When working with cron jobs on Linux systems, you might encounter permission-related errors when attempting to list or edit crontab entries. The specific errors we're examining here manifest as:

crontabs/user/: fopen: Permission denied
crontabs/user/: fdopen: Permission denied

These errors typically occur when there's a mismatch in permissions between the crontab binary, the spool directory, and the user's crontab file itself.

Three critical locations need proper permission settings:

1. The crontab binary:
$ ls -l /usr/bin/crontab
-rwxr-sr-x 1 root libuuid 35896 Aug 24  2010 /usr/bin/crontab

2. The crontab spool directory:
$ ls -ld /var/spool/cron/crontabs
drwx-wx--T 2 root libuuid 4096 Jan 10  2014 /var/spool/cron/crontabs

3. Individual crontab files:
$ ls -l /var/spool/cron/crontabs/
-rw------- 1 1017 libuuid 1120 Dec  3  2013 user
-rw------- 1 root libuuid 1342 Jan 10  2014 root

For proper operation, these should be the permission settings:

# crontab binary (typically correct by default)
chmod 2755 /usr/bin/crontab
chown root:libuuid /usr/bin/crontab

# crontab spool directory
chmod 1730 /var/spool/cron/crontabs
chown root:libuuid /var/spool/cron/crontabs

# individual crontab files (auto-managed by cron)
# Should maintain 600 permissions and proper ownership

Case 1: When the sticky bit is missing from the crontabs directory:

sudo chmod +t /var/spool/cron/crontabs

Case 2: When the group ownership is incorrect:

sudo chown :libuuid /var/spool/cron/crontabs

Case 3: When the directory permissions are too restrictive:

sudo chmod 1730 /var/spool/cron/crontabs

For system administrators managing multiple servers, here's a bash script to verify and fix crontab permissions:

#!/bin/bash

# Check and fix crontab binary permissions
CRONTAB_BIN="/usr/bin/crontab"
if [ "$(stat -c '%a' $CRONTAB_BIN)" != "2755" ]; then
    echo "Fixing crontab binary permissions..."
    sudo chmod 2755 $CRONTAB_BIN
    sudo chown root:libuuid $CRONTAB_BIN
fi

# Check and fix crontab directory permissions
CRONTAB_DIR="/var/spool/cron/crontabs"
if [ "$(stat -c '%a' $CRONTAB_DIR)" != "1730" ]; then
    echo "Fixing crontab directory permissions..."
    sudo chmod 1730 $CRONTAB_DIR
    sudo chown root:libuuid $CRONTAB_DIR
fi

# Verify individual crontab files
for file in $CRONTAB_DIR/*; do
    if [ -f "$file" ] && [ "$(stat -c '%a' "$file")" != "600" ]; then
        echo "Fixing permissions for $file"
        sudo chmod 600 "$file"
    fi
done

• Use strace crontab -l to see exactly where the permission check fails
• Check system logs: grep cron /var/log/syslog
• Verify your user's group membership with groups command
• Test with root user to isolate permission issues: sudo crontab -u username -l

To avoid these issues in the future:

1. Regularly audit cron-related permissions
2. Include permission checks in your server provisioning scripts
3. Monitor cron job execution in your system monitoring
4. Document the proper permission settings for your team

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The crontab system relies on several key permission settings to function properly:

/usr/bin/crontab permissions:
-rwxr-sr-x 1 root libuuid

/var/spool/cron/ structure:
drwxr-xr-x 3 root root
drwx-wx--T 2 root libuuid crontabs/

When you encounter the "fopen: Permission denied" error during crontab operations, it typically indicates one of these scenarios:

• The sticky bit (T) on the crontabs directory is set incorrectly
• The user lacks proper group membership (should be in libuuid group)
• The individual crontab file permissions are corrupted

First, verify the user's group membership:

groups username
id username

If the user isn't in the libuuid group, add them:

sudo usermod -a -G libuuid username

Next, check and correct directory permissions:

sudo chmod 1730 /var/spool/cron/crontabs
sudo chown root:libuuid /var/spool/cron/crontabs

After making these changes, test crontab access:

sudo -u username crontab -l

If successful, you should see the user's crontab contents or an empty file.

To avoid future permission issues:

# Create a maintenance script
#!/bin/bash
chmod 1730 /var/spool/cron/crontabs
chown root:libuuid /var/spool/cron/crontabs
chmod g+s /usr/bin/crontab

Schedule this script to run periodically or include it in your system maintenance routines.