How to store a environment variable for use by another script without source

Question

I'm trying to create two scripts. One called backup, the other called cancel_backup. When backup is ran, it checks if a directory is available, if so it begins the backup. If not, it tries again every 30 seconds until the directory is found.

If cancel_backup is ran, the backup script should terminate.

Here's my code so far:

#/bin/bash!

backup_location='/media/tom/EXTERNAL/Backups/Areca'  

while [ ! -d $backup_location ]
do
  if [ ! $CANCEL_BACKUP ]
  then
    notify-send --icon=/home/tom/icons/dialog-warning.png "Backup Cancelled." "Backup has been cancelled by the user."
    exit 0
  else
    notify-send --icon=/home/tom/icons/dialog-error.png "Backup Directory Inaccessible" "External backup directory is inaccessible. Retrying in 30 seconds."
    sleep 30
  fi
done

notify-send --icon=/home/tom/icons/dialog-info.png "Beginning Backup" "External backup directory located, beginning backup."

I've attempted to use export, however the variable is still not accessible to any other scripts unless they I use source within the above script.

I need to be able to run the scripts independently, but still update $CANCEL_BACKUP from a different script.

Child processes cannot change variables of their parents, that's why you need to source. Maybe decide based on exit codes of your scripts? — PesaThe
– PesaThe, Commented Jan 12, 2018 at 2:01
Environment variables are only inherited by children, there's no way to propagate variables the other way. Use a file as a flag instead of a variable. — Barmar
– Barmar, Commented Jan 12, 2018 at 2:12

Barmar · Accepted Answer · 2018-01-12 02:29:53Z

2

Use a file as the flag rather than a variable.

cancel_file=/tmp/cancel_backup

if [ ! -f $cancel_file ]

The cancel_backup script can then do touch /tmp/cancel_backup. And you remove the file when you want to enable backups again.

/tmp might not be the best place for the file, since some other user could create the file there. Better would be a directory used specifically for the application, which only the application user has access to. The above was just a simple example.

edited Jan 12, 2018 at 2:29

answered Jan 12, 2018 at 2:15

Barmar

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6 Comments

Jonathan Leffler Over a year ago

I might argue about the location of the file in /tmp, but the rest is what I'd suggest doing. Given backup_location='/media/tom/EXTERNAL/Backups/Areca' in the main script, I'd suggest cancel_file='/media/tom/EXTERNAL/Backups/cancel_backup as a more appropriate location.

David C. Rankin Over a year ago

@JonathanLeffler Umm..., just curious, but what would you suggest? LSB would suggest /tmp for general temporary files, and it would avoid the system /run directory (or /var/run or /lib/run on some distros), but perhaps /usr/local/run setgid to a group for those that have privilege to run backup? (I'm thinking like a pid-file)

Jonathan Leffler Over a year ago

@DavidC.Rankin: I added a suggestion. The fixed name in /tmp is a security liability on a multi-user system (a rarity, but it is worth remembering them). Placing the fixed name near where the operations are occurring seems appropriate to me. Also, it isn't a temporary file; it is a control file. If it must go under /tmp, I'd think about how to make it safer — and less likely to interfere with other people also trying to run their own backups. Options include a file in a sub-directory under /tmp with appropriate permissions, but it can't be a fixed name, which complicates things.

Barmar Over a year ago

@JonathanLeffler I suspect /media/tom is a removable device (that's why the directory might or might not be avaiable), so it's probably not the right place for the flag. Or maybe it is -- if the device isn't mounted, the cancel file is not relevant.

David C. Rankin Over a year ago

I gotcha and concur. The run-control is what I was thinking. If only a single user runs the file, then the current directory is fine, but if it is run from a bin directory (either the users personal collection where more than one needs to be able to run it or /usr/local/bin,) then a local run/ex_backup dir could be created or a pid-file like file in a safer location.

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