rsync is ideal for this scenario. It copies only changed files (efficient for daily runs), preserves permissions and timestamps (-a), provides verbose output (-v), and can clean up deleted files (–delete). Tar creates full archives daily, which is less efficient for small changes. A simple copy doesn’t handle deletions efficiently, and dd is for block-level copying, not directory trees.

The ‘Permission denied’ error for an executable in PATH typically indicates the file lacks the execute bit. ‘chmod u+x’ adds execute permission for the file’s owner, which is the correct and minimal privilege needed. Using sudo is unnecessary and bad practice for a script you own. Moving it defeats the purpose of a system-wide PATH. Adding sudo to the shebang is syntactically invalid and a major security anti-pattern.

The atd service is controlled by allow/deny files. If /etc/at.allow exists, only users listed in it can use the ‘at’ command. If the user isn’t listed, their jobs will be silently rejected, which matches the symptom. While checking if atd is running is good practice, a stopped daemon usually gives a more explicit error. Cron does not interfere with at. An alias would cause an immediate command error, not silent job ignoring.

The key requirements are: modified in last 48 hours (48*60 = 2880 minutes) and exclude symlinks (! -type l). Option 2 uses ‘-mmin -2880’ for minutes and ‘! -type l’ correctly. Option 1 uses ‘-mtime -2’ (2*24 hours) but does not exclude symlinks. Option 3’s syntax is incorrect for the time check. Option 4 searches for directories (-type d), not files, and uses a different time format.

The bash -x option (or set -x within the script) runs the script in debug mode, printing each command and its expanded arguments (showing variable values) to stderr before execution. This is exactly what’s needed to trace execution flow and variable expansion. ‘set -v’ (verbose) shows lines as read but does not expand variables. ‘bash -n’ only checks syntax. ‘bash -e’ exits on error but doesn’t provide tracing.

The ‘|&’ operator in Bash is a shortcut for ‘2>&1 |’, which merges stderr into stdout and pipes it to the next command. ‘tee’ then writes that combined stream to the file and to the terminal (its stdout). Option 1 pipes to tee but the syntax is correct for the older style. Option 2 incorrectly tries to redirect and pipe simultaneously. Option 3 runs the command in the background without terminal output.

This is the most robust method. ‘command -v nginx’ checks for the executable in PATH, a reliable test for an installed package. Using ‘-z’ checks if the result is empty. Before installing, it’s good practice to run ‘apt-get update’ to refresh package lists. The ‘-y’ flag provides non-interactive installation. Option 1 is fragile (grepping dpkg output). Option 2 lacks ‘-y’ and may fail on fresh systems. Option 3 uses simulation unnecessarily.

User-specific cron jobs are managed via the ‘crontab’ command and are stored in user-specific files (usually in /var/spool/cron/). The direct and correct way to view a user’s jobs is ‘crontab -l -u username’ (requires appropriate privileges). /etc/crontab and /etc/cron.d/ are for system-wide jobs. /etc/cron.hourly/ is for scripts run by the system’s cron daemon, not necessarily tied to a specific user’s schedule.

The safe way to handle arbitrary filenames (including spaces, newlines) is to use null characters as delimiters. ‘find … -print0’ outputs filenames terminated by null, and ‘xargs -0’ reads them correctly. This is the standard robust pattern. While quoting is crucial in shell scripts, it doesn’t solve the problem when find passes results via pipe or to xargs by default (which uses whitespace). Changing IFS or using -execdir doesn’t address the core parsing issue between find and xargs.

rsync is ideal for this scenario. It copies only changed files (efficient for daily runs), preserves permissions and timestamps (-a), provides verbose output (-v), and can clean up deleted files (–delete). Tar creates full archives daily, which is less efficient for small changes. A simple copy doesn’t handle deletions efficiently, and dd is for block-level copying, not directory trees.0