A script run by a timer inherits nothing you set up for yourself

"It works when I run it" is the most expensive sentence in operations, because it is true. The person saying it is logged in, in an interactive shell that has read a start-up file, sitting in a directory they chose, with a PATH somebody extended years ago. The timer that runs the same script at five in the morning has none of that: a short PATH, the root directory as its working directory, no terminal, and no start-up files at all.

nightly-report depends on two of those things without saying so — a helper found only through a PATH added in root's .bashrc, and a configuration file found only because the same .bashrc changes into /opt/reports. On top of that it cannot fail: whatever goes wrong, it empties the report and exits 0.

What you should be able to do after this

List what a systemd service inherits — environment, working directory, start-up files — and how that differs from an interactive login shell.
Explain when bash reads .bashrc, .profile and /etc/environment, and why none of them reaches a timer's run.
Write a script that names everything it needs: an explicit PATH, absolute paths, configuration by absolute default.
Explain the search rule of the . (source) command for a name without a slash.
Keep the previous output when a run fails, by writing to a temporary file and renaming it.
Reproduce a timer's environment by hand with env -i and systemd-run.

The mechanism

Two kinds of shell, and what each one reads

Bash decides which start-up files to read from how it was started. A login shell reads /etc/profile and the first of ~/.bash_profile, ~/.bash_login, ~/.profile. An interactive non-login shell reads ~/.bashrc. A non-interactive shell — bash script, bash -c '…', the interpreter named in a script's #! line — reads neither (only a file named in $BASH_ENV, if set).

Ubuntu's default ~/.bashrc also begins by checking whether the shell is interactive and returning immediately if it is not. Anything appended to the end of the file, as the PATH for the reporting tools was, exists only in shells where a person is typing.

What a service gets instead

systemd starts a service's process directly, not through a shell. The environment is built from the manager's own small default — a PATH of the standard system directories — plus whatever the unit sets with Environment= or EnvironmentFile=. The working directory is / unless WorkingDirectory= says otherwise, and HOME cannot be relied on — on this machine it arrives empty. There is no terminal. /etc/environment is read by PAM for logins, not by systemd for services.

env -i reproduces the empty end of that range from any shell: it starts a program with no environment at all. systemd-run --wait --pipe runs a command as a transient service, so it gets exactly what a real unit would.

How a script finds a command

A command without a slash is searched for in each directory of PATH, in order. When PATH is unset, bash substitutes a built-in default of standard directories. A helper installed in /opt/reports/tools/bin is therefore found only by callers whose PATH includes that directory, which is to say only by the shells that read that one .bashrc.

A script that must run anywhere sets its own PATH at the top, or calls its helpers by absolute path. Both make the dependency visible in the script, instead of in someone's shell configuration.

How `.` finds a file

. report.conf does not simply read ./report.conf. For a name with no slash, bash first searches PATH for a readable file of that name, and only then (outside POSIX mode) looks in the current directory. Under the timer the current directory is /, so the configuration is not found at all; in an unlucky PATH, a different report.conf could be found instead. . "$conf" with an absolute default removes both surprises.

A redirection happens first

In { …; } > "$out", the shell opens $out for writing — truncating it — before the commands in the group run. If they then fail, the report is already gone. Writing to a temporary file in the same directory and renaming it over the report with mv only when everything worked means a failure leaves yesterday's report in place, and a reader never sees a half-written one. rename(2) within one file system is atomic.

Exit status, again

Nothing in the original script checks that the configuration was read, and its last command is echo "report written…", which succeeds. So the service's main process exits 0, and systemd records Result=success for a run that produced an empty report. set -euo pipefail plus an explicit test of the configuration file turn "nothing to report" into a failure the unit can show.

A failure, walked through

Run on the lab's own machine: ubuntu-26.04-devops, GNU bash 5.3.9, before any change.

Starting the service the way the timer does, and reading its journal:

$ sudo systemctl start nightly-report.service; sudo journalctl -u nightly-report.service -b --no-pager -o cat | tail -4
/opt/reports/bin/nightly-report: line 4: report.conf: No such file or directory
report written to /var/lib/reports/latest.txt
nightly-report.service: Deactivated successfully.
Finished nightly-report.service - Disk use per project, for the morning mail.

The script told the journal what went wrong and then told systemd that all was well. The report it left is a header and nothing else. In an interactive shell, the same script works:

$ sudo bash -ic '/opt/reports/bin/nightly-report' 2>/dev/null; sudo cat /var/lib/reports/latest.txt
report written to /var/lib/reports/latest.txt
Disk use per project, 2026-09-16
/srv/projects/alpha                             300 KiB
/srv/projects/beta                              124 KiB

A non-interactive shell as root — no -i — fails exactly like the timer, and the start of root's .bashrc shows why:

$ sudo bash -c '/opt/reports/bin/nightly-report' 2>&1; echo "exit status: $?"
/opt/reports/bin/nightly-report: line 4: report.conf: No such file or directory
report written to /var/lib/reports/latest.txt
exit status: 0
$ sudo head -12 /root/.bashrc | grep -n 'interactive' -A3
5:# If not running interactively, don't do anything
6-[ -z "$PS1" ] && return
7-

What a service actually starts with can be seen directly, with a transient unit:

$ sudo systemd-run --wait --pipe -q sh -c 'echo "PATH=$PATH"; pwd; echo "HOME=${HOME-unset}"'
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin
/
HOME=

No /opt/reports/tools/bin, and / as the working directory. The search rule of . is easy to demonstrate as well — a report.conf found through PATH wins over none at all:

$ cd /tmp && echo 'echo found the one in PATH' > /tmp/report.conf && PATH=/tmp:$PATH bash -c '. report.conf'
found the one in PATH

Finally, what a missing configuration does to the previous report:

$ sudo sh -c 'echo "Disk use per project, yesterday" > /root/old.txt; REPORT_CONF=/nonexistent REPORT_OUT=/root/old.txt /opt/reports/bin/nightly-report 2>&1; echo "exit status: $?"; wc -c /root/old.txt'
/opt/reports/bin/nightly-report: line 4: /nonexistent: No such file or directory
report written to /root/old.txt
exit status: 0
33 /root/old.txt

Yesterday's 32-byte report was replaced by today's header, and the exit status is still 0.

After the fix — PATH set inside the script, the configuration by absolute default and checked before use, the report written to a temporary file and renamed — an empty environment is enough:

$ sudo env -i REPORT_OUT=/tmp/new.txt /opt/reports/bin/nightly-report; echo "exit status: $?"; cat /tmp/new.txt
report written to /tmp/new.txt
exit status: 0
Disk use per project, 2026-09-16
/srv/projects/alpha                             300 KiB
/srv/projects/beta                              124 KiB
$ sudo sh -c 'echo "Disk use per project, yesterday" > /root/old.txt; REPORT_CONF=/nonexistent REPORT_OUT=/root/old.txt /opt/reports/bin/nightly-report 2>&1; echo "exit status: $?"; cat /root/old.txt'
nightly-report: cannot read /nonexistent
exit status: 1
Disk use per project, yesterday

Common wrong turns

Adding the PATH to another start-up file — /etc/profile, /etc/environment, root's .profile. None of them is read by a service. The script still depends on its caller.
Fixing only the unit. Environment=PATH=… and WorkingDirectory=/opt/reports make the timer's run work, and the next person to run the script from cron, from Ansible or from another directory meets the same failure. Setting them in the unit as well is fine; relying on them is not.
cd "$(dirname "$0")" at the top. It finds the directory of the script, /opt/reports/bin, which is not where the configuration lives, and $0 is not reliable when a script is sourced or started through a symlink.
Testing with sudo -i. An interactive login shell is the one environment the timer never has. Test with env -i and systemd-run.
set -e alone. It stops the script at a failed ., before the report is touched — but a failure inside the redirected group, such as a helper that is not found, comes after the report has been truncated. The temporary file is what keeps yesterday's report in every case.
Trusting Result=success. It reports an exit status. Read the report, or make the script's exit status mean something.

Cheat sheet

# at the top of a script that runs unattended
set -euo pipefail
PATH=/opt/reports/tools/bin:/usr/sbin:/usr/bin:/sbin:/bin
conf=${REPORT_CONF:-/opt/reports/report.conf}
[ -r "$conf" ] || { echo "cannot read $conf" >&2; exit 1; }
. "$conf"

# replace output only on success
tmp=$(mktemp "$out.XXXXXX"); trap 'rm -f -- "$tmp"' EXIT
generate > "$tmp" && mv -f -- "$tmp" "$out"

env -i /path/to/script                        # nothing inherited
sudo systemd-run --wait --pipe -q cmd         # exactly what a service gets
systemctl show unit -p Result -p ExecMainStatus
sudo journalctl -u unit -b -o cat             # what the run printed

In a unit, when it has to: Environment=, EnvironmentFile=, WorkingDirectory=, User=.

Going deeper

man 1 bash, INVOCATION, for exactly which files each kind of shell reads.
man 5 systemd.exec, Environment variables in spawned processes and WorkingDirectory=.
man 1 env and man 1 systemd-run, for reproducing an unattended environment by hand.
The monitoring journal, a check is an exit status.

Review

Which of root's start-up files does a service started by systemd read?

None. systemd starts the process directly, with its own environment; .bashrc, .profile and /etc/environment belong to shells and logins.
Why did the report work in sudo bash -ic but not in sudo bash -c?

Only the interactive shell reads .bashrc, and Ubuntu's .bashrc returns early in a non-interactive shell, before the lines that extend PATH and change directory.
What is a service's working directory unless the unit says otherwise?

The root directory, /.
Where does . report.conf look for the file?

In the directories of PATH first, then (outside POSIX mode) in the current directory.
Why was yesterday's report emptied even though the run failed?

The output redirection truncates the file before the commands that fill it run; writing to a temporary file and renaming it on success keeps the old one.
How do you run a command with exactly the environment a systemd service would have?

systemd-run --wait --pipe runs it as a transient service; env -i gives the empty-environment extreme from any shell.