diff --git a/cylc/flow/scripts/reinstall.py b/cylc/flow/scripts/reinstall.py
index d81d3e6949e..bb471cc05e9 100644
--- a/cylc/flow/scripts/reinstall.py
+++ b/cylc/flow/scripts/reinstall.py
@@ -30,18 +30,56 @@
   # Or, to reinstall a specific run:
   $ cylc reinstall myflow/run2
 
-  # View the changes reinstall would make:
-  $ cylc reinstall myflow --dry-run
+  # If the workflow is running:
+  $ cylc reinstall myflow  # reinstall as usual
+  $ cylc reload myflow     # pick up changes in the workflow config
+
+What reinstall does:
+  Reinstall synchronises files between the workflow source and the specified
+  run directory.
+
+  Any files which have been added, updated or removed in the source directory
+  will be added, updated or removed in the run directory. Cylc uses "rsync"
+  to do this (run in "--debug" mode to see the exact command used).
+
+How changes are displayed:
+  Reinstall will first perform a dry run showing the files it would change.
+  This is displayed in "rsync" format e.g:
+
+    <g>send foo</g>   # this means the file "foo" would be added/updated
+    <r>del. bar</r>   # this means the file "bar" would be deleted
+
+How to prevent reinstall deleting files:
+  Reinstall will delete any files which are not present in the source directory
+  (i.e. if you delete a file from the source directory, a reinstall would
+  remove the file from the run directory too). The "work/" and "share/"
+  directory are excluded from this. These are the recommended locations for any
+  files created at runtime.
+
+  You can extend the list of "excluded" paths by creating a ".cylcignore" file.
+  For example the following file would exclude "data/" and any ".csv" files
+  from being overwritten by a reinstallation:
+
+    $ cat .cylcignore
+    data
+    *.csv
+
+  Note any paths listed in ".cylcignore" will not be installed by
+  "cylc install" even if present in the source directory.
 """
 
 from pathlib import Path
 import sys
-from typing import Optional, TYPE_CHECKING
+from typing import Optional, TYPE_CHECKING, List
 
 from ansimarkup import parse as cparse
 
 from cylc.flow import iter_entry_points
-from cylc.flow.exceptions import PluginError, WorkflowFilesError
+from cylc.flow.exceptions import (
+    PluginError,
+    ServiceFileError,
+    WorkflowFilesError,
+)
 from cylc.flow.id_cli import parse_id
 from cylc.flow.option_parsers import (
     WORKFLOW_ID_ARG_DOC,
@@ -50,9 +88,10 @@
 from cylc.flow.pathutil import get_workflow_run_dir
 from cylc.flow.workflow_files import (
     get_workflow_source_dir,
+    load_contact_file,
     reinstall_workflow,
 )
-from cylc.flow.terminal import cli_function
+from cylc.flow.terminal import cli_function, DIM, is_terminal
 
 if TYPE_CHECKING:
     from optparse import Values
@@ -78,48 +117,19 @@ def get_option_parser() -> COP:
             dest="clear_rose_install_opts"
         )
 
-    parser.add_option(
-        '--dry', '--dry-run',
-        action='store_true',
-        help='Show the changes reinstallation would make.'
-    )
-
     return parser
 
 
-def format_rsync_out(out):
-    """Format rsync stdout for presenting to users.
-
-    Note: Output formats of different rsync implementations may differ so keep
-          this code simple and robust.
-
-    """
-    lines = []
-    for line in out.splitlines():
-        if line[0:4] == 'send':
-            # file added or updated
-            lines.append(cparse(f'<green>{line}</green>'))
-        elif line[0:4] == 'del.':
-            # file deleted
-            lines.append(cparse(f'<red>{line}</red>'))
-        elif line == 'cannot delete non-empty directory: opt':
-            # These "cannot delete non-empty directory" messages can arise
-            # as a result of excluding files within sub-directories.
-            # This opt dir message is likely to occur when a rose-suit.conf
-            # file is present.
-            continue
-        else:
-            # other uncategorised log line
-            lines.append(line)
-    return lines
-
-
 @cli_function(get_option_parser)
 def main(
     parser: COP,
     opts: 'Values',
     args: Optional[str] = None
 ) -> None:
+    """Implement cylc reinstall.
+
+    This is the bit which contains all the CLI logic.
+    """
     run_dir: Optional[Path]
     workflow_id: str
     workflow_id, *_ = parse_id(
@@ -141,7 +151,62 @@ def main(
             ' symlink to continue.'
         )
 
-    if not opts.dry:
+    try:
+        usr: str = ''
+        if is_terminal():
+            # interactive mode - perform dry-run and prompt
+            reinstall(opts, workflow_id, source, run_dir, dry_run=True)
+            print(
+                cparse(
+                    f'\n<{DIM}>TIP: You can "exclude" files/dirs to prevent'
+                    ' Cylc from installing or overwriting\n     them by adding'
+                    'them to the .cylcignore file. See cylc reinstall --help.'
+                    f'</{DIM}>\n'
+                    '\n<bold>Reinstall would make the above changes.</bold>'
+                )
+            )
+            # prompt for permission to continue
+            while usr not in ['y', 'n']:
+                usr = input(
+                    cparse('<bold>Continue [y/n]: </bold>')
+                ).lower()
+        else:
+            # non interactive-mode - no dry-run, no prompt
+            usr = 'y'
+    except KeyboardInterrupt:
+        # ensure the "reinstall canceled" message shows for ctrl+c
+        usr = 'n'  # cancel the reinstall
+        print()    # clear the traceback line
+
+    if usr == 'y':
+        reinstall(opts, workflow_id, source, run_dir, dry_run=False)
+        print(cparse('<green>Successfully reinstalled.</green>'))
+        if is_workflow_running(workflow_id):
+            print(cparse(
+                '\n<blue>'
+                'Run "cylc reload {workflow_id}" to pick up changes.'
+                '</blue>'
+            ))
+    else:
+        print(
+            cparse('<magenta>Reinstall canceled, no changes made.</magenta>')
+        )
+
+
+def reinstall(
+    opts: 'Values',
+    workflow_id: str,
+    source: str,
+    run_dir: Path,
+    dry_run: bool = False,
+) -> None:
+    """Perform reinstallation.
+
+    This is the purely functional bit without any CLI logic.
+    """
+    # run pre_configure plugins
+    if not dry_run:
+        # don't run plugins in dry-mode
         for entry_point in iter_entry_points(
             'cylc.pre_configure'
         ):
@@ -156,28 +221,36 @@ def main(
                     exc
                 ) from None
 
-    stdout = reinstall_workflow(
+    # reinstall from source
+    stdout: str = reinstall_workflow(
         source=Path(source),
         named_run=workflow_id,
         rundir=run_dir,
-        dry_run=opts.dry,
+        dry_run=dry_run,
     )
 
-    if Path(source, 'rose-suite.conf').is_file():
-        print(
-            cparse(
-                '<blue>'
-                'NOTE: Files created by Rose file installation will show as'
-                ' deleted.'
-                '\n      They will be re-created during the reinstall'
-                ' process.'
-                '</blue>',
-            ),
-            file=sys.stderr,
-        )
-    print('\n'.join(format_rsync_out(stdout)), file=sys.stderr)
+    # display changes
+    if dry_run:
+        # only write rsync output in dry-mode
+        if Path(source, 'rose-suite.conf').is_file():
+            # TODO: remove this in combination with
+            # https://github.com/cylc/cylc-rose/issues/149
+            print(
+                cparse(
+                    f'\n<{DIM}>'
+                    'NOTE: Files created by Rose file installation will show'
+                    ' as deleted.'
+                    '\n      They will be re-created during the reinstall'
+                    ' process.'
+                    f'</{DIM}>\n',
+                ),
+                file=sys.stderr,
+            )
+        print('\n'.join(format_rsync_out(stdout)), file=sys.stderr)
 
-    if not opts.dry:
+    # run post_install plugins
+    if not dry_run:
+        # don't run plugins in dry-mode
         for entry_point in iter_entry_points(
             'cylc.post_install'
         ):
@@ -195,3 +268,53 @@ def main(
                     entry_point.name,
                     exc
                 ) from None
+
+
+def format_rsync_out(out: str) -> List[str]:
+    r"""Format rsync stdout for presenting to users.
+
+    Note: Output formats of different rsync implementations may differ so keep
+          this code simple and robust.
+
+    Example:
+        >>> format_rsync_out(
+        ...     'send foo\ndel. bar\nbaz'
+        ... ) == [
+        ...     cparse('<green>send foo</green>'),
+        ...     cparse('<red>del. bar</red>'),
+        ...     'baz',
+        ... ]
+        True
+
+    """
+    lines = []
+    for line in out.splitlines():
+        if line[0:4] == 'send':
+            # file added or updated
+            lines.append(cparse(f'<green>{line}</green>'))
+        elif line[0:4] == 'del.':
+            # file deleted
+            lines.append(cparse(f'<red>{line}</red>'))
+        elif line == 'cannot delete non-empty directory: opt':
+            # These "cannot delete non-empty directory" messages can arise
+            # as a result of excluding files within sub-directories.
+            # This opt dir message is likely to occur when a rose-suit.conf
+            # file is present.
+            continue
+        else:
+            # other uncategorised log line
+            lines.append(line)
+    return lines
+
+
+def is_workflow_running(workflow_id: str) -> bool:
+    """Quick and simple way to tell if a workflow is running.
+
+    It would be better to "ping" the workflow, however, this is sufficient
+    for our purposes.
+    """
+    try:
+        load_contact_file(workflow_id)
+    except ServiceFileError:
+        return False
+    return True
diff --git a/cylc/flow/workflow_files.py b/cylc/flow/workflow_files.py
index 26e63992faf..76e3570ea49 100644
--- a/cylc/flow/workflow_files.py
+++ b/cylc/flow/workflow_files.py
@@ -1514,7 +1514,7 @@ def reinstall_workflow(
         dry_run=dry_run,
     )
     reinstall_log.info(cli_format(rsync_cmd))
-    # print(cli_format(rsync_cmd))
+    LOG.debug(cli_format(rsync_cmd))
     proc = Popen(rsync_cmd, stdout=PIPE, stderr=PIPE, text=True)  # nosec
     # * command is constructed via internal interface
     stdout, stderr = proc.communicate()