Not sure if cron job is run

Question

I have the following cron job in /etc/cron.d/backup:

*/1 * * * * backupbot /home/backupbot/bin/backup-script.sh

Basically, I want the backup-script.sh to run every minute (and the user backupbot should be executing the job).

The /home/backupbot/bin/backup-script.sh file is owned by backupbot (and he has "x" permission on it). The file looks as follows:

#!/bin/bash
set -e
{
BACKUP_DIR=/var/app/backups
STORAGE_ACCOUNT_URL=https://myserver/backups
BACKUP_FILE=$(ls $BACKUP_DIR -t | head -1)
if [ -z "$BACKUP_FILE" ]; then
    echo "There are no backups to synchronize"
    exit 0
fi
azcopy login --identity
azcopy copy $BACKUP_DIR/$BACKUP_FILE $STORAGE_ACCOUNT_URL/$BACKUP_FILE
} >/tmp/cron.backup-script.$$ 2>&1

Normally, any output should be logged into /tmp/cron.backup-script.xxxx. Such a file is never created.

The only evidence that the job is being noticed by Cron is the following output of systemctl status cron.service:

● cron.service - Regular background program processing daemon
   Loaded: loaded (/lib/systemd/system/cron.service; enabled; vendor preset: enabled)
   Active: active (running) since Tue 2021-04-13 09:59:08 UTC; 6h ago
     Docs: man:cron(8)
 Main PID: 1086 (cron)
    Tasks: 1 (limit: 4915)
   CGroup: /system.slice/cron.service
           └─1086 /usr/sbin/cron -f
Apr 13 16:00:01 my-vm CRON[17201]: pam_unix(cron:session): session closed for user root
Apr 13 16:00:01 my-vm CRON[17198]: pam_unix(cron:session): session closed for user root
Apr 13 16:00:01 my-vm CRON[17199]: pam_unix(cron:session): session closed for user root
Apr 13 16:01:01 my-vm CRON[17402]: pam_unix(cron:session): session opened for user root by (uid=0)
Apr 13 16:01:01 my-vm CRON[17403]: (root) CMD ([ -f /etc/krb5.keytab ] && [ ( ! -f /etc/opt/omi/creds/omi.keytab ) -o ( /etc/krb5.keytab -nt /etc/opt/omi/creds/omi.keytab ) ] &&
Apr 13 16:01:01 my-vm CRON[17401]: pam_unix(cron:session): session opened for user backupbot by (uid=0)
Apr 13 16:01:01 my-vm CRON[17404]: (backupbot) CMD (/home/backupbot/bin/backup-script.sh)
Apr 13 16:01:01 my-vm CRON[17402]: pam_unix(cron:session): session closed for user root
Apr 13 16:01:01 my-vm CRON[17401]: (CRON) info (No MTA installed, discarding output)
Apr 13 16:01:01 my-vm CRON[17401]: pam_unix(cron:session): session closed for user backupbot

It mentions something about sessions for backupbot. How can I investigate further?

Answer 1

According to comments, the script is only executable by the owner. I.e., it is not readable. This stops the owner from executing the script.

Example:

$ chmod 500 script
$ ls -l script
-r-x------ 1 myself myself 24 Apr 14 09:21 script
$ ./script
hello

$ chmod 100 script
$ ls -l script
---x------ 1 myself myself 24 Apr 14 09:21 script
$ ./script
/bin/bash: ./script: Permission denied

If the script file is not readable, it can't be read by the bash shell interpreter (which is running as the current user).

Make the script both executable and readable.

---

Apart from the above, consider using proper double quoting of variable expansions, and don't parse the output from ls (it's only really useful to look at).

Your script, modified:

#!/bin/bash
backup_dir=/var/app/backups
storage_account_url=https://myserver/backups
{
    # Find most recently modified file in "$backup_dir".
    # Assumes that there are only files there, no subdirectories.
set -- "$backup_dir"/*
backup_file_path=$1
shift

for pathname do
    if [ "$pathname" -nt "$backup_file_path" ]; then
        backup_file_path=$pathname
    fi
done

if [ ! -e "$backup_file_path" ]; then
    echo 'There are no backups to synchronize'
    exit 1
fi

# Perform backup.

azcopy login --identity || exit 1
azcopy copy "$backup_file_path" "$storage_account_url/$(basename "$backup_file_path")"


} >/tmp/cron.backup-script.$$ 2>&1

Answer 2

Jobs run through cron, or at, or batch, aren't run in the same runtime environment that you have on your desktop. None of your PATH changes, or other environment variable settings are automatically propagated to your cron job. For example, there's no $DISPLAY, so GUI programs need special treatment (read man xhost).

One can set environment variables for all one's cron jobs in the crontab file Read man 5 crontab.

Look at the results of echo "=== set ===";set;echo "=== env ===";env | sort;echo "=== alias ===";alias in each of your environments.

Since the command part of the crontab line is, by default, interpreted by /bin/sh, which has a simpler syntax than /bin/bash, I recommend having command be a call to a bash script (executable, mounted, starts with #!/bin/bash) which sets up the environment, then calls the desired program.

Always use absolute paths (paths starting at /, the root directory) in scripts run through cron, at, batch.