Some of the supernatural powers of ${} in the shell

Suppose we define a variable as:
File=/dir1/dir2/dir3/my.file.txt
We can use${ }Replace each to get different values:
${file#*/}: Take out the first one./And the string to its left:Dir1/dir2/dir3/my.file.txt
${file##*/}: Take the last one off./And the string to its left:My.file.txt
${file#*.}: Take out the first one.And the string to its left:File.txt
${file##*.}: Take out the last one.And the string to its left:Txt
${file%/*}: Take out the last bar/And the string to the right:/dir1/dir2/dir3
${file%%/*}: Take out the first one./And the string to the right:(Null value)
${file%.*}: Take out the last one. /dir1/dir2/dir3/my.file 
${file%%.*} : Take out the first  /dir1/dir2/dir3/my 
< Span style= "LINE-HEIGHT:31PX;" > Memory method:

#is to remove the left(On the Kam disk #  in  $  to the left of Span style= "line-height:31px;" lang= "en-us" >)  
%  is to remove the right  %< Span class= "Apple-converted-space" >  in  $  right )  

 ${file:0:5}: Extracts the leftmost 5 bytes:/dir1
${file:5:5}: Extracts the 5 bytes to the right of the first 5 bytes :/dir2
We can also replace the string in the value of the variable:
${file/dir/path}: the firstDirExchange forPath/path1/dir2/dir3/my.file.txt
${file//dir/path}: Will AllDirExchange forPath/path1/path2/path3/my.file.txt

Use${ }You can also assign values for different variable states(no set, null value, non-null value)：
${file-my.file.txt}: If$fileis a null value, use theMy.file.txt as the default value.(Reservation not set and non-null value)
${file:-my.file.txt} : If$fileIf there is no setting or null value, use theMy.file.txtAs the default value.(leave non-null values)
${file+my.file.txt}: No matter$fileWhy value, all useMy.file.txt as the default value.(No values are preserved)
${file:+my.file.txt}: Unless$fileis a null value, otherwise use theMy.file.txtAs the default value.(Leave empty values)
${file=my.file.txt}: If$fileNot set, then useMy.file.txtAs the default value, and the$fileDefined as a non-null value.(leave null and non-null values)
${file:=my.file.txt}: If$fileis not set or null, use theMy.file.txtAs the default value, and the$fileDefined as a non-null value.(leave non-null values)
${file?my.file.txt}: If$fileis not set, it will beMy.file.txtOutput toSTDERR.(leave null and non-null values))
${file:?my.file.txt}: If$fileIf no value is set or NULL, theMy.file.txtOutput toSTDERR.(leave non-null values) 

Also, ${#var}  to calculate the length of the variable value:  
${#file}  can get  , because  /dir1/dir2/dir3/my.file.txt  just 27   bytes

Some psychic powers of ${} in the shell