Add folder name ($date) to top of log then cut line 21 from same log and add as a value in running script

Question

I am trying to create a script and I am stuck on the final part I want to add.

The script syncs some paths on my server to my google drive. If there are any deleted or modified files on my server it is moved to /old/date/ path on gdrive.

oldlog="/boot/logs/old-log.txt"
del=""
date=$(date +"%Y-%m-%d-%H%M")
--backup-dir "${dest}/old/${date}"

This works really well, but I have to delete files in /old/ manually and that is what I want to change.

So what I need is for $date to be written to $oldlog and always at line 1 in the log and move all other lines in the log one line down. The $date written to log has to match the $date in --backup-dir "${dest}/old/${date}" so they are identical.

So the log needs to work like this after xx day 1 day

2019-07-01-1800

2 day

2019-07-02-1800
2019-07-01-1800

3 day

2019-07-03-1800
2019-07-02-1800
2019-07-01-1800

So for the next part. After the log is updated like in the example above I need to grab line 21 from oldlog.txt, delete the line and add it to the script as $del.

oldlog="/boot/logs/old-log.txt"
del=
rclone purge "${dest}/old/${del}"

Now, if the line is empty I need it to be just a random text so it wont purge everything inside /old/ or maybe I more advanced approach is possible for it? Something like if del=empty then show message nothing to purge. If del=value from oldlog.txt then run rclone purge "${dest}/old/${del}"

oldlog="/boot/logs/old-log.txt"
del=
delempty=echo "nothing to purge today"
delvalue=rclone purge "${dest}/old/${del}"

Is any of this possible to make in a script without it being to complex?

Answer 1

You have four things you're trying to accomplish: 1) insert a line at the beginning of a file, 2) reading a specific line of a file, 3) conditionally branching if that specific line does not exist, and 4) deleting a specific line of a file.

You can use the stream editor utility sed to do all of this. I'll assume GNU sed in my answer, but the exact syntax may be slightly different with other versions of sed.

To insert the line at the beginning of the file:

sed -i "1i\${date}" ${oldlog}

The -i option tells sed to work on the file in place. 1 tells sed to only operate on the first line. i\ tells sed to insert before the line.

To retrieve the 21st line:

del="$(sed -n 21p "${oldlog}")"

-n puts sed in quiet mode only so the only output is what is explicitly printed. 21 is the line number and p is the print command.

To determine if there is a 21st line:

sed -n 21q1 ${oldlog}

Again -n puts sed in quiet mode, 21 selects the 21st line, q tells sed to quit, and 1 is the exit code.

To delete the 21st line:

sed -i 21d ${oldlog}

Again, -i to operate on the file in place, 21 selects line 21 and d deletes the line.

Putting it all together:

sed -i "1i\${date}" ${oldlog}
del="$(sed -n 21p "${oldlog}")"
sed -n 21q1 ${oldlog} || { echo "Deleting ${dest}/old/${del}" && rclone purge "${dest}/old/${del}" && sed -i 21d ${oldlog} }