A utility for running external programs

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Paul M. Rodriguez <>




A utility for running external programs, built on uiop:launch-program.

Cmd is designed to:

  1. Be natural to use.
  2. Protect against shell interpolation.
  3. Be usable from multi-threaded programs.
  4. Support Windows.

Argument handling

Arguments to cmd are never passed to a shell for interpretation.

Arguments are usually:

  • strings
  • keywords
  • lists of strings and keywords

Some other types get special handling. Nested lists are not allowed.

Arguments are handled as follows:

  1. A string is tokenized (using cl-shlex) and added to the list of arguments.

    (cmd "ls -al")
    (uiop:wait-process (uiop:launch-program '("ls" "-al")))
    (cmd "echo 'hello world'")
    (uiop:wait-process (uiop:launch-program '("echo" "hello world")))

    Redirection operators in the tokenized string (such as <, >, or |) are translated into keywords (see below).

    (cmd "echo 'hello world' > myfile")
    (cmd '("echo" "hello world" :> "myfile"))
  2. A list is added directly to the list of arguments (not tokenized). (Putting a string in a list is “escaping” it.)

    (cmd "bash -c 'exit 1'")
    (cmd "bash -c" '("exit 1"))

    Keywords in the list are treated exactly like keywords as arguments.

  3. Keywords that are subcommand dividers (like |) are handled internally by cmd. Otherwise, a keyword, along with the next value, is used as a keyword argument to UIOP.

    (cmd "bash -c 'exit 1'" :ignore-error-status t)
    (cmd :ignore-error-status t "bash -c 'exit 1'")
    (cmd :check nil "bash -c 'exit 1'")
    (cmd "bash -c" :ignore-error-status t '("exit 1"))
    (cmd "bash -c" :check nil '("exit 1"))

    Note that unlike normal Lisp functions, keyword arguments can appear anywhere, not just at the end.

    Also note that :check is accepted as an alias for :ignore-error-status, although the value is negated before being passed to UIOP.

  4. Any character, integer, or pathname is directly added to the list of arguments, as if it were an escaped string. (It is an error if a pathname begins with -.)

  5. Cmd supports a basic form of process substitution, running processes as input to commands that expect files. To construct a process substitution, use the psub Lisp function.

    (cmd? "diff" (psub "echo x") (psub "echo x"))
    => T
    (cmd? "diff" (psub "echo x") (psub "echo y"))
    => NIL

    (For this specific case, however – passing a string to a command that expects a file – use psub-echo or psub-format, which don’t actually call an external program.)

The external program’s working directory

Cmd is designed with multi-threaded programs in mind. It always runs programs with their working directory relative to *default-pathname-defaults*. This is because the OS-level working directory a program, on both Windows and Unix, is the working directory of the entire process, not the individual thread, and changing it changes it for all threads.

You can also specify the directory for a particular command with the keyword argument :in:

(cmd "ls" :in #p"/")
(cmd :in #p"/" "ls")
=> /bin /home /tmp /usr ...

For convenience Cmd supplies the macro with-working-directory:

(with-working-directory (dir)
  (cmd ...)
  (cmd ...))
    (cmd :in dir ...)
    (cmd :in dir ...))

The external program’s environment

For Unix users only, the variable *cmd-env* holds an alist of extra environment variables to set for each call to cmd.

(let ((*cmd-env* (acons "GIT_PAGER" "cat" *cmd-env*)))
  (cmd "git diff" ...))

We are currently very restrictive about what we consider a valid environment variable name.

Controlling PATH

For controlling the PATH environment variable, the Lisp variable *cmd-path* can be used:

(let ((*cmd-path* (cons #p"~/.local/bin" *cmd-path*)))

Directories in *cmd-path* are prepended to PATH.

This uses the same mechanism as *cmd-env*, so it also only works on Unix.

Entry points

The cmd package offers several entry points:

  • cmd runs an external program synchronously, returning the exit code. By default, on a non-zero exit it signals an error.

    (cmd "cat /etc/os-release")
    NAME="Ubuntu" [...]
    => 0
  • $cmd returns the output of the external program as a string, stripping any trailing newline. (Much like $(cmd) in a shell.) The exit code is returned as a second value.

    ($cmd "date")
    => "Sun Sep 27 15:43:01 CDT 2020", 0
  • cmd! runs an external program purely for side effects, discarding all output and returning nothing. If the program exits non-zero, however, it will still signal an error.

  • cmd? returns t if the external program returned 0, and nil otherwise, with the exit code as a second value. As other variants by default signal an error if the process exists non-zero, cmd? is useful for programs that are expected to fail.

    (cmd? "kill -0" pid)
    => T, 0   ;; PID is a live process
    => NIL, 1 ;; PID is not a live process
  • cmd& runs an external program asynchronously (with uiop:launch-program) and returns a UIOP process-info object.

    (cmd& "cp -a" src dest)
    => #<PROCESS-INFO ...>

Error handling

By default, Cmd stores the stderr of a process, and if there is an error (due to non-zero exit) it presents the stderr as part of the error message.

Accordingly cmd errors are a subclass of uiop:subprocess-error. The stored stderr can be accessed with cmd:cmd-error-stderr.


Redirection is accomplished via either tokenized strings or keyword arguments. These should be self-explanatory to anyone who has used a shell.

;;; Using keyword arguments.
(cmd "echo 'hello world'" :> "hello.txt")
(cmd "cat hello.txt")
=> hello world
;; Append
(cmd "echo 'goodbye world'" :>> "hello.txt")
(cmd "cat hello.txt")
=> hello world
   goodbye world
(cmd "tar cf - hello.txt" :> #p"hello.tar")
(cmd "rm hello.txt")
(cmd "tar xf hello.tar")
(cmd "cat hello.txt")
=> hello world
goodbye world

;;; Using tokenized strings.
(cmd "echo 'hello world' > hello.txt")
(cmd "cat hello.txt")
=> hello world
;; Append
(cmd "echo 'goodbye world' >> hello.txt")
(cmd "cat hello.txt")
=> hello world
goodbye world
(cmd "tar cf - hello.txt > hello.tar")
(cmd "rm hello.txt")
(cmd "tar xf hello.tar")
(cmd "cat hello.txt")
=> hello world
goodbye world

Redirection with keyword arguments is usually more readable when the arguments are computed.

Supported directions include:

  • :< Redirect stdin.
  • :>, :1> Redirect stdout.
  • :>>, :1>> Append stdout.
  • :2> Redirect stderr.
  • :2>> Append stderr.
  • :&>, :>& Redirect stdout and stderr.
  • :&>>, :>>& Append stdout and stderr.
  • :<<< Provide input from a “here string”.

Note that redirections are interpreted according to the rules for Lisp keywords (only the first occurrence of a keyword argument matters), not the side-effecting rules for redirections in POSIX shells.


The simplest way to set up pipelines is to use tokenized strings:

(cmd "cat /usr/share/dict/words | sort | uniq -c | sort -nrs | head -3")
=>    1 a
      1 A
      1 Aachen

Alternately you can use keywords. While :|\|| is acceptable, you can write "|" instead. (Remember "|" will be tokenized to '(:|\||).)

(cmd "cat /usr/share/dict/words"
     "|" '("sort")
     "|" '("uniq" "-c")
     "|" '("sort" "-nrs")
     "|" '("head" "-3"))
=>    1 a
      1 A
      1 Aachen

Again, separating out the pipeline symbols is usually more readable when the subcommands are computed.

Controlling cmd with hooks

There are two hooks you can use to control cmd. These are exported from the cmd/hooks package (so you can :use :cmd without having to worry about them.) Both hooks expect a list of functions of one argument.

The hook *message-hook* is called with the external program and its arguments, quoted as a shell command line. This can be useful for logging commands as they are run.

The hook *proc-hook* is called with the process object (as returned by uiop:launch-program). This can be useful if you want to be able to track what is being run in a particular dynamic extent.


On Windows only, the first argument (the program name) has .exe appended to it automatically if it doesn’t already have a file extension.


While cmd does not use a shell to interpret its arguments, it may still have to run a shell (sh on Unix, cmd.exe on Windows) in order to change the working directory of the program.

How inefficient this is depends on what your distribution uses as a shell; it is faster when sh is, say, dash, than when it is bash.

Recent versions of GNU env support a -C switch to do this directly. When that is supported (support is detected dynamically) then env -C is used in place of a shell and overhead is negligible.


Cmd is a spinoff of Overlord, a Common Lisp build system, and was inspired by the cmd function in Shake, a Haskell build system, as well as the Julia language’s shell command facility. The psub function is inspired by the builtin of the same name in the Fish shell.


  • Pipelines should have “pipefail” behavior.
  • Pipelines should support stderr as well (2|, &|).
  • Efferent process substitution should also be supported.
  • There should be a special variable holding an alist of extra environment variables to set when running a command. (The problem here is Windows.)

Dependencies (7)

  • alexandria
  • cl-shlex
  • fiveam
  • serapeum
  • trivia
  • trivial-garbage
  • uiop

Dependents (2)

  • GitHub
  • Quicklisp