ((var+=1)) will not copy back to the main shell.
Actually this patchmunge() function is in RH/CentOS 5 /etc/profile. And is risky, at least one must quote

if ! echo "$PATH" | /bin/egrep -q "(^|:)$1($|:)")

And also needs an external program /bin/egrep.
It is improved in RH/CentOS 6:

pathmunge () {
    case ":${PATH}:" in
        *:"$1":*)
            ;;
        *)
            if [ "$2" = "after" ] ; then
                PATH=$PATH:$1
            else
                PATH=$1:$PATH
            fi
    esac
}

The functionality of pathmunge is:
it prepends the 1st argument to the PATH (if not yet exists), unless the 2nd argument is "after" where it appends the 1st argument to the PATH (if not yet exists).