diff --git a/src/Streamly/Coreutils/Id.hs b/src/Streamly/Coreutils/Id.hs
new file mode 100644
index 0000000..d22c845
--- /dev/null
+++ b/src/Streamly/Coreutils/Id.hs
@@ -0,0 +1,385 @@
+{-# OPTIONS_GHC -Wno-missing-signatures #-}
+-- |
+-- Module      : Streamly.Coreutils.Id
+-- Copyright   : (c) 2022 Composewell Technologies
+-- License     : BSD-3-Clause
+-- Maintainer  : streamly@composewell.com
+-- Stability   : experimental
+-- Portability : GHC (POSIX only)
+--
+-- Experimental alternative wrapper API over "System.Posix.User".
+--
+-- Provides the read-only functionality of the @id@, @whoami@, and @logname@
+-- coreutils commands, intended for programmatic use.
+
+module Streamly.Coreutils.Id
+    (
+    -- * Options
+    -- | Start from 'defaultConfig' (effective user) and compose modifiers.
+      IdOptions
+    , idReal
+    , idEffective
+    , idUser
+    , idGroup
+
+    -- * Owner ids of the current process
+    , idNum
+    , idName
+    -- , idInfo -- get (Num, Name)
+
+    -- * Groups of the current process user
+    , groupIds
+    , groupNames
+
+    -- * Login name of the current process user
+    , loginName
+
+    -- XXX FromUserId is common in all these, we can assume that as implicit
+    -- and remove it and maybe prefix these with get.
+    --
+    -- XXX To save db lookups we should perhaps get the entire user record and
+    -- use that to print whatever fields we want. We can build a formatter to
+    -- use the fields.
+
+    -- * Lookups by user id
+    , userNameFromId
+    , groupNameFromId
+    , primaryGroupIdFromUserId
+    , primaryGroupNameFromUserId
+    , groupIdsFromUserId
+    , groupNamesFromUserId
+
+    {-
+    -- * Numeric ids of the current process
+    , realUserId
+    , effectiveUserId
+    , realGroupId
+    , effectiveGroupId
+
+    -- * Names for the current process
+    , realUserName
+    , effectiveUserName
+    , realGroupName
+    , effectiveGroupName
+    -}
+    )
+where
+
+import Control.Exception (try, SomeException)
+import Data.List (nub)
+import qualified System.Posix.User as Posix
+
+-- = Design notes
+--
+-- * __Scope: current process only.__ This module only reports identity
+--   information about the /current process/. Looking up identity details for
+--   an arbitrary named user (i.e. @id \<username\>@) is a separate concern
+--   that requires reading the user/group database (@/etc/passwd@,
+--   @/etc/group@, NSS, etc.). That functionality will live in a separate
+--   module (e.g. @Streamly.Coreutils.UserDB@) and is not implemented here.
+--
+-- * __Scope: read-only.__ Setting the uid/gid of the current process
+--   (@setuid@, @setgid@) is the domain of @sudo@-style utilities and has
+--   significant security implications. It is intentionally not exposed from
+--   this module.
+--
+-- * __Portability.__ This is a POSIX-only module. A portable alternative
+--   exposing a minimal common subset (e.g. @idNum@, @idName@) could be added
+--   later.
+--
+-- * __Int for ids.__ User and group ids are exposed as 'Int' for
+--   convenience and brevity at call sites. Alternatives considered:
+--   'System.Posix.Types.UserID' / 'System.Posix.Types.GroupID' (which are
+--   @CUid@ / @CGid@ newtypes) would be more type-safe (a uid cannot be
+--   confused with a gid) and avoid any downcast concerns on platforms where
+--   these are wider than 'Int'. If type safety or exotic-platform
+--   correctness becomes a priority, switch to those. Downcasts via
+--   'fromIntegral' are safe on all mainstream 64-bit platforms where
+--   uid_t/gid_t are 32 bits.
+--
+-- * __'Maybe' for DB lookups.__ Functions that may fail to find a matching
+--   entry return 'Maybe' rather than throwing, which is friendlier for
+--   callers. The underlying @System.Posix.User@ primitives throw on a miss;
+--   we catch and convert.
+--
+-- * __Individual functions over a bundled record.__ Each piece of
+--   information is exposed as its own function rather than a combined
+--   @IdInfo@ record. A record would not save syscalls here (each field
+--   corresponds to a distinct primitive), so the extra surface area isn't
+--   justified. Revisit if a batched primitive becomes available.
+--
+-- * __@whoami@ is just @effectiveUserName@__ and is not exposed as a
+--   separate function to avoid redundancy.
+--
+-- * __Scalar router ('idNum' \/ 'idName').__ In addition to the individual
+--   named functions, a pair of \"router\" functions driven by 'IdOptions'
+--   is provided. They are convenient when the choice of real\/effective
+--   and user\/group is determined dynamically (e.g. from parsed CLI flags).
+--   The individual functions remain the preferred style when the choice
+--   is known statically — they avoid the runtime dispatch and are more
+--   self-describing at the call site.
+--
+-- * __Router is scalar-only.__ 'idNum' and 'idName' only cover the
+--   scalar queries (real\/effective × user\/group). The list-returning
+--   queries ('groupIds', 'groupNames') and 'loginName' do not fit the
+--   router's return type and are kept as standalone functions rather than
+--   forced into a polymorphic or awkwardly-typed router.
+--
+
+------------------------------------------------------------------------------
+-- Lookups by user / group id
+------------------------------------------------------------------------------
+
+-- These functions are user/group-DB lookups. They logically belong in a
+-- future Streamly.Coreutils.UserDB module and will likely move there; a
+-- re-export from here may be kept for backwards compatibility when that
+-- happens.
+
+-- | Look up the user name for a uid. Returns 'Nothing' if no matching
+-- user-db entry exists.
+userNameFromId :: Int -> IO (Maybe String)
+userNameFromId i = do
+    r <- try (Posix.getUserEntryForID (fromIntegral i))
+    return $ case r of
+        Left (_ :: SomeException) -> Nothing
+        Right ue -> Just (Posix.userName ue)
+
+-- | Look up the group name for a gid. Returns 'Nothing' if no matching
+-- group-db entry exists.
+groupNameFromId :: Int -> IO (Maybe String)
+groupNameFromId i = do
+    r <- try (Posix.getGroupEntryForID (fromIntegral i))
+    return $ case r of
+        Left (_ :: SomeException) -> Nothing
+        Right ge -> Just (Posix.groupName ge)
+
+-- Internal: look up the full passwd entry for a uid, returning Nothing on
+-- miss rather than throwing.
+userEntryFromId i = do
+    r <- try (Posix.getUserEntryForID (fromIntegral (i :: Int)))
+    return $ case r of
+        Left (_ :: SomeException) -> Nothing
+        Right ue -> Just ue
+
+-- | Primary group id of the user with the given uid. This is @pw_gid@
+-- from the user's passwd entry. Corresponds to @id -g \<uid\>@.
+--
+-- Returns 'Nothing' if no passwd entry exists for the uid.
+primaryGroupIdFromUserId :: Int -> IO (Maybe Int)
+primaryGroupIdFromUserId i =
+    fmap (fromIntegral . Posix.userGroupID) <$> userEntryFromId i
+
+-- | Primary group name of the user with the given uid. Corresponds to
+-- @id -gn \<uid\>@.
+--
+-- Returns 'Nothing' if no passwd entry exists for the uid, or if the
+-- primary gid has no matching group-db entry.
+primaryGroupNameFromUserId :: Int -> IO (Maybe String)
+primaryGroupNameFromUserId i = do
+    mGid <- primaryGroupIdFromUserId i
+    case mGid of
+        Nothing  -> return Nothing
+        Just gid -> groupNameFromId gid
+
+-- | All group ids the user with the given uid belongs to: the primary
+-- group from the passwd entry, plus every group in the group database
+-- whose member list contains the user name. Deduplicated. Corresponds
+-- to @id -G \<uid\>@.
+--
+-- Returns 'Nothing' if no passwd entry exists for the uid.
+--
+-- Note: group membership lookup walks 'Posix.getAllGroupEntries', which
+-- reflects only @/etc/group@. Groups provided exclusively via NSS
+-- backends (LDAP, SSSD, etc.) will not be seen. A future version could
+-- FFI to @getgrouplist(3)@ for full NSS coverage.
+--
+-- Note: in contrast to 'groupIds' (current process, which cannot fail),
+-- this returns 'Maybe' to distinguish a non-existent user from a user
+-- whose only group is the primary.
+groupIdsFromUserId :: Int -> IO (Maybe [Int])
+groupIdsFromUserId i = do
+    mUe <- userEntryFromId i
+    case mUe of
+        Nothing -> return Nothing
+        Just ue -> do
+            let uname   = Posix.userName ue
+                primary = fromIntegral (Posix.userGroupID ue)
+            allGroups <- Posix.getAllGroupEntries
+            let supp = [ fromIntegral (Posix.groupID g)
+                       | g <- allGroups
+                       , uname `elem` Posix.groupMembers g
+                       ]
+            return $ Just (nub (primary : supp))
+
+-- | Names of all groups the user with the given uid belongs to, in the
+-- same order as 'groupIdsFromUserId'. Corresponds to @id -Gn \<uid\>@.
+--
+-- Returns 'Nothing' if no passwd entry exists for the uid. Entries for
+-- which no group-db record exists are silently dropped.
+--
+-- See 'groupIdsFromUserId' for the note on NSS coverage.
+groupNamesFromUserId :: Int -> IO (Maybe [String])
+groupNamesFromUserId i = do
+    mGids <- groupIdsFromUserId i
+    case mGids of
+        Nothing   -> return Nothing
+        Just gids -> do
+            mNames <- mapM groupNameFromId gids
+            return $ Just [ n | Just n <- mNames ]
+
+------------------------------------------------------------------------------
+-- Current process: numeric ids
+------------------------------------------------------------------------------
+
+-- | Real user id of the current process. Corresponds to @id -ru@.
+realUserId :: IO Int
+realUserId = fromIntegral <$> Posix.getRealUserID
+
+-- | Effective user id of the current process. Corresponds to @id -u@.
+effectiveUserId :: IO Int
+effectiveUserId = fromIntegral <$> Posix.getEffectiveUserID
+
+-- | Real group id of the current process. Corresponds to @id -rg@.
+realGroupId :: IO Int
+realGroupId = fromIntegral <$> Posix.getRealGroupID
+
+-- | Effective group id of the current process. Corresponds to @id -g@.
+effectiveGroupId :: IO Int
+effectiveGroupId = fromIntegral <$> Posix.getEffectiveGroupID
+
+-- | All group ids the current process belongs to: the effective primary
+-- group plus all supplementary groups, deduplicated. Corresponds to @id -G@.
+--
+-- Note: @getgroups(2)@ alone returns only the supplementary list, and
+-- whether that list includes the primary gid is OS-dependent. This function
+-- explicitly merges the primary gid with the supplementary list to match
+-- the @id -G@ command's output.
+groupIds :: IO [Int]
+groupIds = do
+    primary <- effectiveGroupId
+    supp <- map fromIntegral <$> Posix.getGroups
+    return $ nub (primary : supp)
+
+------------------------------------------------------------------------------
+-- Current process: names
+------------------------------------------------------------------------------
+
+{-
+-- | Real user name of the current process. Corresponds to @id -unr@.
+--
+-- 'Nothing' if there is no user-db entry for the real uid.
+realUserName :: IO (Maybe String)
+realUserName = realUserId >>= userNameFromId
+
+-- | Effective user name of the current process. Corresponds to @id -un@
+-- and @whoami@.
+--
+-- 'Nothing' if there is no user-db entry for the effective uid.
+effectiveUserName :: IO (Maybe String)
+effectiveUserName = effectiveUserId >>= userNameFromId
+
+-- | Real group name of the current process. Corresponds to @id -gnr@.
+--
+-- 'Nothing' if there is no group-db entry for the real gid.
+realGroupName :: IO (Maybe String)
+realGroupName = realGroupId >>= groupNameFromId
+
+-- | Effective group name of the current process. Corresponds to @id -gn@.
+--
+-- 'Nothing' if there is no group-db entry for the effective gid.
+effectiveGroupName :: IO (Maybe String)
+effectiveGroupName = effectiveGroupId >>= groupNameFromId
+-}
+
+-- | Names of all groups the current process belongs to, in the same order
+-- as 'groupIds'. Corresponds to @id -Gn@.
+--
+-- Entries for which no group-db record exists are silently dropped.
+groupNames :: IO [String]
+groupNames = do
+    gs <- groupIds
+    mNames <- mapM groupNameFromId gs
+    return [ n | Just n <- mNames ]
+
+-- | Original login name of the session the current process belongs to.
+--
+-- Note: this can differ from 'effectiveUserName' after operations like
+-- @su@ or @setuid@ — the login name reflects who originally logged in,
+-- not who the process is currently acting as. Corresponds to the
+-- @logname@ command.
+loginName :: IO String
+loginName = Posix.getLoginName
+
+------------------------------------------------------------------------------
+-- Option-driven router
+------------------------------------------------------------------------------
+
+-- | Options for the 'idNum' and 'idName' router functions. Use
+-- 'defaultConfig' together with the modifiers 'idReal', 'idEffective',
+-- 'idUser', 'idGroup' to configure.
+--
+-- The default is /effective user/, matching the behaviour of @id -u@ and
+-- @id -un@ when invoked without @-r@ or @-g@.
+data IdOptions = IdOptions
+    { idoReal  :: Bool  -- ^ True = real id, False = effective id
+    , idoGroup :: Bool  -- ^ True = group, False = user
+    }
+
+-- | Default configuration: effective user.
+defaultConfig :: IdOptions
+defaultConfig = IdOptions { idoReal = False, idoGroup = False }
+
+-- | Select the real id instead of the effective id.
+idReal :: IdOptions -> IdOptions
+idReal cfg = cfg { idoReal = True }
+
+-- | Select the effective id (the default). Useful when composing modifiers
+-- dynamically and you need to explicitly override a prior 'idReal'.
+idEffective :: IdOptions -> IdOptions
+idEffective cfg = cfg { idoReal = False }
+
+-- | Select the group id instead of the user id.
+idGroup :: IdOptions -> IdOptions
+idGroup cfg = cfg { idoGroup = True }
+
+-- | Select the user id (the default). Useful when composing modifiers
+-- dynamically and you need to explicitly override a prior 'idGroup'.
+idUser :: IdOptions -> IdOptions
+idUser cfg = cfg { idoGroup = False }
+
+-- TODO: After proper inlining this should get simplified such that there is
+-- not dynamic dispatch for choosing the options. So we can keep this as a
+-- simplified single point function instead of separate functions. Check GHC
+-- core if it gets simplified.
+
+-- | Return a numeric id (uid or gid, real or effective) of the current
+-- process, based on the given options.
+--
+-- > idNum id                          -- effective uid  (id -u)
+-- > idNum idReal                      -- real uid       (id -ru)
+-- > idNum idGroup                     -- effective gid  (id -g)
+-- > idNum (idReal . idGroup)          -- real gid       (id -rg)
+idNum :: (IdOptions -> IdOptions) -> IO Int
+idNum f =
+    case (idoReal cfg, idoGroup cfg) of
+        (False, False) -> effectiveUserId
+        (True,  False) -> realUserId
+        (False, True ) -> effectiveGroupId
+        (True,  True ) -> realGroupId
+  where
+    cfg = f defaultConfig
+
+-- | Return the name corresponding to a uid or gid (real or effective) of
+-- the current process, based on the given options. Returns 'Nothing' if
+-- the corresponding user- or group-db entry does not exist.
+--
+-- > idName id                         -- effective user name  (id -un)
+-- > idName idReal                     -- real user name       (id -unr)
+-- > idName idGroup                    -- effective group name (id -gn)
+-- > idName (idReal . idGroup)         -- real group name      (id -gnr)
+idName :: (IdOptions -> IdOptions) -> IO (Maybe String)
+idName f = do
+    n <- idNum f
+    if idoGroup cfg then groupNameFromId n else userNameFromId n
+  where
+    cfg = f defaultConfig
diff --git a/streamly-coreutils.cabal b/streamly-coreutils.cabal
index b9268e6..3892dd1 100644
--- a/streamly-coreutils.cabal
+++ b/streamly-coreutils.cabal
@@ -144,6 +144,7 @@ library
   else
       exposed-modules:
           Streamly.Coreutils.FileTest.Posix
+        , Streamly.Coreutils.Id
   other-modules:
       Streamly.Coreutils.String
     , Streamly.Coreutils.Uniq