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CLESH — A Unix shell interface for Common Lisp

Christian von Essen

This manual documents CLESH (Common Lisp Embedded Shell) , a very short and simple program, written in Common Lisp, that extends Common Lisp to embed shell code in a manner similar to perl's backtick. It has been forked from from SHELISP by Alexandru Dan Corlana

Quick guide to clesh

Load any Common Lisp implemenation, and on the REPL load clesh by invoking

(load "clesh.lisp")
(use-package 'named-readtables)
(in-readtable clesh:syntax)

The bang (!) escape to shell

For convenience, and for use from the REPL, you can emit single line calls to the shell using the bang.

You can say (the '*' is already put there by your CL):

* !ls

And it will execute the shell ls command (by running a bash instance and passing the command to it. Of course, you are actually in Lisp. You can try this:

* (defun factorial (x) (if (zerop x) 1 (* x (factorial (1- x)))))
FACTORIAL
* (factorial 33) 8683317618811886495518194401280000000

So, if you enter '!' the rest of the line (until the first end of line that is not escaped with a “\”) is interpreted as a bash command and the result is printed on the standard output. Now try:

* !echo ?(+ 2 3) zuzu 5 zuzu

The '?' is the 'lisp escape'. It is followed by an s-expression which is read, executed and printed (with princ) and the printed result replaces the '?' and the expression in the shell command. It can be any Lisp expression.

* !echo ?(+ 2/3 2/11) " <- this is a fraction" 28/33 <- this is a
fraction
* !echo ?(factorial 100) " <- this is a beegnum"
933262154439441526816992388562667004907159682643816\
21468592963895217599993229915608941463\
97615651828625369792082722375825118521091686400\
0000000000000000000000 <- this is a beegnum

Provided that you already entered the factorial definition above. You may escape the '?' with a '\' to have it transfered to the shell command. for example:

* !echo \?\(+ 2 3\) ?(+ 2 3) 4

Embedded shell scripts

Anything written between square brackets is interpreted as a shell script. What the script prints on the standard output, however, is not displayed, but collected in a string and returned as a result of the bracketed expression. For example:

* [echo hi there!] "hi there! " "" 0

One thing that you can't ordinarily do in bash:

* (dotimes (i 7) (princ [echo ?i])) 0 1 2 3 4 5 6

You can now say:

* (defun count-to (x) (dotimes (i x) (princ [echo ?i ]))) COUNT-TO
* (COUNT-TO 3) 0 1 2

Or, for example:

* (defun c-c-count-to (x) (dotimes (i x) (princ
    [echo ?i | sed 's/\(.\)/\1-\1-\1/' ]) )) C-C-COUNT-TO
* (c-c-count-to 3) 0-0-0 1-1-1 2-2-2
  

Switching to shell mode (double-bang, '!!')

If you enter a double bang ('!!') then the prompter is changed to '$' and you can issue unescaped shell command until you start a line with '!!' again - then you revert to 'lisp mode'. Constructs with '?' are honored and are read and evaluated immediately by Lisp. Results of commands are printed immediately after being issued. For example:

* !! $ ls Makefile clesh.lisp clesh_mn.aux clesh_mn.log
clesh_mn.pdf clesh_mn.tex clesh_sc.aux clesh_sc.log clesh_sc.pdf
clesh_sc.tex clesh.tex spec.txt $ # ?(setq bb 33.34) $ echo ?bb "
is " ?(sqrt bb) " squared." 33.34 is 5.77408 squared. $ echo ?bb "
is " ?(sqrt bb) " squared." >somefile.txt $ cat somefile.txt 33.34
is 5.77408 squared. $ echo "I am almost sure that " 'cat
somefile.txt' I am almost sure that 33.34 is 5.77408 squared. $ !!
$ NIL *

Notice how purely Lisp commands, such as variable assignement (bindings), can be escaped with '#' char-acters as bash comments.

Run scripts as Lisp calls

The function script takes as argument a string and executes it as a bash script, returning the standard output of the script as a string as first value, the error output of the script as second value and the return value as last value.

* (script "ls") " Makefile clesh.lisp clesh_mn.aux clesh_mn.log
clesh_mn.pdf clesh_mn.tex clesh_sc.aux clesh_sc.log clesh_sc.pdf
clesh_sc.tex clesh.tex spec.txt " "" 0

Templates

A template is a string introduced with '#[' and ended with ']#'. It is treated like an usual string, however '?'-preceded lisp expressions are evaluated and their result printed inside the string. For example:

(defvar *title* "Title of an empty page")
    
...

(prin1
#[Content-type: text/html <html> <head><title> ?*title* </title></head> <body></body> </html> ]#)

Will print to '*standard-output*':

Content-type: text/html <html> <head><title>Title of an empty
page</title></head> <body></body> </html>

Storable templates

One problem with templates is that we might desire to run them at a later time, in a different context. For example, we might want to define a variable with a generic web-page template and then generate actual web pages at later times, with various contents. We use '#{' and '}#' for this purpose. In the example below notice that each time the value of variable A is evaluated, the BB in the evaluation context is used.

*(setf bb 9) 9
* (setf a #{ plus: ?bb :sulp }#) (MIXED-TEMPLATE " plus: " BB "
:sulp ")
* (setf bb 10) 10
* (eval a) " plus: 10 :sulp "
* (setf bb 22) 22
* (eval a) " plus: 22 :sulp "
* (defun calc-a (bb) (eval a)) CALC-A
* (calc-a 88) " plus: 88 :sulp "
* (eval a) " plus: 22 :sulp "

Other fun things to do

The program that is called for the embedded script is controlled by the variable *shell* in package clesh. If you want to use another shell (or indeed anything else), then set this variable to the path of the program to use.

For example, you can set it to cat -n, and you will get everything you enclose in brackets back with line numbers in front:

  [ foo
    bar
    baz ]

will return as first string

 1 foo
 2 bar
 3 baz

Furthermore, this easily allows to talk to remote PCs via an ssh connection or via netcat.

Escape policy

We use the backslash chatacter '\' to escape special parts of an input string. Special characters are: '?' and whatever is used as end-markers (i.e., ']' or '}'). If they are seen with a backslash before, then the backslash will vanish and the usual, special meaning of the character will be ignored. In all other cases, the backslash will just be left alone.

When you have multiple backslashes in front of a special char, then a double back-slash will be replaced with a single-backslash. If a single backslash remains after this replacement, then it escapes the special character. Otherwise, the special character keeps its usual meaning.

Assume x is bound to 1.

#[?x]#    ==> "1"
#[\?x]#   ==> "?x"
#[\\?x]#  ==> '\?x'
#[\\\?x]# ==> '\1'

Technical issues

Expressions preceded with '?' in the embedded shell scripts (with the '[]' syntax) are evaluated in the context where they appear, at 'eval' time; expressions in the 'bang' context (with the '!' or the '!!' syntax) are evaluated at read time, in the context of the last top level form before the form containing the bangs. This is because the bangs are intended for shell-only commands, normally given at the top level (command line) with immediate results. The embedded scripts are supposed to become part of functions or more complex forms, are parsed at read time and prepared to be executed at runtime.

In the 'bang' forms only simple shell commands can be issued as the reader does not detect the circumstances when a construct (such as ca 'case') occupies more than one line. In the embedded form or with the script command, any script can be executed.