--- name: "mdo" section: "1" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["freebsd"] url: "https://new.carta.tech/mdo" --- # mdo(1) execute commands with specific credentials ## NAME **mdo** – execute commands with specific credentials ## SYNOPSIS **mdo** \[**-u** *user* \| **-k**\] \[**-i**\] \[**-g** *group*\] \[**-G** *group1,group2,...*\] \[**-s** *groups\_mod1,groups\_mod2,...*\] \[**-h**\] \[**--ruid** *user*\] \[**--svuid** *user*\] \[**--euid** *user*\] \[**--rgid** *group*\] \[**--svgid** *group*\] \[**--egid** *group*\] \[--\] \[*command* \[*args* *...*\]\] ## DESCRIPTION The **mdo** utility executes the passed *command* with the requested process credentials or, if no *command* was specified, the program whose path is the value of the SHELL environment variable or */bin/sh* if that variable is unset. The calling user must either be the superuser (effective user ID of 0) or the credentials transition from the caller's to the requested ones must be authorized by a MAC module such as [mac\_do(4)](/mac_do/4.md). The target process credentials are applied atomically using [setcred(2)](/setcred/2.md). Process credentials comprise the real, effective and saved user IDs, the real, effective and saved group IDs, hereby called the “primary” groups, and the supplementary groups as a set of group IDs. Below, the “user” phrase implies that the real, effective and saved user IDs all have or are going to be set to the same value. The “primary group” phrase is used similarly with respect to primary groups. The target process credentials have to be fully specified, either explicitly by listing all attributes and their requested values, or indirectly by establishing a baseline that provides a default value for each attribute, which can then be amended by additional options. Possible baselines are either the full set of credentials established at login for a specific named user, the current credentials, or the current credentials' primary and supplementary groups which implies some user is specified explicitly. They are respectively established by using either option **-u** with a named user argument, option **-k**, or option **-i** in conjunction with **-u** or no other options. If no other option than **-i** appears, a default of **-u** **root** is implied. The primary group can be set or amended with option **-g**, whereas the supplementary groups can be either fully replaced with an explicit list using option **-G** or amended through set-like operations with option **-s**. Any of the individual real, effective and saved user and group IDs can be overridden separately if desired through the options **--ruid**, **--euid**, and **--svuid** for users, and **--rgid**, **--egid**, and **--svgid** for groups respectively. The options are: - **--euid** *user* Override the effective user. As for **-u**, *user* may either be a name or a numerical ID. - **--egid** *group* Override the effective group. As for **-g**, *group* may either be a name or a numerical ID. - **-G** *group1,group2,...* Set or replace the full set of supplementary groups. As for **-g**, groups can be specified by name or numerical ID. Groups must be separated by commas, and spaces around commas are not allowed. - **-g** *group* Set or amend the primary group. *group* may be the name of a group in the group database, else will be interpreted as a numerical group ID. - **-h** Display usage information and exit. - **-i** Uses the current credentials' primary and supplementary groups as the baseline. If no other option is present, the target user is assumed to be “root”. Otherwise, **-u** or **-k** must be specified. - **-k** Use the current credentials as the baseline. Incompatible with **-u**. Implies **-i**. - **--ruid** *user* Override the real user. As for **-u**, *user* may either be a name or a numerical ID. - **--rgid** *group* Override the real group. As for **-g**, *group* may either be a name or a numerical ID. - **-s** *groups\_mod1,groups\_mod2,...* Incrementally modify the supplementary groups set. The argument is a comma-separated list of directives: - **@** Reset the set to the empty set. When present, must be the first directive. - **+***group* Include a group. - **-***group* Exclude a group. If **-G** is also specified, **-s** applies on the list installed by it. In this case, the **@** directive cannot be used (this limitation may be lifted in the future). - **--svuid** *user* Override the saved user. As for **-u**, *user* may either be a name or a numerical ID. - **--svgid** *group* Override the saved group. As for **-g**, *group* may either be a name or a numerical ID. - **-u** *user* Specify a target user. If *user* is the name of some user in the user database, this option establishes his full login credentials, as specified by the user and group databases, as the baseline. Else, *user* is interpreted as a numerical user ID, and that ID is used to set the target user only. ## EXAMPLES Run a command as another user: ``` mdo -u alice id ``` Run with explicit primary and supplementary groups: ``` mdo -u 1001 -g wheel -G staff,operator /bin/sh ``` Modify only supplementary groups for the current user: ``` mdo -k -s +wheel,+operator /usr/bin/id ``` Emulate the effect of a set-user-ID bit on the process image file, assuming its user ID is “root”: ``` mdo -k --euid root --svuid root id ``` ## SEE ALSO [su(1)](/su/1.md), [setcred(2)](/setcred/2.md), [mac\_do(4)](/mac_do/4.md), [security(7)](/security/7.md) ## HISTORY The **mdo** command first appeared in FreeBSD 14.2. Support for specifying or amending groups, group-only transitions and fine-grained control of real, effective and saved variants of user and primary group first appeared in FreeBSD 15.0. ## AUTHORS The **mdo** program was originally created by Baptiste Daroussin ⟨*bapt@FreeBSD.org*⟩. It was modified to use the [setcred(2)](/setcred/2.md) system call by Olivier Certner ⟨*olce@FreeBSD.org*⟩, who designed the group-related and fine-grained-control-of-target-credentials functionalities and supervised Kushagra Srivastava ⟨*kushagra1403@gmail.com*⟩ to add them during Google Summer of Code 2025. ## SECURITY CONSIDERATIONS The **mdo** program is geared to role-based scenarios. Consequently, it does not ask for any password or request other form of authentication before trying to establish new credentials, instead relying solely on the requester's credentials for this purpose. Specific unprivileged uses may be enabled by using the [mac\_do(4)](/mac_do/4.md) security policy.