How do I list all cron jobs for all users?

You would have to run this as root, but:

for user in $(cut -f1 -d: /etc/passwd); do crontab -u $user -l; done

will loop over each user name listing out their crontab. The crontabs are owned by the respective users so you won't be able to see another user's crontab w/o being them or root.

Editif you want to know which user a crontab belongs to, use echo $user

for user in $(cut -f1 -d: /etc/passwd); do echo $user; crontab -u $user -l; done

I ended up writing a script (I'm trying to teach myself the finer points of bash scripting, so that's why you don't see something like Perl here). It's not exactly a simple affair, but it does most of what I need. It uses Kyle's suggestion for looking up individual users' crontabs, but also deals with /etc/crontab (including the scripts launched by run-parts in /etc/cron.hourly, /etc/cron.daily, etc.) and the jobs in the /etc/cron.d directory. It takes all of those and merges them into a display something like the following:

mi     h    d  m  w  user      command09,39  *    *  *  *  root      [ -d /var/lib/php5 ] && find /var/lib/php5/ -type f -cmin +$(/usr/lib/php5/maxlifetime) -print0 | xargs -r -0 rm47     */8  *  *  *  root      rsync -axE --delete --ignore-errors / /mirror/ >/dev/null17     1    *  *  *  root      /etc/cron.daily/apt17     1    *  *  *  root      /etc/cron.daily/aptitude17     1    *  *  *  root      /etc/cron.daily/find17     1    *  *  *  root      /etc/cron.daily/logrotate17     1    *  *  *  root      /etc/cron.daily/man-db17     1    *  *  *  root      /etc/cron.daily/ntp17     1    *  *  *  root      /etc/cron.daily/standard17     1    *  *  *  root      /etc/cron.daily/sysklogd27     2    *  *  7  root      /etc/cron.weekly/man-db27     2    *  *  7  root      /etc/cron.weekly/sysklogd13     3    *  *  *  archiver  /usr/local/bin/offsite-backup 2>&132     3    1  *  *  root      /etc/cron.monthly/standard36     4    *  *  *  yukon     /home/yukon/bin/do-daily-stuff5      5    *  *  *  archiver  /usr/local/bin/update-logs >/dev/null

Note that it shows the user, and more-or-less sorts by hour and minute so that I can see the daily schedule.

So far, I've tested it on Ubuntu, Debian, and Red Hat AS.

#!/bin/bash# System-wide crontab file and cron job directory. Change these for your system.CRONTAB='/etc/crontab'CRONDIR='/etc/cron.d'# Single tab character. Annoyingly necessary.tab=$(echo -en "\t")# Given a stream of crontab lines, exclude non-cron job lines, replace# whitespace characters with a single space, and remove any spaces from the# beginning of each line.function clean_cron_lines() {    while read line ; do        echo "${line}" |            egrep --invert-match '^($|\s*#|\s*[[:alnum:]_]+=)' |            sed --regexp-extended "s/\s+/ /g" |            sed --regexp-extended "s/^ //"    done;}# Given a stream of cleaned crontab lines, echo any that don't include the# run-parts command, and for those that do, show each job file in the run-parts# directory as if it were scheduled explicitly.function lookup_run_parts() {    while read line ; do        match=$(echo "${line}" | egrep -o 'run-parts (-{1,2}\S+ )*\S+')        if [[ -z "${match}" ]] ; then            echo "${line}"        else            cron_fields=$(echo "${line}" | cut -f1-6 -d' ')            cron_job_dir=$(echo  "${match}" | awk '{print $NF}')            if [[ -d "${cron_job_dir}" ]] ; then                for cron_job_file in "${cron_job_dir}"/* ; do  # */ <not a comment>                    [[ -f "${cron_job_file}" ]] && echo "${cron_fields} ${cron_job_file}"                done            fi        fi    done;}# Temporary file for crontab lines.temp=$(mktemp) || exit 1# Add all of the jobs from the system-wide crontab file.cat "${CRONTAB}" | clean_cron_lines | lookup_run_parts >"${temp}" # Add all of the jobs from the system-wide cron directory.cat "${CRONDIR}"/* | clean_cron_lines >>"${temp}"  # */ <not a comment># Add each user's crontab (if it exists). Insert the user's name between the# five time fields and the command.while read user ; do    crontab -l -u "${user}" 2>/dev/null |        clean_cron_lines |        sed --regexp-extended "s/^((\S+ +){5})(.+)$/\1${user} \3/" >>"${temp}"done < <(cut --fields=1 --delimiter=: /etc/passwd)# Output the collected crontab lines. Replace the single spaces between the# fields with tab characters, sort the lines by hour and minute, insert the# header line, and format the results as a table.cat "${temp}" |    sed --regexp-extended "s/^(\S+) +(\S+) +(\S+) +(\S+) +(\S+) +(\S+) +(.*)$/\1\t\2\t\3\t\4\t\5\t\6\t\7/" |    sort --numeric-sort --field-separator="${tab}" --key=2,1 |    sed "1i\mi\th\td\tm\tw\tuser\tcommand" |    column -s"${tab}" -trm --force "${temp}"

Under Ubuntu or debian, you can view crontab by /var/spool/cron/crontabs/ and then a file for each user is in there. That's only for user-specific crontab's of course.

For Redhat 6/7 and Centos, the crontab is under /var/spool/cron/.