So, I am building a bash script which iterates through folders named by numbers from 1 to 9. The script depends on getting the folder names by user input. My intention is to use a for loop using read input to get a folder name or a range of folder names and then do some stuff.
Example:
Let's assume I want to make a backup with rsync -a of a certain range of folders. Usually I would do:
for p in {1..7}; do
rsync -a $p/* backup.$p
done
The above would recursively backup all content in the directories 1 2 3 4 5 6 and 7 and put them into folders named as 'backup.{index-number}'. It wouldn't catch folders/files with a leading . but that is not important right now.
Now I have a similar loop in an interactive bash script. I am using select and case statements for this task. One of the options in case is this loop and it shall somehow get a range of numbers from user input. This now becomes a problem.
Problem:
If I use read to get the range then it fails when using {1..7} as input. The input is taken literally and the output is just:
{1..7}
I really would like to know why this happens. Let me use a more descriptive example with a simple echo command.
var={1..7} # fails and just outputs {1..7}
for p in $var; do echo $p;done
read var # Same result as above. Just outputs {1..7}
for p in $var; do echo $p;done
for p in {1..7}; do echo $p;done # works fine and outputs the numbers 1-7 seperated with a newline.
I've found a workaround by storing the numbers in an array. The user can then input folder names seperated by a space character like this: 1 2 3 4 5 6 7
read -a var # In this case the output is similar to the 3rd loop above
for p in ${var[@]}; do echo $p; done
This could be a way to go but when backing up 40 folders ranging from 1-40 then adding all the numbers one-by-one completely makes my script redundant. One could find a solution to one of the millennium problems in the same time.
Is there any way to read a range of numbers like {1..9} or could there be another way to get input from terminal into the script so I can iterate through the range within a for-loop?
This sounds like a question for google but I am obviously using the wrong patterns to get a useful answer. Most of similar looking issues on SO refer to brace and parameter expansion issues but this is not exactly the problem I have. However, to me it feels like the answer to this problem is going in a similar direction. I fail to understand why when a for-loop for assigning {1..7} to a variable works but doing the same like var={1..7} doesn't. Plz help -.-
EDIT: My bash version:
$ echo $BASH_VERSION
4.2.25(1)-release
EDIT2: The versatility of a brace expansion is very important to me. A possible solution should include the ability to define as many ranges as possible. Like I would like to be able to choose between backing up just 1 folder or a fixed range between f.ex 4-22 and even multiple options like folders 1,2,5,6-7
Copyright Notice：Content Author:「tobi78」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/38313141/read-range-of-numbers-into-a-for-loop