How to subsitute contents of a file contained within the output of a variable

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In the end I went with

for i in root*/ do if [ "$i" != 'root*/' ] then sudo touch "$i"/test fi done 

Using sudo here cuz those snapshot folders are owned by root.

So now I understand my first step which is to create, rename, delete, change permissions, etc. on a file within a list of folders returned inside a variable.

I am now bringing this a step deeper, which is to edit a file within each of the folders returned by the variable, and replace a fixed value by a variable one which equals the value returned by the variable at the start.

I can't understand how to integrate a variable in the value I want to replace.

I changed my snapshots structure to make the script easier (for me). So now the variable listing all folders within a folder returns the following:

echo $SNAPSHOTS 2020-12-22_19H05/ 2020-12-23_19H05/ 2020-12-24_19H05/ 2020-12-25_19H05/ 2020-12-26_19H05/ 2020-12-27_19H05/ 2020-12-28_19H05/ 

Within each of those folders, for which the directory names change every day (oldest day gets deleted and today gets added), there is a file located at /etc/fstab, so 2020-12-22_19H05/etc/fstab as an example.

I need to edit that file, search for the fixed value "@home" and replace it with the variable value part of a fixed prefix string, like:

@snapshots/daily/home/$VARIABLE 

The $VARIABLE = the directory names returned by the first variable listed above.

So within folder 2020-12-22_19H05/ the file /etc/fstab would be edited to replace @home by @snapshots/daily/home/2020-12-22_19H05 without the / at the end, it needs to be omited/truncated. And so on for all folders found by the first variable.

I am expecting sed and possibly % to be used, but I am really out of clue as to what to search on internet to 1-get an answer but mostly 2-understand the answer.

Thanks!

Probably SNAPSHOTS contains more than one directory.

You can see the problem when you insert a line set -x before the echo command. This will show the expansion of $SNAPSHOTS.

Use a loop to call the echo command for every matching directory.

for i in root*/ do if [ "$i" != 'root*/' ] then touch "$i"/test fi done 

The additional if avoids running touch 'root*/test' in case there is no matching directory.

Your SNAPSHOTS variable contains spaces. To redirect to its value, double quote it:

echo > "$SNAPSHOTS"/test 

This will most likely not do what you want as you seems to want to store multiple path in SNAPSHOTS. You can't store multiple separate strings in one string and think you can retrieve them back out of that single string correctly. It should really be an array and you don't need ls at all here:

shopt -s nullglob SNAPSHOTS=(root*/) for snap in "${SNAPSHOTS[@]}"; do echo >"$snap"/test done 

or just

shopt -s nullglob for snap in root*/; do echo >"$snap"/test done 

Setting the nullglob shell option ensures that the pattern is removed completely if it doesn't match anything. Without nullglob, the pattern is retained as is if it doesn't match anything.

If it's not many thousands of files that you want to put an empty line into, you could also do

shopt -s nullglob SNAPSHOTS=(root*/) echo | tee "${SNAPSHOTS[@]/%//test}" >/dev/null 

If you really just want to create the files (not write an empty line into them), then use touch:

shopt -s nullglob SNAPSHOTS=(root*/) touch "${SNAPSHOTS[@]/%//test}" 2>/dev/null