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Bash parameter expansions
=========================

NOTE:
  The `:' character used in many of the expansions is optional.
  Using the `:' checks for param that is unset or null.
  Without the `:' checks for param that is unset.

| Expansion                | Note                            |
|--------------------------+---------------------------------|
| ${param}                 | Use value                       |
| ${!param}                | Variable indirection            |
| ${param:-word}           | Use default value               |
| ${param:=word}           | Assign default value            |
| ${param:?word}           | Display error                   |
| ${param:+word}           | Use alternate value             |
| ${param:offset}          | Substring expansion             |
| ${param:offset:length}   | Substring expansion with length |
| ${!prefix*}              | Name expansion with prefix      |
| ${#param}                | Length in characters            |
| ${param#word}            | Remove shortest match at start  |
| ${param##word}           | Remove longest match at start   |
| ${param%word}            | Remove shortest match at end    |
| ${param%%word}           | Remove longest match at end     |
| ${param/pattern/string}  | Replace first matching pattern  |
| ${param//pattern/string} | Replace all matching patterns   |

Examples:

$ x=yes
$ echo ${x}         #=> yes
$ echo ${x:-no}     #=> yes
$ x=
$ echo ${x:-no}     #=> no
$ echo ${x-no}      #=> "" <an empty string>
$ unset x
$ echo ${x:-no}     #=> no
$ echo ${x-no}      #=> no

$ path='/some/path/to/file.txt'
$ echo ${path##*/}  #=> file.txt
$ echo ${path%/*}   #=> /some/path/to

$ x=yes
$ y=x
$ echo ${!y}        #=> yes

$ x=0123456789
$ echo ${x:0}       #=> 0123456789
$ echo ${x:3}       #=> 3456789
$ echo ${x:3:2}     #=> 34

$ xval1=first
$ xval2=second
$ xval3=third
$ echo ${!xval*}    #=> xval1 xval2 xval3