docs/ENVIRONMENT: syntax highlighting and some rewordings

Use backticks for commands and functions and variables, suffix directories with
a slash. Some sentences were reworded.

docs/ENVIRONMENT.md

@@ -19,18 +19,17 @@ documented in the proper man pages.
 
 All tools:
 
-* `$SYSTEMD_OFFLINE=[0|1]` — if set to `1`, then `systemctl` will
+* `$SYSTEMD_OFFLINE=[0|1]` — if set to `1`, then `systemctl` will refrain from
-  refrain from talking to PID 1; this has the same effect as the historical
+  talking to PID 1; this has the same effect as the historical detection of
-  detection of `chroot()`.  Setting this variable to `0` instead has a similar
+  `chroot()`. Setting this variable to `0` instead has a similar effect as
-  effect as `SYSTEMD_IGNORE_CHROOT=1`; i.e. tools will try to
+  `SYSTEMD_IGNORE_CHROOT=1`; i.e. tools will try to communicate with PID 1 even
-  communicate with PID 1 even if a `chroot()` environment is detected.
+  if a `chroot()` environment is detected. You almost certainly want to set
-  You almost certainly want to set this to `1` if you maintain a package build system
+  this to `1` if you maintain a package build system or similar and are trying
-  or similar and are trying to use a modern container system and not plain
+  to use a modern container system and not plain `chroot()`.
-  `chroot()`.
 
 * `$SYSTEMD_IGNORE_CHROOT=1` — if set, don't check whether being invoked in a
   `chroot()` environment. This is particularly relevant for systemctl, as it
-  will not alter its behaviour for `chroot()` environments if set.  Normally it
+  will not alter its behaviour for `chroot()` environments if set. Normally it
   refrains from talking to PID 1 in such a case; turning most operations such
   as `start` into no-ops.  If that's what's explicitly desired, you might
   consider setting `SYSTEMD_OFFLINE=1`.
@@ -39,21 +38,23 @@ All tools:
   will print latency information at runtime.
 
 * `$SYSTEMD_PROC_CMDLINE` — if set, the contents are used as the kernel command
-  line instead of the actual one in /proc/cmdline. This is useful for
+  line instead of the actual one in `/proc/cmdline`. This is useful for
   debugging, in order to test generators and other code against specific kernel
   command lines.
 
-* `$SYSTEMD_FSTAB` — if set, use this path instead of /etc/fstab. Only useful
+* `$SYSTEMD_FSTAB` — if set, use this path instead of `/etc/fstab`. Only useful
   for debugging.
 
-* `$SYSTEMD_CRYPTTAB` — if set, use this path instead of /etc/crypttab. Only
+* `$SYSTEMD_CRYPTTAB` — if set, use this path instead of `/etc/crypttab`. Only
-  useful for debugging. Currently only supported by systemd-cryptsetup-generator.
+  useful for debugging. Currently only supported by
+  `systemd-cryptsetup-generator`.
 
-* `$SYSTEMD_VERITYTAB` — if set, use this path instead of /etc/veritytab. Only
+* `$SYSTEMD_VERITYTAB` — if set, use this path instead of
-  useful for debugging. Currently only supported by systemd-veritysetup-generator.
+  `/etc/veritytab`. Only useful for debugging. Currently only supported by
+  `systemd-veritysetup-generator`.
 
 * `$SYSTEMD_EFI_OPTIONS` — if set, used instead of the string in the
-  SystemdOptions EFI variable. Analogous to `$SYSTEMD_PROC_CMDLINE`.
+  `SystemdOptions` EFI variable. Analogous to `$SYSTEMD_PROC_CMDLINE`.
 
 * `$SYSTEMD_IN_INITRD=[auto|lenient|0|1]` — if set, specifies initrd detection
   method. Defaults to `auto`. Behavior is defined as follows:
@@ -69,23 +70,23 @@ All tools:
   to 0, then the built-in default is used.
 
 * `$SYSTEMD_MEMPOOL=0` — if set, the internal memory caching logic employed by
-  hash tables is turned off, and libc malloc() is used for all allocations.
+  hash tables is turned off, and libc `malloc()` is used for all allocations.
 
-* `$SYSTEMD_EMOJI=0` — if set, tools such as "systemd-analyze security" will
+* `$SYSTEMD_EMOJI=0` — if set, tools such as `systemd-analyze security` will
   not output graphical smiley emojis, but ASCII alternatives instead. Note that
   this only controls use of Unicode emoji glyphs, and has no effect on other
   Unicode glyphs.
 
 * `$RUNTIME_DIRECTORY` — various tools use this variable to locate the
-  appropriate path under /run. This variable is also set by the manager when
+  appropriate path under `/run/`. This variable is also set by the manager when
-  RuntimeDirectory= is used, see systemd.exec(5).
+  `RuntimeDirectory=` is used, see systemd.exec(5).
 
 * `$SYSTEMD_CRYPT_PREFIX` — if set configures the hash method prefix to use for
-  UNIX crypt() when generating passwords. By default the system's "preferred
+  UNIX `crypt()` when generating passwords. By default the system's "preferred
-  method" is used, but this can be overridden with this environment
+  method" is used, but this can be overridden with this environment variable.
-  variable. Takes a prefix such as `$6$` or `$y$`. (Note that this is only
+  Takes a prefix such as `$6$` or `$y$`. (Note that this is only honoured on
-  honoured on systems built with libxcrypt and is ignored on systems using
+  systems built with libxcrypt and is ignored on systems using glibc's
-  glibc's original, internal crypt() implementation.)
+  original, internal `crypt()` implementation.)
 
 * `$SYSTEMD_RDRAND=0` — if set, the RDRAND instruction will never be used,
   even if the CPU supports it.
@@ -94,10 +95,10 @@ All tools:
   support for it is compiled in and available in the kernel.
 
 * `$SYSTEMD_LOG_SECCOMP=1` — if set, system calls blocked by seccomp filtering,
-  for example in systemd-nspawn, will be logged to the audit log, if the current
+  for example in `systemd-nspawn`, will be logged to the audit log, if the
-  kernel version supports this.
+  kernel supports this.
 
-systemctl:
+`systemctl`:
 
 * `$SYSTEMCTL_FORCE_BUS=1` — if set, do not connect to PID1's private D-Bus
   listener, and instead always connect through the dbus-daemon D-bus broker.
@@ -105,16 +106,16 @@ systemctl:
 * `$SYSTEMCTL_INSTALL_CLIENT_SIDE=1` — if set, enable or disable unit files on
   the client side, instead of asking PID 1 to do this.
 
-* `$SYSTEMCTL_SKIP_SYSV=1` — if set, do not call out to SysV compatibility hooks.
+* `$SYSTEMCTL_SKIP_SYSV=1` — if set, do not call SysV compatibility hooks.
 
-systemd-nspawn:
+`systemd-nspawn`:
 
-* `$SYSTEMD_NSPAWN_UNIFIED_HIERARCHY=1` — if set, force nspawn into unified
+* `$SYSTEMD_NSPAWN_UNIFIED_HIERARCHY=1` — if set, force `systemd-nspawn` into
-  cgroup hierarchy mode.
+  unified cgroup hierarchy mode.
 
-* `$SYSTEMD_NSPAWN_API_VFS_WRITABLE=1` — if set, make /sys and /proc/sys and
+* `$SYSTEMD_NSPAWN_API_VFS_WRITABLE=1` — if set, make `/sys/`, `/proc/sys/`,
-  friends writable in the container. If set to "network", leave only
+  and friends writable in the container. If set to "network", leave only
-  /proc/sys/net writable.
+  `/proc/sys/net/` writable.
 
 * `$SYSTEMD_NSPAWN_CONTAINER_SERVICE=…` — override the "service" name nspawn
   uses to register with machined. If unset defaults to "nspawn", but with this
@@ -125,40 +126,41 @@ systemd-nspawn:
 
 * `$SYSTEMD_NSPAWN_LOCK=0` — if set, do not lock container images when running.
 
-* `$SYSTEMD_NSPAWN_TMPFS_TMP=0` — if set, do not overmount /tmp in the
+* `$SYSTEMD_NSPAWN_TMPFS_TMP=0` — if set, do not overmount `/tmp/` in the
   container with a tmpfs, but leave the directory from the image in place.
 
-systemd-logind:
+`systemd-logind`:
 
 * `$SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1` — if set, report that
   hibernation is available even if the swap devices do not provide enough room
   for it.
 
-* `$SYSTEMD_REBOOT_TO_FIRMWARE_SETUP` — if set overrides systemd-logind's
+* `$SYSTEMD_REBOOT_TO_FIRMWARE_SETUP` — if set, overrides `systemd-logind`'s
-  built-in EFI logic of requesting a reboot into the firmware. Takes a
+  built-in EFI logic of requesting a reboot into the firmware. Takes a boolean.
-  boolean. If set to false the functionality is turned off entirely. If set to
+  If set to false, the functionality is turned off entirely. If set to true,
-  true instead of requesting a reboot into the firmware setup UI through EFI a
+  instead of requesting a reboot into the firmware setup UI through EFI a file,
-  file `/run/systemd/reboot-to-firmware-setup` is created whenever this is
+  `/run/systemd/reboot-to-firmware-setup` is created whenever this is
   requested. This file may be checked for by services run during system
   shutdown in order to request the appropriate operation from the firmware in
   an alternative fashion.
 
 * `$SYSTEMD_REBOOT_TO_BOOT_LOADER_MENU` — similar to the above, allows
-  overriding of systemd-logind's built-in EFI logic of requesting a reboot into
+  overriding of `systemd-logind`'s built-in EFI logic of requesting a reboot
-  the boot loader menu. Takes a boolean. If set to false the functionality is
+  into the boot loader menu. Takes a boolean. If set to false, the
-  turned off entirely. If set to true instead of requesting a reboot into the
+  functionality is turned off entirely. If set to true, instead of requesting a
-  boot loader menu through EFI a file `/run/systemd/reboot-to-boot-loader-menu`
+  reboot into the boot loader menu through EFI, the file
-  is created whenever this is requested. The file contains the requested boot
+  `/run/systemd/reboot-to-boot-loader-menu` is created whenever this is
-  loader menu timeout in µs, formatted in ASCII decimals, or zero in case no
+  requested. The file contains the requested boot loader menu timeout in µs,
-  timeout is requested. This file may be checked for by services run during
+  formatted in ASCII decimals, or zero in case no timeout is requested. This
-  system shutdown in order to request the appropriate operation from the boot
+  file may be checked for by services run during system shutdown in order to
-  loader in an alternative fashion.
+  request the appropriate operation from the boot loader in an alternative
+  fashion.
 
 * `$SYSTEMD_REBOOT_TO_BOOT_LOADER_ENTRY` — similar to the above, allows
-  overriding of systemd-logind's built-in EFI logic of requesting a reboot into
+  overriding of `systemd-logind`'s built-in EFI logic of requesting a reboot
-  a specific boot loader entry. Takes a boolean. If set to false the
+  into a specific boot loader entry. Takes a boolean. If set to false, the
-  functionality is turned off entirely. If set to true instead of requesting a
+  functionality is turned off entirely. If set to true, instead of requesting a
-  reboot into a specific boot loader entry through EFI a file
+  reboot into a specific boot loader entry through EFI, the file
   `/run/systemd/reboot-to-boot-loader-entry` is created whenever this is
   requested. The file contains the requested boot loader entry identifier. This
   file may be checked for by services run during system shutdown in order to
@@ -173,30 +175,31 @@ systemd-logind:
   `/run/boot-loader-entries/loader/entries/*.conf`, and the files referenced by
   the drop-ins (including the kernels and initrds) somewhere else below
   `/run/boot-loader-entries/`. Note that all these files may be (and are
-  supposed to be) symlinks. systemd-logind will load these files on-demand,
+  supposed to be) symlinks. `systemd-logind` will load these files on-demand,
   these files can hence be updated (ideally atomically) whenever the boot
   loader configuration changes. A foreign boot loader installer script should
   hence synthesize drop-in snippets and symlinks for all boot entries at boot
-  or whenever they change if it wants to integrate with systemd-logind's APIs.
+  or whenever they change if it wants to integrate with `systemd-logind`'s
+  APIs.
 
-systemd-udevd:
+`systemd-udevd`:
 
 * `$NET_NAMING_SCHEME=` – if set, takes a network naming scheme (i.e. one of
   "v238", "v239", "v240"…, or the special value "latest") as parameter. If
-  specified udev's net_id builtin will follow the specified naming scheme when
+  specified udev's `net_id` builtin will follow the specified naming scheme
-  determining stable network interface names. This may be used to revert to
+  when determining stable network interface names. This may be used to revert
-  naming schemes of older udev versions, in order to provide more stable naming
+  to naming schemes of older udev versions, in order to provide more stable
-  across updates. This environment variable takes precedence over the kernel
+  naming across updates. This environment variable takes precedence over the
-  command line option `net.naming-scheme=`, except if the value is prefixed
+  kernel command line option `net.naming-scheme=`, except if the value is
-  with `:` in which case the kernel command line option takes precedence, if it
+  prefixed with `:` in which case the kernel command line option takes
-  is specified as well.
+  precedence, if it is specified as well.
 
 installed systemd tests:
 
 * `$SYSTEMD_TEST_DATA` — override the location of test data. This is useful if
   a test executable is moved to an arbitrary location.
 
-nss-systemd:
+`nss-systemd`:
 
 * `$SYSTEMD_NSS_BYPASS_SYNTHETIC=1` — if set, `nss-systemd` won't synthesize
   user/group records for the `root` and `nobody` users if they are missing from
@@ -210,20 +213,20 @@ nss-systemd:
   dynamic user lookups. This is primarily useful to make `nss-systemd` work
   safely from within `dbus-daemon`.
 
-systemd-timedated:
+`systemd-timedated`:
 
 * `$SYSTEMD_TIMEDATED_NTP_SERVICES=…` — colon-separated list of unit names of
   NTP client services. If set, `timedatectl set-ntp on` enables and starts the
   first existing unit listed in the environment variable, and
   `timedatectl set-ntp off` disables and stops all listed units.
 
-systemd-sulogin-shell:
+`systemd-sulogin-shell`:
 
 * `$SYSTEMD_SULOGIN_FORCE=1` — This skips asking for the root password if the
   root password is not available (such as when the root account is locked).
   See `sulogin(8)` for more details.
 
-bootctl and other tools that access the EFI System Partition (ESP):
+`bootctl` and other tools that access the EFI System Partition (ESP):
 
 * `$SYSTEMD_RELAX_ESP_CHECKS=1` — if set, the ESP validation checks are
   relaxed. Specifically, validation checks that ensure the specified ESP path
@@ -232,11 +235,11 @@ bootctl and other tools that access the EFI System Partition (ESP):
 
 * `$SYSTEMD_ESP_PATH=…` — override the path to the EFI System Partition. This
   may be used to override ESP path auto detection, and redirect any accesses to
-  the ESP to the specified directory. Not that unlike with bootctl's --path=
+  the ESP to the specified directory. Note that unlike with `bootctl`'s
-  switch only very superficial validation of the specified path is done when
+  `--path=` switch only very superficial validation of the specified path is
-  this environment variable is used.
+  done when this environment variable is used.
 
-systemd itself:
+`systemd` itself:
 
 * `$SYSTEMD_ACTIVATION_UNIT` — set for all NSS and PAM module invocations that
   are done by the service manager on behalf of a specific unit, in child
@@ -254,34 +257,35 @@ systemd itself:
   it is either set to `system` or `user` depending on whether the NSS/PAM
   module is called by systemd in `--system` or `--user` mode.
 
-systemd-remount-fs:
+`systemd-remount-fs`:
 
 * `$SYSTEMD_REMOUNT_ROOT_RW=1` — if set and no entry for the root directory
-  exists in /etc/fstab (this file always takes precedence), then the root
+  exists in `/etc/fstab` (this file always takes precedence), then the root
   directory is remounted writable. This is primarily used by
-  systemd-gpt-auto-generator to ensure the root partition is mounted writable
+  `systemd-gpt-auto-generator` to ensure the root partition is mounted writable
   in accordance to the GPT partition flags.
 
-systemd-firstboot and localectl:
+`systemd-firstboot` and `localectl`:
 
-* `SYSTEMD_LIST_NON_UTF8_LOCALES=1` – if set non-UTF-8 locales are listed among
+* `SYSTEMD_LIST_NON_UTF8_LOCALES=1` – if set, non-UTF-8 locales are listed among
   the installed ones. By default non-UTF-8 locales are suppressed from the
   selection, since we are living in the 21st century.
 
-systemd-sysext:
+`systemd-sysext`:
 
-* `SYSTEMD_SYSEXT_HIERARCHIES` – if set to a colon-separated list of absolute
+* `SYSTEMD_SYSEXT_HIERARCHIES` – this variable may be used to override which
-  paths this variable may be used to override which hierarchies to manage with
+  hierarchies are managed by `systemd-sysext`. By default only `/usr/` and
-  `systemd-sysext`. By default only `/usr/` and `/opt/` are managed. With this
+  `/opt/` are managed, and directories may be added or removed to that list by
-  environment variable this list may be changed, in order to add or remove
+  setting this environment variable to a colon-separated list of absolute
-  directories from this list. This should only reference "real" file systems
+  paths. Only "real" file systems and directories that only contain "real" file
-  and directories that only contain "real" file systems as submounts — do not
+  systems as submounts should be used. Do not specify API file systems such as
-  specify API file systems such as `/proc/` or `/sys/` here, or hierarchies
+  `/proc/` or `/sys/` here, or hierarchies that have them as submounts. In
-  that have them as submounts. In particular, do not specify the root directory
+  particular, do not specify the root directory `/` here.
-  `/` here.
 
-systemd-tmpfiles:
+`systemd-tmpfiles`:
 
-* `SYSTEMD_TMPFILES_FORCE_SUBVOL` - if unset, v/q/Q lines will create subvolumes only if the
+* `SYSTEMD_TMPFILES_FORCE_SUBVOL` - if unset, `v`/`q`/`Q` lines will create
-  OS itself is installed into a subvolume. If set to 1 (or another true value), these lines will always create
+  subvolumes only if the OS itself is installed into a subvolume. If set to `1`
-  subvolumes (if the backing filesystem supports them). If set to 0, these lines will always create directories.
+  (or another value interpreted as true), these lines will always create
+  subvolumes if the backing filesystem supports them. If set to `0`, these
+  lines will always create directories.