The magical application of ${} in the shell

1. Truncation function

 file=dir1/dir2/dir3/my. file.txt   
 ${file  #*/}: Remove the first/its left string: dir2/dir3/my. file  .txt${ file  ##*/}: Take out the last/and left string: my.< Span style= "color: #0000ff;"       >file  .txt${ file  #*.}: Take out the first. and its left string: file  .txt${ file  ##* file %/*  }: Take off the last bar/its right string:/dir1/dir2/dir3$ {file%%/*}: Remove the first/its right string: (null) ${file%.*}: Take off the last one. and its right string:/dir1/dir2/dir3/my.file${file%%.*}: Take off the first. and the string to the right:/ Dir1/dir2/dir3/my  

The methods of memory are:

#是去掉左边 # #最后一个% is the first one to remove the right

2. String extraction

The single symbol is the minimum match, and the two symbol is the maximum match.
File=dir1/dir2/dir3/my. file.txt${file:0:55 bytes:/dir1${file :5:555 bytes:/dir2

3. String substitution

${file/dirdir for path:/path1/dir2/dir3/my. file . txt${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 if $file is set to a null or non-null value.file. txt as the return 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. (not processed when not set and null value) ${file=my.file. txt}: If $file is not set, use My.file. txt as the return value while the $fileAssign to 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 while the $fileAssign to My.file. txt. (Non-null value is not processed) ${file? My.file. txt}: If $file is not set, it will be 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, then my.file. txt output to stderr. (Non-null value is not processed)

Attention:

" :+ " is not included in the case of a null value. " :- " " := " as long as there is a number, it contains a null value (null).

5. Length of the variable

${#File}

6. Array operations

A=(a b C def) ${a[@]} or ${a[*]} can get a B C def (all groups) ${a[0]} can get a (first group number), ${a[1]} Number of second groups ... ${#A [@]} or ${#A [4  (number of total groups) ${#A [01 (that is, the length of the first group (a), ${#A [3 3 (length of fourth Group (DEF))

The magical application of ${} in the shell