Schemesh is an interactive shell scriptable in Lisp.
It is primarily intended as a user-friendly Unix login shell, replacing bash, zsh, pdksh etc.
As such, it supports interactive line editing, autocompletion, history and the familiar Unix shell syntax:
it can start commands, including redirections, pipelines, job concatenation with ; && ||,
groups surrounded by { }, subshells surrounded by [ ], and manage foreground/background jobs.
For more complex tasks, it seamlessly integrates a full Lisp REPL backed by Chez Scheme.
Schemesh can be used as:
-
a replacement for traditional interactive Unix shell, as for example bash/zsh/pdksh etc.
-
a Unix shell scriptable in Chez Scheme.
-
a Scheme REPL with additional syntax and functions to start, redirect and manage Unix processes.
-
a Scheme library for starting, redirecting and managing Unix processes.
For scripting and serious programming, schemesh completely replaces the slow, clumsy and error-prone scripting language of a traditional shell (yes, the author has opinions) with a full-featured Lisp REPL, backed by the fast and open-source Chez Scheme compiler that generates highly optimized native code.
As a traditional Unix shell: type a command and press Enter.
As a Lisp REPL: type an expression starting with ( and press Enter.
If the parentheses/braces/brackets/quotes are balanced,
schemesh will execute the command and show any failure, or evaluate the expression and pretty-print its value.
If the parentheses/braces/brackets/quotes are not balanced,
schemesh will create a second line where you can continue typing.
You can move between lines with the cursor keys, and use all the classical line-editing features including cut-and-paste.
Switching between shell syntax and Lisp syntax is extremely simple, and can be done basically everywhere:
-
open parenthesis
(temporarily switches to Lisp syntax until the corresponding). -
open brace i.e.
{temporarily switches to shell syntax until the corresponding}. -
open bracket i.e.
[starts a new sub-form in current syntax until the corresponding].
If found in Lisp syntax, it is equivalent to(.
If found in shell syntax, it is similar to{with the difference that commands will be executed in a subshell. -
the directives
#!scheme#!chezschemeand#!r6rstemporarily switch to Scheme syntax (with the appropriate flavor) until the end of current( ),[ ]or{ }.
If entered at top level, they change the default syntax until another directive is entered at top level. -
the directive
#!shelltemporarily switches to shell syntax until the end of current( ),[ ]or{ }.
If entered at top level, it changes the default syntax until another directive is entered at top level. -
shell syntax creates first-class Lisp
sh-jobobjects, which can be started/stopped/managed from both syntaxes. -
sh-jobobjects are discoverable and pretty-printable:
(values '{SOME-SHELL-SYNTAX})shows how shell syntax is converted toshell...macros,
(expand '{SOME-SHELL-SYNTAX})shows howshell...macros are expanded tosh...functions for creating jobs,
(values {SOME-SHELL-SYNTAX})- without quotes - pretty-prints the createdsh-jobobjects.
The most common mechanisms to start/stop/manage jobs from shell syntax are:
- CTRL+C interrupt the current foreground job
- CTRL+Z suspend the current foreground job
bg job-idresume a job in backgroundfg job-idresume a job in foreground
The analogous job control mechanisms from Scheme syntax are:
- CTRL+C as above
- CTRL+Z as above
(sh-start job-object)start a job in background, return immediately(sh-run/i job-object)start a job in foreground, wait until job finishes or is suspended(sh-run job-object)start a job in foreground, wait until job finishes(sh-bg job-or-id)resume a job in background, return immediately(sh-fg job-or-id)resume a job in foreground, wait until job finishes or is suspended(sh-wait job-or-id)resume a job in foreground, wait until job finishes
Some more advanced Scheme functions:
(sh-run/string job-object)start a job in foreground, wait until job finishes, return its output as a Scheme string(sh-start/fd-stdout job-object)start a job in background, return a file descriptor fixnum for reading its standard output - for example with(open-fd-input-port fd)
You can mix shell command execution with Lisp control structures, loops and functions as for example:
(if (some_expression arg1 (sub_expression2))
(sh-run/i {then_run_this_command foo bar $VAR})
(sh-run/i {else_run_this_command foo bar $VAR}))instead of typical shell syntax, which is error prone as it's based on string expansion and splitting, and geared toward command execution, as for example:
# Note: this is POSIX shell syntax for `if-then-else`. It will NOT work in schemesh.
if some_command "$arg1" "$(sub_command)"
then
then_run_this_command foo bar $VAR
else
else_run_this_command foo bar $VAR
fimore examples mixing shell and Lisp syntax:
find (lisp-function-returning-some-string) -type f | grep ^lib | wc -l &
fg 1(define job {ls -l > ls.out || echo "ls failed"})
(sh-start job)
(sh-fg job)(define txt (sh-run/string {git log}))
(display txt)- REPL with multi-line editing and parentheses highlighting
- dual syntax parsing, allows mixing Scheme and shell expressions
- shortcuts to execute commands by pressing a single key: KP- KP+
- history searchable with PageUp and PageDown keys
- cut-and-paste
- context-sensitive autocompletion - some improvements pending
- UTF-8b for losslessly converting byte sequences that are not valid UTF-8
- shell commands, including
;&!&&||{}[] - shell job control
- shell aliases
- shell builtins
- shell environment variables
- shell pipelines
||& - shell redirections
<><>>><&>&$()`` - shell wildcard expansion
- customizable prompt, set by environment variable
$SCHEMESH_PS1 - if the directory
$HOME/.cache/schemesh/exists,
history is automatically saved to and loaded from a filehistory.txtinside such directory - if the file
$HOME/.config/schemesh/repl_init.ssexists,
it is automatically executed when starting the REPL - if the file
$HOME/.config/schemesh/repl_quit.ssexists,
it is automatically executed when exiting the REPL
On Debian Linux, execute the following commands:
sudo apt update
sudo apt install build-essential chezscheme-dev liblz4-dev libncurses-dev git uuid-dev zlib1g-dev
git clone https://github.com/cosmos72/schemesh
cd schemesh
make -j
sudo make installOn Fedora Linux:
sudo dnf install gcc gcc-c++ make chez-scheme-devel lz4-devel ncurses-devel git libuuid-devel zlib-devel
git clone https://github.com/cosmos72/schemesh
cd schemesh
make -j
sudo make installFor other systems, the instructions above can (hopefully) be adapted as needed.
If all went well, you can execute schemesh
In case your environment variable $PATH does not contain /usr/local/bin,
the command schemesh will not suffice - you will need to run /usr/local/bin/schemesh
Troubleshooting:
if make -j fails, a possible error is that it fails to autodetect Chez Scheme installation directory.
In such case you can manually specify it, as for example make -j CHEZ_SCHEME_DIR="/usr/local/lib/csv10.0.0/ta6le"
- autocomplete shell paths and scheme strings: unescape stems before searching for completions, escape completions
- autocomplete shell paths and scheme strings: when autocompleting inside single or double quotes, the stem starts at the quotes.
- autocomplete shell paths starting with ~
- maybe add missing shell builtins "kill"
- implement function
(string->sh-patterns)