How do I do string manipulations in bash?
Bash can do string operations. LOTS of string operations. This is an introduction to bash string operations for those new to Bash's special tool/feature called Parameter Expansion (PE), with a focus on typical string operations. (Note: much of this material is in FAQ #73. It is being presented here with an approach inteneded to make it more accessable and more easily learned by people who have not had a lot of exposure to Bash.)
Note Bash's PE capability is a lot more powerful than the typical string manipulation calls you may be used to. There are some twists in the road up ahead. 
Here is a list of some typical string manipulation functions/subroutines that you may already be familiar with:
- strlen returns the length of the string
- leftstr returns a string N chars long starting from the left hand side
- rightstr returns a string N chars long starting from the right hand side
- midstr returns a string N chars long starting from an offset K chars from the beginning/end
- substr substitutes all instances of a pattern with a new string
- basename returns the last component of a pathname (everything after the last "/")
- dirname returns everything in the pathname up to, but not including the last "/"
Two that you haven't heard of but would want to use all the time when scripting on *NIX:
- getext returns a filename's extension (eg. "txt", "mp3", "doc", "sxc", "html", etc. ... )
- dropext returns the filename with the extension stripped off the end of the name.
This article will cover how to do all of these using the Bash and will introduce the more powerful actions available with Bash's PE's. Please note there is a BashFaq about PE's already. FAQ #73 covers a larger scope of PE capabilities, where this one focuses on string operations.
Lest say we have a bash variable named fullpath that contains "/usr/home/JosephBaldwin/Its_only_Rock_and_Roll.mp3"
Naturally in scripting we would want to manipulate cerain pieces of the path, like just the file name,
For a basename we use the PE expression: ${fullpath##*/} which returns "Its_only_Rock_and_Roll.mp3".
To find the dirname we use the PE expression: ${fullpath%/*} which produces "/usr/home/JosephBaldwin".
To drop the filename extesnion, we use the PE expression: ${fullpath%.*} giving out "/usr/home/JosephBaldwin/Its_only_Rock_and_Roll"
To get the filename's extension we use the PE expression: ${fullpath##*.} generating only "mp3".
To find the strlen the PE expression: ${#fullpath} finds it and its 49.
To get a leftstr, the PE expression: ${fullpath:0:20} grabs the first 20 chars of fullpath to make "usr/home/JosephBaldw".usr/home/JosephBaldwin/Its_only_Roll_and_Roll.mp3
To perform a rightstr in bash we use the following PE expression: ${fullpath:$(( 0 - 20 ))} which gets the last 20 chars, "ly_Rock_and_Roll.mp3".
To perform a midstr in bash we use the following PE expression: ${fullpath:10:20} making "osephBaldwin/Its_onl"
To perform a substr in bash we use the following PE expression: ${fullpath//Rock/Roll} rolling it into "usr/home/JosephBaldwin/Its_only_Roll_and_Roll.mp3".
Why aren't the PE things named more nicely
Can't I just have these a regular functions with nice names?
not totally generalizable