Expansions and substitutions
Before executing your commands, Bash checks whether there are any syntax elements in the command line that should be interpreted rather than taken literally. After splitting the command line into tokens (words), Bash scans for these special elements and interprets them, resulting in a changed command line: the elements are said to be expanded to or substituted to new text and maybe new tokens (words).
The most simple example of this behaviour is a referenced variable:
mystring="Hello world" echo "$mystring"The
echoprogram definitely doesn't care about what a shell variable is. It is Bash's job to deal with the variable. Bash expands the string "
$mystring" to "
Hello world", so that
echowill only see
Hello world, not the variable or anything else!
After all these expansions and substitutions are done, all quotes that are not meant literally (i.e. the quotes that marked contiguous words, as part of the shell syntax) are removed from the commandline text, so the called program won't see them. This step is called quote-removal.
Saw a possible expansion syntax but don't know what it is? Here's a small list.
$(( EXPRESSION ))
$[ EXPRESSION ]
$( COMMAND )
` COMMAND `
<( COMMAND )
>( COMMAND )
Bash performs expansions and substitutions in a defined order. This explains why globbing (pathname expansion), for example, is safe to use on filenames with spaces (because it happens after the final word splitting!).
The order is (from first to last):
- The following expansions happen at the same time, in a left-to-right fashion on the commandline (see below)
i=1 echo $i $((i++)) $iwill output
1 1 2and not
1 1 1.