${in Shell}, # #和%% use example _linux shell

Today read a script file when there are some places do not understand, find an article to read some, feel good, keep down.

Let's say we've defined a variable as:

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File=/dir1/dir2/dir3/my.file.txt

You can replace each of the different values with ${}:

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${file#*/}: Delete the first/and left string: dir1/dir2/dir3/my.file.txt
${file##*/}: Delete the last/and left string: my.file.txt
${file#*.}  : Delete the first one. And the string to the left: file.txt
${file##*.}  : Delete the last one. And the string to the left: txt
${file%/*}: Delete the last/And right string:/dir1/dir2/dir3
${file%%/*}: Delete the first/and the right string: (null value)
${file%.*}: Delete the last one. And the string to the right:/dir1/dir2/dir3/my.file
${file%%.*}: Delete the first one. And the string to the right:/dir1/dir2/dir3/my

The method of memory is:

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# is to remove the left (on the keyboard # on the left of the $)
% is to remove the right (on the keyboard% on the right side of the $)
A single symbol is a minimum match; two symbols is the maximum match
${file:0:5}: Extract leftmost 5 bytes:/dir1
${file:5:5}: Fetches 5 consecutive bytes to the right of the 5th byte:/dir2

You can also replace the string in the value of the variable:

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${file/dir/path}: Replaces the first dir with the Path:/path1/dir2/dir3/my.file.txt
${file//dir/path}: Replace all dir with Path:/path1/path2/path3/my.file.txt

${} can also be assigned to different variable state (no set, null value, Non-null value):

${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 value of the return. (non-null values are not processed)
${file+my.file.txt}: If $file is set to null or Non-null values, use My.file.txt as the return value. (not to be processed when not set)
${file:+my.file.txt}: If the $file is a non-null value, the my.file.txt is used as the return value. (not set and null value when not processed)
${file=my.file.txt}: If the $file is not set, then use My.file.txt as the return value, at the same time will $file the value of My.file.txt. (null and Non-null values are not processed)
${file:=my.file.txt}: If $file is not set or NULL, the my.file.txt is used as the return value, and the $file value is my.file.txt. (non-null values are not processed)
${file?my.file.txt}: If $file is not set, the my.file.txt is 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 is exported to STDERR. (non-null values are not processed)
${#var} to calculate the length of the variable value:

${#file} can get 27 because/dir1/dir2/dir3/my.file.txt is 27 bytes
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