Assuming a modern Linux host, is there any convenient way to spawn an interactive shell in the exact context systemd would use for a unit/service?
Alternatively, any way to attach a shell to a running process started by systemd?
The goal is to interactively test restrictions and capabilities, starting env, working path, etc.
systemd v258 added a new unit-shell verb to the systemd-analyze command that does exactly this:
systemd-analyze unit-shell SERVICE [command...]
The given command runs on the namespace of the specified running service. If no command is given, spawn and attach a shell with the namespace to the service.
Demonstration:
# cat <<EOF > /etc/systemd/system/my-test-unit.service [Service] Type=exec ExecStart=/usr/bin/sleep infinity TemporaryFileSystem=/etc/ BindPaths=/usr/lib/systemd/systemd:/etc/some-folder/some-file EOF # systemctl daemon-reload && systemctl restart my-test-unit.service # /etc/some-folder/some-file --version bash: /etc/some-folder/some-file: No such file or directory # systemd-analyze unit-shell my-test-unit.service > /etc/some-folder/some-file --version systemd 258 (258.2-1.fc43) +PAM +AUDIT +SELINUX -APPARMOR +IMA +IPE +SMACK +SECCOMP -GCRYPT +GNUTLS +OPENSSL +ACL +BLKID +CURL +ELFUTILS +FIDO2 +IDN2 -IDN -IPTC +KMOD +LIBCRYPTSETUP +LIBCRYPTSETUP_PLUGINS +LIBFDISK +PCRE2 +PWQUALITY +P11KIT +QRENCODE +TPM2 +BZIP2 +LZ4 +XZ +ZLIB +ZSTD +BPF_FRAMEWORK +BTF +XKBCOMMON +UTMP +SYSVINIT +LIBARCHIVE > exit # /etc/some-folder/some-file --version bash: /etc/some-folder/some-file: No such file or directory Note that systemd v258 was only released a few months ago (September 2025), so you'll probably have to wait a bit until this is available in whichever distro you use.
Use systemd-run with the properties copied out of the .service file into -p options. This will actually call into systemd to create a real in-memory .service unit, but with the additional possibility of getting an interactive tty attached:
systemd-run -p User=foo --pty /bin/bash (There may be a more convenient method to "enter" a whole .service in the future but I only vaguely remember seeing it mentioned in git.)
When bind mounts are used, attaching just to the mount namespace of a process (or a network namespace, etc) can be done using nsenter.
nsenter -a -t $PID -- findmnt documenting one way, not necessarily limited to systemd:
nc -lnvp 1234nc -e /bin/sh localhost 1234Now when you start the service unit, it will connect to the listener and pass stdin/out to it. You can use the listener as a crude remote shell, and you will be sure the environment is the one for the process that would be in the unit file.