Usage of ${} in Shell

The following content is basically extracted from the Shell13 question.
1. Truncation function
${file#*/}: Take out the first/its left string: dir1/dir2/dir3/my.file.txt
${file##*/}: Take out the last/and left string: my.file.txt
${file#*.}: Take out the first. and its left string: file.txt
${file##*.}: Take out the last one. and its left string: txt
${file%/*}: Take off the last bar/its right string:/dir1/dir2/dir3
${file%%/*}: Remove the first/its right string: (null value)
${FILE%.*}: Take out the last one. and the string to the right:/dir1/dir2/dir3/my.file
${FILE%%.*}: Take out the first. and its right string:/dir1/dir2/dir3/my
The methods of memory are:
[List] #是去掉左边, # #最后一个
% is removed right, percent of the first
2. String Extraction
The single symbol is the minimum match, and the two symbol is the maximum match.
${file:0:5}: Extract the leftmost 5 bytes:/dir1
${file:5:5}: Extracts the 5th byte to the right of 5 consecutive bytes:/DIR2
3. String substitution
${file/dir/path}: Change the first dir to Path:/path1/dir2/dir3/my.file.txt
${file//dir/path}: Change all dir to Path:/path1/path2/path3/my.file.txt
4. Assign values to different variable states (no settings, null values, non-null values):
${file-my.file.txt}: If $file is not set, use My.file.txt as the return value. (null and non-null values are not processed)
${file:-my.file.txt}: If $file is not set or null, use My.file.txt as the return value. (Non-null value is not processed)
${file+my.file.txt}: Use My.file.txt as the return value if $file is set to a null or non-null value. (not processed when not set)
${file:+my.file.txt}: If $file is a non-null value, use My.file.txt as the return value. (No processing when not set and null value)
${file=my.file.txt}: If $file is not set, My.file.txt is used as the return value, and $file is assigned my.file.txt. (null and non-null values are not processed)
${file:=my.file.txt}: If $file is not set or null, use My.file.txt as the return value and assign the $file to My.file.txt. (Non-null value is not processed)
${file?my.file.txt}: If $file is not set, the My.file.txt output to STDERR. (null and non-null values are not processed)
${file:?my.file.txt}: If $file is not set or null, the my.file.txt output to stderr. (Non-null value is not processed)
Attention:
The ": +" case does not contain a null value.
":-", ": =" as long as there is a number is a null value (NULL).
5. Length of the variable
${#file}
6. Array Operations
A= (a b c def)
${a[@]} or ${a[*]} to get A b C def (total number of groups)
${a[0]} can get a (number of first group), ${a[1]} is the second group number ...
${#A [@]} or ${#A [*]} to get 4 (total number of groups)
${#A [0]} can get 1 (that is, the length of the first group (a), ${#A [3]} can get 3 (fourth group number (def) length)

Usage of ${} in Shell