Fix --format parsing extensions with dots

Also improve error reporting for `--format` with malformed or
unsupported extensions

This commit is very messy, as it also does an refac in the project,
which should ideally be in a separated commit

src/check.rs

@@ -10,7 +10,7 @@ use std::{
 
 use crate::{
     error::FinalError,
-    extension::{build_archive_file_suggestion, Extension, PRETTY_SUPPORTED_ALIASES, PRETTY_SUPPORTED_EXTENSIONS},
+    extension::{build_archive_file_suggestion, Extension},
     utils::{
         logger::{info_accessible, warning},
         pretty_format_list_of_paths, try_infer_extension, user_wants_to_continue, EscapedPathDisplay,
@@ -160,10 +160,8 @@ pub fn check_missing_formats_when_decompressing(files: &[PathBuf], formats: &[Ve
         ));
     }
 
-    error = error
+    error = error.detail("Decompression formats are detected automatically from file extension");
-        .detail("Decompression formats are detected automatically from file extension")
+    error = error.hint_all_supported_formats();
-        .hint(format!("Supported extensions are: {}", PRETTY_SUPPORTED_EXTENSIONS))
-        .hint(format!("Supported aliases are: {}", PRETTY_SUPPORTED_ALIASES));
 
     // If there's exactly one file, give a suggestion to use `--format`
     if let &[path] = files_with_broken_extension.as_slice() {

src/commands/mod.rs

@@ -15,10 +15,10 @@ use crate::{
     cli::Subcommand,
     commands::{compress::compress_files, decompress::decompress_file, list::list_archive_contents},
     error::{Error, FinalError},
-    extension::{self, parse_format},
+    extension::{self, parse_format_flag},
     list::ListOptions,
     utils::{
-        self, colors::*, is_path_stdin, logger::info_accessible, to_utf, EscapedPathDisplay, FileVisibilityPolicy,
+        self, colors::*, is_path_stdin, logger::info_accessible, path_to_str, EscapedPathDisplay, FileVisibilityPolicy,
     },
     CliArgs, QuestionPolicy,
 };
@@ -68,7 +68,7 @@ pub fn run(
             // Formats from path extension, like "file.tar.gz.xz" -> vec![Tar, Gzip, Lzma]
             let (formats_from_flag, formats) = match args.format {
                 Some(formats) => {
-                    let parsed_formats = parse_format(&formats)?;
+                    let parsed_formats = parse_format_flag(&formats)?;
                     (Some(formats), parsed_formats)
                 }
                 None => (None, extension::extensions_from_path(&output_path)),
@@ -111,7 +111,7 @@ pub fn run(
                 // having a final status message is important especially in an accessibility context
                 // as screen readers may not read a commands exit code, making it hard to reason
                 // about whether the command succeeded without such a message
-                info_accessible(format!("Successfully compressed '{}'.", to_utf(&output_path)));
+                info_accessible(format!("Successfully compressed '{}'.", path_to_str(&output_path)));
             } else {
                 // If Ok(false) or Err() occurred, delete incomplete file at `output_path`
                 //
@@ -139,7 +139,7 @@ pub fn run(
             let mut formats = vec![];
 
             if let Some(format) = args.format {
-                let format = parse_format(&format)?;
+                let format = parse_format_flag(&format)?;
                 for path in files.iter() {
                     let file_name = path.file_name().ok_or_else(|| Error::NotFound {
                         error_title: format!("{} does not have a file name", EscapedPathDisplay::new(path)),
@@ -199,7 +199,7 @@ pub fn run(
             let mut formats = vec![];
 
             if let Some(format) = args.format {
-                let format = parse_format(&format)?;
+                let format = parse_format_flag(&format)?;
                 for _ in 0..files.len() {
                     formats.push(format.clone());
                 }

src/error.rs

@@ -4,15 +4,21 @@
 
 use std::{
     borrow::Cow,
+    ffi::OsString,
     fmt::{self, Display},
+    io,
 };
 
-use crate::{accessible::is_running_in_accessible_mode, utils::colors::*};
+use crate::{
+    accessible::is_running_in_accessible_mode,
+    extension::{PRETTY_SUPPORTED_ALIASES, PRETTY_SUPPORTED_EXTENSIONS},
+    utils::os_str_to_str,
+};
 
 /// All errors that can be generated by `ouch`
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub enum Error {
-    /// Not every IoError, some of them get filtered by `From<io::Error>` into other variants
+    /// An IoError that doesn't have a dedicated error variant
     IoError { reason: String },
     /// From lzzzz::lz4f::Error
     Lz4Error { reason: String },
@@ -33,9 +39,9 @@ pub enum Error {
     /// Custom and unique errors are reported in this variant
     Custom { reason: FinalError },
     /// Invalid format passed to `--format`
-    InvalidFormat { reason: String },
+    InvalidFormatFlag { text: OsString, reason: String },
     /// From sevenz_rust::Error
-    SevenzipError(sevenz_rust::Error),
+    SevenzipError { reason: String },
     /// Recognised but unsupported format
     // currently only RAR when built without the `unrar` feature
     UnsupportedFormat { reason: String },
@@ -62,6 +68,8 @@ pub struct FinalError {
 
 impl Display for FinalError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use crate::utils::colors::*;
+
         // Title
         //
         // When in ACCESSIBLE mode, the square brackets are suppressed
@@ -122,56 +130,72 @@ impl FinalError {
         self.hints.push(hint.into());
         self
     }
+
+    /// Adds all supported formats as hints.
+    ///
+    /// This is what it looks like:
+    /// ```
+    /// hint: Supported extensions are: tar, zip, bz, bz2, gz, lz4, xz, lzma, sz, zst
+    /// hint: Supported aliases are: tgz, tbz, tlz4, txz, tzlma, tsz, tzst
+    /// ```
+    pub fn hint_all_supported_formats(self) -> Self {
+        self.hint(format!("Supported extensions are: {}", PRETTY_SUPPORTED_EXTENSIONS))
+            .hint(format!("Supported aliases are: {}", PRETTY_SUPPORTED_ALIASES))
+    }
 }
 
-impl fmt::Display for Error {
+impl From<Error> for FinalError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn from(err: Error) -> Self {
-        let err = match self {
+        match err {
-            Error::WalkdirError { reason } => FinalError::with_title(reason.to_string()),
+            Error::WalkdirError { reason } => FinalError::with_title(reason),
-            Error::NotFound { error_title } => FinalError::with_title(error_title.to_string()).detail("File not found"),
+            Error::NotFound { error_title } => FinalError::with_title(error_title).detail("File not found"),
             Error::CompressingRootFolder => {
                 FinalError::with_title("It seems you're trying to compress the root folder.")
                     .detail("This is unadvisable since ouch does compressions in-memory.")
                     .hint("Use a more appropriate tool for this, such as rsync.")
             }
-            Error::IoError { reason } => FinalError::with_title(reason.to_string()),
+            Error::IoError { reason } => FinalError::with_title(reason),
-            Error::Lz4Error { reason } => FinalError::with_title(reason.to_string()),
+            Error::Lz4Error { reason } => FinalError::with_title(reason),
-            Error::AlreadyExists { error_title } => {
+            Error::AlreadyExists { error_title } => FinalError::with_title(error_title).detail("File already exists"),
-                FinalError::with_title(error_title.to_string()).detail("File already exists")
+            Error::InvalidZipArchive(reason) => FinalError::with_title("Invalid zip archive").detail(reason),
+            Error::PermissionDenied { error_title } => FinalError::with_title(error_title).detail("Permission denied"),
+            Error::UnsupportedZipArchive(reason) => FinalError::with_title("Unsupported zip archive").detail(reason),
+            Error::InvalidFormatFlag { reason, text } => {
+                FinalError::with_title(format!("Failed to parse `--format {}`", os_str_to_str(&text)))
+                    .detail(reason)
+                    .hint_all_supported_formats()
+                    .hint("")
+                    .hint("Examples:")
+                    .hint("  --format tar")
+                    .hint("  --format gz")
+                    .hint("  --format tar.gz")
             }
-            Error::InvalidZipArchive(reason) => FinalError::with_title("Invalid zip archive").detail(*reason),
-            Error::PermissionDenied { error_title } => {
-                FinalError::with_title(error_title.to_string()).detail("Permission denied")
-            }
-            Error::UnsupportedZipArchive(reason) => FinalError::with_title("Unsupported zip archive").detail(*reason),
-            Error::InvalidFormat { reason } => FinalError::with_title("Invalid archive format").detail(reason.clone()),
             Error::Custom { reason } => reason.clone(),
-            Error::SevenzipError(reason) => FinalError::with_title("7z error").detail(reason.to_string()),
+            Error::SevenzipError { reason } => FinalError::with_title("7z error").detail(reason),
             Error::UnsupportedFormat { reason } => {
                 FinalError::with_title("Recognised but unsupported format").detail(reason.clone())
             }
             Error::InvalidPassword { reason } => FinalError::with_title("Invalid password").detail(reason.clone()),
-        };
+        }
+    }
+}
 
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let err = FinalError::from(self.clone());
         write!(f, "{err}")
     }
 }
 
 impl From<std::io::Error> for Error {
     fn from(err: std::io::Error) -> Self {
+        let error_title = err.to_string();
+
         match err.kind() {
-            std::io::ErrorKind::NotFound => Self::NotFound {
+            io::ErrorKind::NotFound => Self::NotFound { error_title },
-                error_title: err.to_string(),
+            io::ErrorKind::PermissionDenied => Self::PermissionDenied { error_title },
-            },
+            io::ErrorKind::AlreadyExists => Self::AlreadyExists { error_title },
-            std::io::ErrorKind::PermissionDenied => Self::PermissionDenied {
+            _other => Self::IoError { reason: error_title },
-                error_title: err.to_string(),
-            },
-            std::io::ErrorKind::AlreadyExists => Self::AlreadyExists {
-                error_title: err.to_string(),
-            },
-            _other => Self::IoError {
-                reason: err.to_string(),
-            },
         }
     }
 }
@@ -201,7 +225,9 @@ impl From<unrar::error::UnrarError> for Error {
 
 impl From<sevenz_rust::Error> for Error {
     fn from(err: sevenz_rust::Error) -> Self {
-        Self::SevenzipError(err)
+        Self::SevenzipError {
+            reason: err.to_string(),
+        }
     }
 }
 

src/extension.rs

@@ -3,8 +3,8 @@
 use std::{ffi::OsStr, fmt, path::Path};
 
 use bstr::ByteSlice;
+use CompressionFormat::*;
 
-use self::CompressionFormat::*;
 use crate::{error::Error, utils::logger::warning};
 
 pub const SUPPORTED_EXTENSIONS: &[&str] = &[
@@ -34,7 +34,10 @@ pub const PRETTY_SUPPORTED_ALIASES: &str = "tgz, tbz, tlz4, txz, tzlma, tsz, tzs
 
 /// A wrapper around `CompressionFormat` that allows combinations like `tgz`
 #[derive(Debug, Clone)]
+// Keep `PartialEq` only for testing because two formats are the same even if
+// their `display_text` does not match (beware of aliases)
 #[cfg_attr(test, derive(PartialEq))]
+// Should only be built with constructors
 #[non_exhaustive]
 pub struct Extension {
     /// One extension like "tgz" can be made of multiple CompressionFormats ([Tar, Gz])
@@ -144,17 +147,30 @@ fn split_extension(name: &mut &[u8]) -> Option<Extension> {
     Some(ext)
 }
 
-pub fn parse_format(fmt: &OsStr) -> crate::Result<Vec<Extension>> {
+pub fn parse_format_flag(input: &OsStr) -> crate::Result<Vec<Extension>> {
-    let fmt = <[u8] as ByteSlice>::from_os_str(fmt).ok_or_else(|| Error::InvalidFormat {
+    let format = input.as_encoded_bytes();
-        reason: "Invalid UTF-8".into(),
+
+    let format = std::str::from_utf8(format).map_err(|_| Error::InvalidFormatFlag {
+        text: input.to_owned(),
+        reason: "Invalid UTF-8.".to_string(),
     })?;
 
-    let mut extensions = Vec::new();
+    let extensions: Vec<Extension> = format
-    for extension in fmt.split_str(b".") {
+        .split('.')
-        let extension = to_extension(extension).ok_or_else(|| Error::InvalidFormat {
+        .filter(|extension| !extension.is_empty())
-            reason: format!("Unsupported extension: {}", extension.to_str_lossy()),
+        .map(|extension| {
-        })?;
+            to_extension(extension.as_bytes()).ok_or_else(|| Error::InvalidFormatFlag {
-        extensions.push(extension);
+                text: input.to_owned(),
+                reason: format!("Unsupported extension '{}'", extension),
+            })
+        })
+        .collect::<crate::Result<_>>()?;
+
+    if extensions.is_empty() {
+        return Err(Error::InvalidFormatFlag {
+            text: input.to_owned(),
+            reason: "Parsing got an empty list of extensions.".to_string(),
+        });
     }
 
     Ok(extensions)
@@ -245,6 +261,7 @@ pub fn build_archive_file_suggestion(path: &Path, suggested_extension: &str) ->
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::utils::logger::spawn_logger_thread;
 
     #[test]
     fn test_extensions_from_path() {
@@ -257,7 +274,9 @@ mod tests {
     }
 
     #[test]
+    /// Test extension parsing for input/output files
     fn test_separate_known_extensions_from_name() {
+        let _handler = spawn_logger_thread();
         assert_eq!(
             separate_known_extensions_from_name("file".as_ref()),
             ("file".as_ref(), vec![])
@@ -287,6 +306,37 @@ mod tests {
         );
     }
 
+    #[test]
+    /// Test extension parsing of `--format FORMAT`
+    fn test_parse_of_format_flag() {
+        assert_eq!(
+            parse_format_flag(OsStr::new("tar")).unwrap(),
+            vec![Extension::new(&[Tar], "tar")]
+        );
+        assert_eq!(
+            parse_format_flag(OsStr::new(".tar")).unwrap(),
+            vec![Extension::new(&[Tar], "tar")]
+        );
+        assert_eq!(
+            parse_format_flag(OsStr::new("tar.gz")).unwrap(),
+            vec![Extension::new(&[Tar], "tar"), Extension::new(&[Gzip], "gz")]
+        );
+        assert_eq!(
+            parse_format_flag(OsStr::new(".tar.gz")).unwrap(),
+            vec![Extension::new(&[Tar], "tar"), Extension::new(&[Gzip], "gz")]
+        );
+        assert_eq!(
+            parse_format_flag(OsStr::new("..tar..gz.....")).unwrap(),
+            vec![Extension::new(&[Tar], "tar"), Extension::new(&[Gzip], "gz")]
+        );
+
+        assert!(parse_format_flag(OsStr::new("../tar.gz")).is_err());
+        assert!(parse_format_flag(OsStr::new("targz")).is_err());
+        assert!(parse_format_flag(OsStr::new("tar.gz.unknown")).is_err());
+        assert!(parse_format_flag(OsStr::new(".tar.gz.unknown")).is_err());
+        assert!(parse_format_flag(OsStr::new(".tar.!@#.gz")).is_err());
+    }
+
     #[test]
     fn builds_suggestion_correctly() {
         assert_eq!(build_archive_file_suggestion(Path::new("linux.png"), ".tar"), None);

src/utils/formatting.rs

@@ -1,4 +1,4 @@
-use std::{borrow::Cow, cmp, fmt::Display, path::Path};
+use std::{borrow::Cow, cmp, ffi::OsStr, fmt::Display, path::Path};
 
 use crate::CURRENT_DIRECTORY;
 
@@ -45,7 +45,11 @@ impl Display for EscapedPathDisplay<'_> {
 /// This is different from [`Path::display`].
 ///
 /// See <https://gist.github.com/marcospb19/ebce5572be26397cf08bbd0fd3b65ac1> for a comparison.
-pub fn to_utf(os_str: &Path) -> Cow<str> {
+pub fn path_to_str(path: &Path) -> Cow<str> {
+    os_str_to_str(path.as_ref())
+}
+
+pub fn os_str_to_str(os_str: &OsStr) -> Cow<str> {
     let format = || {
         let text = format!("{os_str:?}");
         Cow::Owned(text.trim_matches('"').to_string())
@@ -65,15 +69,15 @@ pub fn strip_cur_dir(source_path: &Path) -> &Path {
 /// Converts a slice of `AsRef<OsStr>` to comma separated String
 ///
 /// Panics if the slice is empty.
-pub fn pretty_format_list_of_paths(os_strs: &[impl AsRef<Path>]) -> String {
+pub fn pretty_format_list_of_paths(paths: &[impl AsRef<Path>]) -> String {
-    let mut iter = os_strs.iter().map(AsRef::as_ref);
+    let mut iter = paths.iter().map(AsRef::as_ref);
 
-    let first_element = iter.next().unwrap();
+    let first_path = iter.next().unwrap();
-    let mut string = to_utf(first_element).into_owned();
+    let mut string = path_to_str(first_path).into_owned();
 
-    for os_str in iter {
+    for path in iter {
         string += ", ";
-        string += &to_utf(os_str);
+        string += &path_to_str(path);
     }
     string
 }
@@ -83,7 +87,7 @@ pub fn nice_directory_display(path: &Path) -> Cow<str> {
     if path == Path::new(".") {
         Cow::Borrowed("current directory")
     } else {
-        to_utf(path)
+        path_to_str(path)
     }
 }
 

src/utils/logger.rs

@@ -37,6 +37,7 @@ fn info_with_accessibility(contents: String, accessible: bool) {
     });
 }
 
+#[track_caller]
 pub fn warning(contents: String) {
     logger_thread::send_log_message(PrintMessage {
         contents,
@@ -147,6 +148,16 @@ mod logger_thread {
         }
     }
 
+    #[cfg(test)]
+    // shutdown_and_wait must be called manually, but to keep 'em clean, in
+    // case of tests just do it on drop
+    impl Drop for LoggerThreadHandle {
+        fn drop(&mut self) {
+            send_shutdown_message();
+            self.shutdown_barrier.wait();
+        }
+    }
+
     pub fn spawn_logger_thread() -> LoggerThreadHandle {
         let log_receiver = setup_channel();
 

src/utils/mod.rs

@@ -11,18 +11,19 @@ pub mod io;
 pub mod logger;
 mod question;
 
-pub use file_visibility::FileVisibilityPolicy;
+pub use self::{
-pub use formatting::{
+    file_visibility::FileVisibilityPolicy,
-    nice_directory_display, pretty_format_list_of_paths, strip_cur_dir, to_utf, Bytes, EscapedPathDisplay,
+    formatting::{
+        nice_directory_display, os_str_to_str, path_to_str, pretty_format_list_of_paths, strip_cur_dir, Bytes,
+        EscapedPathDisplay,
+    },
+    fs::{
+        cd_into_same_dir_as, clear_path, create_dir_if_non_existent, is_path_stdin, is_symlink, remove_file_or_dir,
+        try_infer_extension,
+    },
+    question::{ask_to_create_file, user_wants_to_continue, user_wants_to_overwrite, QuestionAction, QuestionPolicy},
+    utf8::{get_invalid_utf8_paths, is_invalid_utf8},
 };
-pub use fs::{
-    cd_into_same_dir_as, clear_path, create_dir_if_non_existent, is_path_stdin, is_symlink, remove_file_or_dir,
-    try_infer_extension,
-};
-pub use question::{
-    ask_to_create_file, user_wants_to_continue, user_wants_to_overwrite, QuestionAction, QuestionPolicy,
-};
-pub use utf8::{get_invalid_utf8_paths, is_invalid_utf8};
 
 mod utf8 {
     use std::{ffi::OsStr, path::PathBuf};

src/utils/question.rs

@@ -11,11 +11,10 @@ use std::{
 
 use fs_err as fs;
 
-use super::{strip_cur_dir, to_utf};
 use crate::{
     accessible::is_running_in_accessible_mode,
     error::{Error, FinalError, Result},
-    utils::{self, colors, io::lock_and_flush_output_stdio},
+    utils::{self, colors, formatting::path_to_str, io::lock_and_flush_output_stdio, strip_cur_dir},
 };
 
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
@@ -44,7 +43,7 @@ pub fn user_wants_to_overwrite(path: &Path, question_policy: QuestionPolicy) ->
         QuestionPolicy::AlwaysYes => Ok(true),
         QuestionPolicy::AlwaysNo => Ok(false),
         QuestionPolicy::Ask => {
-            let path = to_utf(strip_cur_dir(path));
+            let path = path_to_str(strip_cur_dir(path));
             let path = Some(&*path);
             let placeholder = Some("FILE");
             Confirmation::new("Do you want to overwrite 'FILE'?", placeholder).ask(path)
@@ -83,7 +82,7 @@ pub fn user_wants_to_continue(
                 QuestionAction::Compression => "compress",
                 QuestionAction::Decompression => "decompress",
             };
-            let path = to_utf(strip_cur_dir(path));
+            let path = path_to_str(strip_cur_dir(path));
             let path = Some(&*path);
             let placeholder = Some("FILE");
             Confirmation::new(&format!("Do you want to {action} 'FILE'?"), placeholder).ask(path)

tests/integration.rs

@@ -6,7 +6,7 @@ use std::{iter::once, path::PathBuf};
 use fs_err as fs;
 use parse_display::Display;
 use proptest::sample::size_range;
-use rand::{rngs::SmallRng, Rng, RngCore as _, SeedableRng};
+use rand::{rngs::SmallRng, Rng, SeedableRng};
 use tempfile::tempdir;
 use test_strategy::{proptest, Arbitrary};
 

tests/snapshots/ui__ui_test_err_format_flag-2.snap

@@ -0,0 +1,14 @@
+---
+source: tests/ui.rs
+expression: "run_ouch(\"ouch compress input output --format targz\", dir)"
+---
+[ERROR] Failed to parse `--format targz`
+ - Unsupported extension 'targz'
+
+hint: Supported extensions are: tar, zip, bz, bz2, gz, lz4, xz, lzma, sz, zst, rar, 7z
+hint: Supported aliases are: tgz, tbz, tlz4, txz, tzlma, tsz, tzst
+hint: 
+hint: Examples:
+hint:   --format tar
+hint:   --format gz
+hint:   --format tar.gz

tests/snapshots/ui__ui_test_err_format_flag-3.snap

@@ -0,0 +1,14 @@
+---
+source: tests/ui.rs
+expression: "run_ouch(\"ouch compress input output --format .tar.$#!@.rest\", dir)"
+---
+[ERROR] Failed to parse `--format .tar.$#!@.rest`
+ - Unsupported extension '$#!@'
+
+hint: Supported extensions are: tar, zip, bz, bz2, gz, lz4, xz, lzma, sz, zst, rar, 7z
+hint: Supported aliases are: tgz, tbz, tlz4, txz, tzlma, tsz, tzst
+hint: 
+hint: Examples:
+hint:   --format tar
+hint:   --format gz
+hint:   --format tar.gz

tests/snapshots/ui__ui_test_err_format_flag.snap

@@ -0,0 +1,14 @@
+---
+source: tests/ui.rs
+expression: "run_ouch(\"ouch compress input output --format tar.gz.unknown\", dir)"
+---
+[ERROR] Failed to parse `--format tar.gz.unknown`
+ - Unsupported extension 'unknown'
+
+hint: Supported extensions are: tar, zip, bz, bz2, gz, lz4, xz, lzma, sz, zst, rar, 7z
+hint: Supported aliases are: tgz, tbz, tlz4, txz, tzlma, tsz, tzst
+hint: 
+hint: Examples:
+hint:   --format tar
+hint:   --format gz
+hint:   --format tar.gz

tests/snapshots/ui__ui_test_ok_format_flag-2.snap

@@ -0,0 +1,7 @@
+---
+source: tests/ui.rs
+expression: "run_ouch(\"ouch compress input output2 --format .tar.gz\", dir)"
+---
+[INFO] Compressing 'input'.
+[INFO] Successfully compressed 'output2'.
+

tests/snapshots/ui__ui_test_ok_format_flag.snap

@@ -0,0 +1,7 @@
+---
+source: tests/ui.rs
+expression: "run_ouch(\"ouch compress input output1 --format tar.gz\", dir)"
+---
+[INFO] Compressing 'input'.
+[INFO] Successfully compressed 'output1'.
+

tests/ui.rs

@@ -87,6 +87,29 @@ fn ui_test_err_missing_files() {
     ui!(run_ouch("ouch list a b", dir));
 }
 
+#[test]
+fn ui_test_err_format_flag() {
+    let (_dropper, dir) = testdir().unwrap();
+
+    // prepare
+    create_files_in(dir, &["input"]);
+
+    ui!(run_ouch("ouch compress input output --format tar.gz.unknown", dir));
+    ui!(run_ouch("ouch compress input output --format targz", dir));
+    ui!(run_ouch("ouch compress input output --format .tar.$#!@.rest", dir));
+}
+
+#[test]
+fn ui_test_ok_format_flag() {
+    let (_dropper, dir) = testdir().unwrap();
+
+    // prepare
+    create_files_in(dir, &["input"]);
+
+    ui!(run_ouch("ouch compress input output1 --format tar.gz", dir));
+    ui!(run_ouch("ouch compress input output2 --format .tar.gz", dir));
+}
+
 #[test]
 fn ui_test_ok_compress() {
     let (_dropper, dir) = testdir().unwrap();

tests/utils.rs

@@ -52,6 +52,7 @@ pub fn create_files_in(dir: &Path, files: &[&str]) {
 /// Write random content to a file
 pub fn write_random_content(file: &mut impl Write, rng: &mut impl RngCore) {
     let mut data = vec![0; rng.gen_range(0..4096)];
+
     rng.fill_bytes(&mut data);
     file.write_all(&data).unwrap();
 }