Linux Shell ${} simple usage

Suppose we define a variable as:

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

We can replace each other with ${} to get different values:

${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 one. And the string to the left: file.txt

${file##*.} : Take out the last one. And the string to the left: txt

${file%/*}: Take off the last bar/its right string:/dir1/dir2/dir3

${file%%/*}: Remove the first/its right string: (null value)

${FILE%.*}: Take off the last one. And the string to the right:/dir1/dir2/dir3/my.file

${FILE%%.*}: Take out the first one. And the string to the right:/dir1/dir2/dir3/my

The methods of memory are:

# is to remove the left side (on the plate # on the left of the $)

% is removed to the right (on the plate% on the right of the $)

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

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

${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

With ${} You can also 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 to return the value. (null value and non-

Null value is not processed)

${file:-my.file.txt}: If the $file is not set or null, use My.file.txt to return the value. (

Non-null value is not processed)

${file+my.file.txt}: If the $file is set to a null value or a non-null value, the value is returned using My.file.txt. (No

Not processed when set)

${file:+my.file.txt}: If $file is a non-null value, use My.file.txt as the return value. (Not set and empty

Value is not processed)

${file=my.file.txt}: If $file is not set, use My.file.txt to return the value, while assigning $file

The value is 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 to return the value, and

The $file is assigned a value of My.file.txt. (Non-null value is not processed)

${file?my.file.txt}: If the $file is not set, the My.file.txt output to stderr. (null value and non-

Null value is not processed)

${file:?my.file.txt}: If the $file is not set or null, the my.file.txt output to STDERR. (

Non-null value is not processed)

The above understanding is that you must distinguish between Chu unset and null and non-null these three kinds of assignment states.

In general,: null is not affected if not with: null is also affected if the band: null.

And oh, ${#var} to calculate the length of the variable value:

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

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Linux Shell ${} simple usage