CRON bash execution result differs from manual launch [duplicate] - linux

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Run docker command in bash file and run using crontab
(1 answer)
Closed 5 years ago.
I use GOGS as a personal Git server (using Docker image). I made a backup script which runs fine when I launch it manually, but when CRON lauches it, seems to malfunction as backups are strictly identical to previous one.
IMPORTANT PRECISION : usual problems like this are related to $PATH that might be incomplete, but script below only uses docker, ls, head or xargs commands which are under /usr/bin or /usr (so in $PATH).
Example results on 3 weeks (CRON once a week) :
root#1:~# ll /path/to/backup/
total 1724864
-rw-r--r-- 1 root root 588742750 janv. 29 11:30 gogs_2018_01_29.zip
-rw-r--r-- 1 root root 588742750 janv. 29 11:30 gogs_2018_02_02.zip
-rw-r--r-- 1 root root 588742750 janv. 29 11:30 gogs_2018_02_09.zip
Even system dates are identical... While date setup by bash are good related on execution. I don't get it.
After manual launch :
-rw-r--r-- 1 root root 588742750 janv. 29 11:30 gogs_2018_01_29.zip
-rw-r--r-- 1 root root 588742750 janv. 29 11:30 gogs_2018_02_02.zip
-rw-r--r-- 1 root root 589503781 févr. 9 11:21 gogs_2018_02_09.zip
Last backup is bigger, dates matches... Worked.
Why do I have this malfunction on CRON launch only?
Script gogs_backup:
#!/bin/bash
# Make backup
docker exec -i -t gogs_1 /app/gogs/gogs backup --target /tmp --archive-name gogs.zip
# Build file name
now=$(date +"%Y_%m_%d")
file="/path/to/backup/gogs_$now.zip"
# Copy file
docker cp gogs_1:/tmp/gogs.zip $file
# Remove everything from /tmp dir
docker exec -i -t gogs_1 rm -rf /tmp/gogs*
# Keep only 3 in destination folder
ls -drt /path/to/backup/* | head -n-3 | xargs rm
CRON line :
0 3 * * 5 bash /path/to/gogs_backup

Answer 1

From the exec documentation
The docker exec command runs a new command in a running container.
And from the cp documentation
The CONTAINER can be a running or stopped container.
So my guess is that docker exec fails because the container is stoped, but cp works because it works on stoped containers.
Check your logs. And check your return values, there are more bugs in this script.

Answer 2

Thanks to #mncl post, I decided to have a look again on docker commands documentation.
I spotted here that I runned docker exec -i -t gogs_1 /app/gogs/gogs backup --target /tmp --archive-name gogs.zip.
-i seemed useless to me, as it is a CRON launch. I removed it and the CRON launched script runned well.
Nonetheless, the Docker doc about -i puzzle me out. Why keeping STDIN attached to command would be a problem in this case?