Add directory to $PATH if it's not already there

From my .bashrc:

pathadd() {
    if [ -d "$1" ] && [[ ":$PATH:" != *":$1:"* ]]; then
        PATH="${PATH:+"$PATH:"}$1"
    fi
}

Note that PATH should already be marked as exported, so reexporting is not needed. This checks whether the directory exists & is a directory before adding it, which you may not care about.

Also, this adds the new directory to the end of the path; to put at the beginning, use PATH="$1${PATH:+":$PATH"}" instead of the above PATH= line.

Here's something from my answer to this question combined with the structure of Doug Harris' function. It uses Bash regular expressions:

add_to_path ()
{
    if [[ "$PATH" =~ (^|:)"${1}"(:|$) ]]
    then
        return 0
    fi
    export PATH=${1}:$PATH
}

Put this in the comments to the selected answer, but comments don't seem to support PRE formatting, so adding the answer here:

@gordon-davisson I'm not a huge fan of unnecessary quoting & concatenation. Assuming you are using a bash version >= 3, you can instead use bash's built in regexs and do:

pathadd() {
    if [ -d "$1" ] && [[ ! $PATH =~ (^|:)$1(:|$) ]]; then
        PATH+=:$1
    fi
}

This does correctly handle cases where there are spaces in the directory or the PATH. There is some question as to whether bash's built in regex engine is slow enough that this might net be less efficient than the string concatenation and interpolation that your version does, but somehow it just feels more aesthetically clean to me.

idempotent_path_prepend ()
{
    PATH=${PATH//":$1"/} #delete any instances in the middle or at the end
    PATH=${PATH//"$1:"/} #delete any instances at the beginning
    export PATH="$1:$PATH" #prepend to beginning
}

When you need $HOME/bin to appear exactly once at the beginning of your $PATH and nowhere else, accept no substitutes.