ES(1)ES(1)NAMEes - extensible shell
SYNOPSISes [-silevxnpo] [-c command | file] [arguments]
DESCRIPTION
Es is a command interpreter and programming language which combines the
features of other Unix shells and the features of a functional program‐
ming language such as Scheme. The syntax is derived from rc(1). Es is
intended for use both as an interactive shell and a programming lan‐
guage for scripts.
Es is an extremely customizable language. The semantics can be altered
radically by redefining functions that are called to implement internal
operations. This manual page describes the default, initial configura‐
tion. See the section entitled Hook Functions for details on entry
points which can be redefined to give the shell extended semantics.
LANGUAGE
Es is an interpreter which reads commands and executes them. The sim‐
plest form of command in es is a sequence of words separated by white
space (space and tab) characters. A word is either a string or a pro‐
gram fragment (see below). The first word is the command to be exe‐
cuted; the remaining words are passed as arguments to that command. If
the first word is a string, it is a interpreted as the name of a pro‐
gram or shell function to run. If the name is the name of a shell
function, that function is executed. Otherwise, the name is used as
the name of an executable file. If the name begins with /, ./, or ../,
then it is used as the absolute path name of a file; if not, es looks
for an executable file in the directories named by $path.
Commands are terminated by newline or semicolon (;). A command may
also be terminated by an ampersand (&), which causes the command to be
run in the background: the shell does not wait for the command to fin‐
ish before continuing execution. Background processes have an implicit
redirection of /dev/null as their standard input that may be overridden
by an explicit redirection.
Quoting
Es gives several characters special meaning; special characters auto‐
matically terminate words. The following characters, along with space,
tab, and newline, are special:
# $ & ´ ( ) ; < = > \ ^ ` { | }
The single quote (') prevents special treatment of any character other
than itself. Any characters between single quotes, including newlines,
backslashes, and control characters, are treated as an uninterpreted
string. A quote character itself may be quoted by placing two quotes
in a row. A single quote character is therefore represented by the
sequence ''''. The empty string is represented by ''. Thus:
echo 'What''s the plan, Stan?'
prints out
What's the plan, Stan?
The backslash (\) quotes the immediately following character, if it is
one of the special characters, except for newline. In addition, es
recognizes backslash sequences similar to those used in C strings:
\a alert (bell)
\b backspace
\e escape
\f form-feed
\n newline
\r carriage return
\t tab
\xnn hexadecimal character nn
\nnn octal character nnn
Comments
The number sign (#) begins a comment in es. All characters up to but
not including the next newline are ignored.
Line continuation
A long logical line may be continued over several physical lines by
terminating each line (except the last) with a backslash (\). The
backslash-newline sequence is treated as a space. Note that line con‐
tinuation does not work in comments, where the backslash is treated as
part of the comment, and inside quoted strings, where the backslash and
newline are quoted.
Lists
The primary data structure in es is the list, which is a sequence of
words. Parentheses are used to group lists. The empty list is repre‐
sented by (). Lists have no hierarchical structure; a list inside
another list is expanded so that the outer list contains all the ele‐
ments of the inner list. Thus, the following are all equivalent:
one two three
(one two three)
((one) () ((two three)))
Note that the null string, '', and the empty list, (), are two very
different things. Assigning the null string to variable is a valid
operation, but it does not remove its definition.
Concatenation
Two lists may be joined by the concatenation operator (^). A single
word is a list of length one, so
echo foo^bar
produces the output
foobar
For lists of more than one element, concatenation produces the cross
(Cartesian) product of the elements in both lists:
echo (a- b- c-)^(1 2)
produces the output
a-1 a-2 b-1 b-2 c-1 c-2
Variables
A list may be assigned to a variable, using the notation:
var = list
Any sequence of non-special characters, except a sequence including
only digits, may be used as a variable name. Es exports all user-
defined variables into the environment unless it is explicitly told not
to.
The value of a variable is referenced with the notation:
$var
Any variable which has not been assigned a value returns the empty list
when referenced. In addition, multiple references are allowed:
a = foo
b = a
echo $$b
prints
foo
A variable's definition may also be removed by assigning the empty list
to a variable:
var=
Multiple variables may be assigned with a single assignment statment.
The left hand side of the assignment operation consists of a list of
variables which are assigned, one by one, to the values in the list on
the right hand side. If there are more variables than values in the
list, the empty list is assigned to the remaining variables. If there
are fewer variables than elements in the list, the last variable is
bound to all the remaining list values.
For example,
(a b) = 1 2 3
has the same effect as
a = 1
b = 2 3
and
(a b c) = 1 2
is the same as
a = 1
b = 2
c =
Note that when assigning values to more than one variable,
the list of variables must be enclosed in parentheses.
For ``free careting'' (see below) to work correctly,
es
must make certain assumptions
about what characters may appear in a variable name.
Es
assumes that a variable name consists only of alphanumeric characters,
percent
(%),
star
(*),
dash
(-),
and underscore
(_).
To reference a variable with other
characters in its name, quote the variable name.
Thus:
echo $'we$Irdriab!le'
A variable name produced by some complex operation, such as concatena‐
tion, should be enclosed in parentheses:
$(var)
Thus:
Good-Morning = Bonjour
Guten = Good
Morgen = Morning
echo $($Guten^-^$Morgen)
prints
Bonjour
Each element of the list in parentheses is treated as an independent
variable and expanded separately. Thus, given the above definitions,
echo $(Guten Morgen)
prints
Good Morning
To count the number of elements in a variable, use
$#var
This returns a single-element list with the number of elements in $var.
Subscripting
Variables may be indexed with the notation
$var(n)
where n is a list of integers or ranges. Subscript indexes are based
at one. The list of subscripts need not be in order or even unique.
Thus, if
a = one two three
then
echo $a(3 3 3)
prints
three three three
Subscript indices which refer to nonexistent elements expand to the
empty list. Thus, given the definition above
echo $a(3 1 4 1 5 9 2 6 5)
prints
three one one two
Subscript ranges are of the form lo...hi and refer to all the elements
between lo and hi. If lo is omitted, then 1 is used as a default
value; if hi is omitted, the length of the list is used. Thus
* = $*(2 ...)
removes the first element of *, similar to the effect of shift in rc(1)
or sh(1).
The notation $n, where n is an integer, is a shorthand for $*(n).
Thus, es's arguments may be referred to as $1, $2, and so on.
Note that the list of subscripts may be given by any es expression, so
$var(`{awk 'BEGIN{for(i=1;i<=10;i++)print i;exit }'})
returns the first 10 elements of $var.
Free Carets
Es inserts carets (concatenation operators) for free in certain situa‐
tions, in order to save some typing on the user's behalf. For example,
the following are all equivalent:
cc -O -g -c malloc.c alloca.c
cc -^(O g c) (malloc alloca)^.c
opts=O g c; files=malloc alloca; cc -$opts $files.c
Es inserts a free-caret between the ``-'' and $opts, as well as between
$files and .c. The rule for free carets is as follows: if a word or
keyword is immediately followed by another word, keyword, dollar-sign
or backquote without any intervening spaces, then es inserts a caret
between them.
Flattened Lists
To create a single-element list from a multi-element list, with the
components space-separated, use
$^var
Flattening is useful when the normal list concatenation rules need to
be bypassed. For example, to append a single period at the end of
$path, use:
echo $^path.
Wildcard Expansion
Es expands wildcards in filenames if possible. When the characters *,
[ or ? occur in an argument or command, es looks at the argument as a
pattern for matching against files. (Contrary to the behavior some
other shells exhibit, es will only perform pattern matching if a
metacharacter occurs unquoted and literally in the input. Thus,
foo = '*'
echo $foo
will always echo just a star. In order for non-literal metacharacters
to be expanded, an eval statement must be used in order to rescan the
input.) Pattern matching occurs according to the following rules: a *
matches any number (including zero) of characters. A ? matches any
single character, and a [ followed by a number of characters followed
by a ] matches a single character in that class. The rules for charac‐
ter class matching are the same as those for ed(1), with the exception
that character class negation is achieved with the tilde (~), not the
caret (^), since the caret already means something else in es. The
filename component separator, slash (/), must appear explicitly in pat‐
terns. * and ? do not match a dot character (.) at the beginning of
a filename component.
A tilde (~) as the first character of an argument is used to refer to
home directories. A tilde alone or followed by a slash (/) is replaced
by the value of $home, which is usually the home directory of the cur‐
rent user. A tilde followed by a username is replaced with the home
directory of that user, according to getpwent(3).
Pattern Matching
The tilde (~) operator is used in es for matching strings against wild‐
card patterns. The command
~ subject pattern pattern ...
returns a true value if and only if the subject matches any of the pat‐
terns. The matching follows the same rules as wildcard expansion,
except that slashes (/) are not considered significant, leading dots
(.) do not have to be matched explicitly, and home directory expansion
does not occur. Thus
~ foo f*
returns zero (true), while
~ (bar baz) f*
returns one (false). The null list is matched by the null list, so
~ $foo ()
checks to see whether $foo is empty or not. This may also be achieved
by the test
~ $#foo 0
Note that inside a ~ command es does not match patterns against file
names, so it is not necessary to quote the characters *, [ and ?. How‐
ever, es does expand the subject against filenames if it contains
metacharacters. Thus, the command
~ * ?
returns true if any of the files in the current directory have a sin‐
gle-character name. Note that if the ~ command is given a list as its
first argument, then a successful match against any of the elements of
that list will cause ~ to return true. For example:
~ (foo goo zoo) z*
is true.
Pattern Extraction
The double-tilde (~~) operator is used in es for extracting the parts
of strings that match patterns. The command
~~ subject pattern pattern ...
returns the parts of each matching subject which correspond to the
wildcards.
Each subject is checked in order against each pattern; if it matches
the pattern, the parts of the subject which matched each *, ?, or []
character range are extracted, and processing moves on to the next sub‐
ject. If the subject does not match, the next pattern is tried.
For example, the result of the extraction operation
~~ (foo.c foo.x bar.h) *.[ch]
is the list (foo c bar h).
Command Substitution
A list may be formed from the output of a command by using backquote
substitution:
`{ command }
returns a list formed from the standard output of the command in
braces. The characters stored in the variable $ifs (for ``input field
separator'') are used to split the output into list elements. By
default, $ifs has the value space-tab-newline. The braces may be omit‐
ted if the command is a single word. Thus `ls may be used instead of
`{ls}. This last feature is useful when defining functions that expand
to useful argument lists. A frequent use is:
fn src { echo *.[chy] }
followed by
wc `src
(This will print out a word-count of all C and Yacc source files in the
current directory.)
In order to override the value of $ifs for a single command substitu‐
tion, use:
`` ifs-list { command }
$ifs will be temporarily ignored and the command's output will be split
as specified by the list following the double backquote. For example:
`` :\n {cat /etc/passwd}
splits up /etc/passwd into fields.
Return Values
The return value of a command is obtained with the construct
<={ command }
The return value of an external program is its exit status (which in
other shells can be found in special variables such as $? or $status),
as either a small integer or the name of signal. Thus
echo <={test -f /etc/motd} <={test -w /vmunix} <=a.out
might produce the output
0 1 sigsegv+core
along with any output or error messages from the programs.
Es functions and primitives can produce ``rich return values,'' that
is, arbitrary lists as return values.
When return values are interpreted as truth values, an extension of the
normal shell conventions apply. If any element of a list is not equal
to ``0'' (or the empty string), that list is considered false.
The return value of an assignment operation is the assigned value.
Logical Operators
There are a number of operators in es which depend on the exit status
of a command.
command1 && command2
executes the first command and then executes the second command if and
only if the first command has a ``true'' return value.
command1 || command2
executes the first command and then executes the second command if and
only if the first command has a ``false'' return value.
! command
inverts the truth value of the exit status of a command.
Input and output
The standard output of a command may be redirected to a file with
command > file
and the standard input may be taken from a file with
command < file
File descriptors other than 0 and 1 may be specified also. For exam‐
ple, to redirect standard error to a file, use:
command >[2] file
In order to duplicate a file descriptor, use >[n=m]. Thus to redirect
both standard output and standard error to the same file, use
command > file >[2=1]
To close a file descriptor that may be open, use >[n=]. For example,
to close file descriptor 7:
command >[7=]
In order to place the output of a command at the end of an already
existing file, use:
command >> file
If the file does not exist, then it is created.
To open a file for reading and writing, use the <> redirection opera‐
tor; for reading and appending, use <>>. Both of these operators use
file descriptor 0 (standard input) by default. Similarly, >< truncates
a file and opens it for reading and writing, and >>< opens a file for
reading and appending; these operators use file descriptor 1 by
default.
``Here documents'' are supported as in sh(1) with the use of
command << 'eof-marker'
If the end-of-file marker is quoted, then no variable substitution
occurs inside the here document. Otherwise, every variable is substi‐
tuted by its space-separated-list value (see Flat Lists, below), and if
a ^ character follows a variable name, it is deleted. This allows the
unambiguous use of variables adjacent to text, as in
$variable^follow
To include a literal $ in a here document created with an unquoted end-
of-file marker, use $$.
Additionally, es supports ``here strings'', which are like here docu‐
ments, except that input is taken directly from a string on the command
line. Its use is illustrated here:
cat <<< 'this is a here string' | wc
(This feature enables es to export functions that use here documents.)
Pipes
Two or more commands may be combined in a pipeline by placing the ver‐
tical bar (|) between them. The standard output (file descriptor 1) of
the command on the left is tied to the standard input (file descriptor
0) of the command on the right. The notation |[n=m] indicates that
file descriptor n of the left process is connected to file descriptor m
of the right process. |[n] is a shorthand for |[n=0]. As an example,
to pipe the standard error of a command to wc(1), use:
command |[2] wc
The exit status of a pipeline is considered true if and only if every
command in the pipeline exits true.
Input/Output Substitution
Some commands, like cmp(1) or diff(1), take their input from named
files on the command line, and do not use standard input. It is conve‐
nient sometimes to build nonlinear pipelines so that a command like cmp
can read the output of two commands at once. Es does it like this:
cmp <{command1} <{command2}
compares the output of the two commands. Note: on some systems, this
form of redirection is implemented with pipes, and since one cannot
lseek(2) on a pipe, commands that use lseek will hang. For example,
most versions of diff seek on their inputs.
Data can be sent down a pipe to several commands using tee(1) and the
output version of this notation:
echo hi there | tee >{sed 's/^/p1 /'} >{sed 's/^/p2 /'}
Program Fragments
Es allows the intermixing of code with strings. A program fragment,
which is a group of commands enclosed in braces ({ and }), may be used
anywhere a word is expected, and is treated as an indivisible unit.
For example, a program fragment may be passed as an argument, stored in
a variable, or written to a file or pipe. If a program fragment
appears as the first word in a command, it is executed, and any argu‐
ments are ignored. Thus the following all produce the same output:
{ echo hello, world }
{ echo hello, world } foo bar
es-c { echo hello, world }
x = { echo hello, world }; $x
echo { echo hello, world } | es
echo { echo hello, world } > foo; es < foo
Since program fragments in the first position in a command are exe‐
cuted, braces may be used as a grouping mechanism for commands. For
example, to run several commands, with output from all of them redi‐
rected to the same file, one can do
{ date; ps agux; who } > snapshot
In addition, program fragments can continue across multiple physical
lines without explicit line continuations, so the above command could
also be written:
{
date
ps agux
who
} > snapshot
A lambda is a variant on a program fragment which takes arguments. A
lambda has the form
@ parameters { commands }
The parameters are one or more variable names, to which arguments of
the lambda are assigned while the commands are run. The first argument
is assigned to the first variable, the second to the second, and so on.
If there are more arguments than parameters, the last named variable is
assigned all the remaining arguments; if there are fewer, the parame‐
ters for which there are no arguments are bound to the empty list. If
no parameters are listed, the variable named * is assigned all the
arguments of the lambda. Note that @ is a keyword and not a special
character in es, so it must be separated by whitespace from other
words.
As a small example,
@ { echo $* } hi
is a complicated way of producing the output hi. The first word is a
function which echoes its arguments, and the second word is the argu‐
ment to the function, the word hi.
Lambdas, like other program fragments, can appear anywhere in a list.
A more complicated example in the same spirit:
@ cmd arg { $cmd $arg } @ { echo $* } hi
This command executes a lambda which runs its first argument, named
cmd, using its second argument, named arg, as the argument for the
first. The first argument of this function is another lambda, seen
previously, and the second argument is the word hi.
These lambda expressions
@ a b c { echo $c $b $a } 1 2
@ a b c { echo $c $b $a } 1 2 3 4 5
produce this output:
2 1
3 4 5 2 1
Functions
A function in es is introduced with the syntax
fn name parameters { commands }
If the function name appears as the first word of a command, the com‐
mands are run, with the named parameters bound to the arguments to the
function.
The similarity between functions and lambdas is not coincidental. A
function in es is a variable of the form fn-name. If name for which
the appropriate fn- variable exists is found in the first position of a
command, the value of the variable is substituted for the first word.
The above syntax for creating functions is equivalent to the variable
assignment
fn-name = @ parameters { commands }
Functions may be deleted with the syntax
fn name
which is equivalent to the assignment
fn-name=
If, as the most common case, a function variable is bound to a lambda,
when the function is invoked, the variable $0 is bound (dynamically,
see below) to the name of the function.
Lambdas are just another form of code fragment, and, as such, can be
exported in the environment, passed as arguments, etc. The central
difference between the two forms is that lambdas bind their arguments,
while simple brace-enclosed groups just ignore theirs.
Local Variables
Variable assignments may be made local to a set of commands with the
local construct:
local (var = value; var = value ...) command
The command may be a program fragment, so for example:
local (path = /bin /usr/bin; ifs = ) {
...
}
sets path to a minimal useful path and removes ifs for the duration of
one long compound command.
Local-bound variables are exported into the environment, and will
invoke appropriately named settor functions (see below).
Lexically Scoped Variables
In addition to local variables, es supports a different form of tempo‐
rary variable binding, using let-bound, or ``lexically scoped,'' vari‐
ables. (Lexical scoping is the form of binding used by most compiled
programming languages, such as C or Scheme.) A lexically scoped vari‐
able is introduced with a let statement:
let (var = value; var = value ...) command
All references to any of the variables defined in a let statement by
any code located lexically (that is, textually) within the command por‐
tion of the statement will refer to the let-bound variable rather than
any environment or local-bound variable; the immediate text of the let
statement is the complete extent of that binding. That is, lexically
bound variables surrounding code fragments follow those code fragments
around.
An example best shows the difference between let and local (also known
as ``dynamic'') binding: (note that ``; '' is es's default prompt.)
; x = foo
; let (x = bar) {
echo $x
fn lexical { echo $x }
}
bar
; local (x = baz) {
echo $x
fn dynamic { echo $x }
}
baz
; lexical
bar
; dynamic
foo
;
Lexically bound variables are not exported into the environment, and
never cause the invocation of settor functions. Function (lambda)
parameters are lexically bound to their values.
For loops
The command
for (var = list) command
Runs the command once for each element of the list, with the named
variable bound lexically to each element of the list, in order.
If multiple bindings are given in the for statement, the looping occurs
in parallel and stops when all lists are exhausted. When one list is
finished before the others, the corresponding variable is bound to the
empty list for the remaining iterations. Thus the loop
for (i = a b c; j = x y) echo $#i $i $#j $j
produces the output
1 a 1 x
1 b 1 y
1 c 0
Settor Functions
A settor function is a variable of the form set-var, which is typically
bound to a lambda. Whenever a value is assigned to the named variable,
the lambda is invoked with its arguments bound to the new value. While
the settor function is running, the variable $0 is bound to the name of
the variable being assigned. The result of the settor function is used
as the actual value in the assignment.
For example, the following settor function is used to keep the shell
variables home and HOME synchronized.
set-HOME = @ {
local (set-home = )
home = $*
result $*
}
This settor function is called when any assignment is made to the vari‐
able HOME. It assigns the new value to the variable home, but disables
any settor function for home to prevent an infinite recursion. Then it
returns its argument unchanged for use in the actual assignment to
HOME.
Settor functions do not apply to lexically bound variables.
Primitives
Primitives are internal es operations that cannot or should not (for
reasons of performance) be written in the interpreter's language. The
set of primitives makes up the run-time library for es.
Primitives can be used with the syntax
$&name
A primitive can be used anywhere a lambda is expected. The list of
primitives is returned as the result of running the primitive $&primi‐
tives.
For details on specific primitives, see the section entitled PRIMITIVES
below.
Exceptions
Exceptions in es are used for many forms of non-structured control
flow, notably error reporting, signals, and flow of control constructs
such as break and return.
Exceptions are passed up the call chain to catching routines. A
catcher may decide to intercept an exception, retry the code that
caused the exception, or pass the exception along. There can only be
one exception raised at any time.
Exceptions are represented by lists. The first word of an exception
is, by convention, the type of exception being raised. The following
exceptions are known:
break value
Exit from a loop. The return value of the loop is the argument
to the exception.
eof Raised by %parse when the end of input is reached.
error source message
A run-time error. Almost all shell errors are reported with the
error exception. The default interactive loop and the outermost
level of the interpreter catch this exception and print the mes‐
sage. Source is the name of the routine (typically a primitive)
which raised the error.
retry When raised from a signal catcher, causes the body of the catch
clause to be run again.
return value
Causes the current function to exit, with value as the return
value (exit status).
signal signame
Raised when the shell itself receives a signal, and the signal
is listed in the variable signals. Signame is the name of the
signal that was raised.
See the builtin commands catch and throw for details on how to manipu‐
late exceptions.
SPECIAL VARIABLES
Several variables are known to es and are treated specially. Redefin‐
ing these variables can change interpreter semantics. Note that only
dynamically bound (top-level or local-bound) variables are interpreted
in this way; the names of lexically bound variables are unimportant.
* The argument list of es. $1, $2, etc. are the same as $*(1),
$*(2), etc.
$0 Holds the value of argv[0] with which es was invoked. Addition‐
ally, $0 is set to the name of a function for the duration of
the execution of that function, and $0 is also set to the name
of the file being interpreted for the duration of a . command.
apid The process ID of the last process started in the background.
history
The name of a file to which commands are appended as es reads
them. This facilitates the use of a stand-alone history program
(such as history(1)) which parses the contents of the history
file and presents them to es for reinterpretation. If history
is not set, then es does not append commands to any file.
home The current user's home directory, used in tilde (~) expansion,
as the default directory for the builtin cd command, and as the
directory in which es looks to find its initialization file,
.esrc, if es has been started up as a login shell. Like path
and PATH, home and HOME are aliased to each other.
ifs The default input field separator, used for splitting up the
output of backquote commands for digestion as a list. The ini‐
tial value of ifs is space-tab-newline.
noexport
A list of variables which es will not export. All variables
except for the ones on this list and lexically bound variables
are exported.
path This is a list of directories to search in for commands. The
empty string stands for the current directory. Note also that
an assignment to path causes an automatic assignment to PATH,
and vice-versa. If neither path nor PATH are set at startup
time, path assumes a default value suitable for your system.
This is typically /usr/ucb /usr/bin /bin ''.
pid The process ID of the currently running es.
prompt This variable holds the two prompts (in list form) that es
prints. $prompt(1) is printed before each command is read, and
$prompt(2) is printed when input is expected to continue on the
next line. (See %parse for details.) es sets $prompt to ('; '
'') by default. The reason for this is that it enables an es
user to grab commands from previous lines using a mouse, and to
present them to es for re-interpretation; the semicolon prompt
is simply ignored by es. The null $prompt(2) also has its jus‐
tification: an es script, when typed interactively, will not
leave $prompt(2)'s on the screen, and can therefore be grabbed
by a mouse and placed directly into a file for use as a shell
script, without further editing being necessary.
signals
Contains a list of the signals which es traps. Any signal name
which is added to this list causes that signal to raise an es
exception. For example, to run some commands and make sure some
cleanup routine is called even if the user interrupts or discon‐
nects during the script, one can use the form:
local (signals = $signals sighup sigint) {
catch @ e {
cleanup
throw $e
} {
...
}
}
A signal name prefixed by a hyphen (-) causes that signal to be
ignored by es and all of its child processes, unless one of them
resets its handler. A signal prefixed by a slash (/) is ignored
in the current shell, but retains default behavior in child pro‐
cesses. In addition, the signal sigint may be preceeded by the
prefix (.) to indicate that normal shell interrupt processing
(i.e., the printing of an extra newline) occurs. By default es
starts up with the values
.sigint /sigquit /sigterm
in $signals; other values will be on the list if the shell
starts up with some signals ignored.
The values of path and home are derived from the environment values of
PATH and HOME if those values are present. This is for compatibility
with other Unix programs, such as sh(1). $PATH is assumed to be a
colon-separated list.
SYNTACTIC SUGAR
Es internally rewrites much of the syntax presented thus far in terms
of calls to shell functions. Most features of es that resemble tradi‐
tional shell features are included in this category. This rewriting
occurs at parse time, as commands are recognized by the interpreter.
The shell functions that are the results of rewriting are some of the
hook functions documented below.
The following tables list all of the major rewriting which es does,
with the forms typically entered by the user on the left and their
internal form on the right. There is no reason for the user to avoid
using the right-hand side forms, except that they are usually less con‐
venient. To see the internal form of a specific command, a user can
run es with the -n and -x options; when invoked in this way, the shell
prints the internal form of its commands rather than executing them.
Control Flow
! cmd %not {cmd}
cmd & %background {cmd}
cmd1 ; cmd2 %seq {cmd1} {cmd2}
cmd1 && cmd2 %and {cmd1} {cmd2}
cmd1 || cmd2 %or {cmd1} {cmd2}
fn name args { cmd } fn-^name = @ args {cmd}
Input/Output Commands
cmd < file %open 0 file {cmd}
cmd > file %create 1 file {cmd}
cmd >[n] file %create n file {cmd}
cmd >> file %append 1 file {cmd}
cmd <> file %open-write 0 file {cmd}
cmd <>> file %open-append 0 file {cmd}
cmd >< file %open-create 1 file {cmd}
cmd >>< file %open-append 1 file {cmd}
cmd >[n=] %close n {cmd}
cmd >[m=n] %dup m n {cmd}
cmd << tag input tag %here 0 input {cmd}
cmd <<< string %here 0 string {cmd}
cmd1 | cmd2 %pipe {cmd1} 1 0 {cmd2}
cmd1 |[m=n] cmd2 %pipe {cmd1} m n {cmd2}
cmd1 >{ cmd2 } %writeto var {cmd2} {cmd1 $var}
cmd1 <{ cmd2 } %readfrom var {cmd2} {cmd1 $var}
Expressions
$#var <={%count $var}
$^var <={%flatten ' ' $var}
`{cmd args} <={%backquote <={%flatten '' $ifs} {cmd args}}
``ifs {cmd args} <={%backquote <={%flatten '' ifs} {cmd args}}
BUILTINS
Builtin commands are shell functions that exist at shell startup time.
Most builtins are indistinguishable from external commands, except that
they run in the context of the shell itself rather than as a child
process. Many builtins are implemented with primitives (see above).
Some builtin functions have names that begin with a percent character
(%). These are commands with some special meaning to the shell, or are
meant for use only by users customizing the shell. (This distinction
is somewhat fuzzy, and the decisions about which functions have %-names
are somewhat arbitrary.)
All builtins can be redefined and extended by the user.
Builtin Commands
. [-einvx] file [args ...]
Reads file as input to es and executes its contents. The
options are a subset of the invocation options for the shell
(see below).
access [-n name] [-1e] [-rwx] [-fdcblsp] path ...
Tests if the named paths are accessible according to the options
presented. Normally, access returns zero (true) for files which
are accessible and a printable error message (which evaluates as
false, according to shell rules) for files which are not acces‐
sible. If the -1 option is used, the name of the first file
which the test succeeds for is returned; if the test succeeds
for no file, the empty list is returned. However, if the -e
option was used, access raises an error exception. If the -n
option is used, the pathname arguments are treated as a list of
directories, and the name option argument is used as a file in
those directories (i.e., -n is used for path searching).
The default test is whether a file exists. These options change
the test:
-r Is the file readable (by the current user)?
-w Is the file writable?
-x Is the file executable?
-f Is the file a plain file?
-d Is the file a directory?
-c Is the file a character device?
-b Is the file a block device?
-l Is the file a symbolic link?
-s Is the file a socket?
-p Is the file a named pipe (FIFO)?
break value
Exits the current loop. Value is used as the return value for
the loop command.
catch catcher body
Runs body. If it raises an exception, catcher is run and passed
the exception as an argument.
cd [directory]
Changes the current directory to directory. With no argument,
cd changes the current directory to $home.
echo [-n] [--] args ...
Prints its arguments to standard output, terminated by a new‐
line. Arguments are separated by spaces. If the first argument
is -n no final newline is printed. If the first argument is --,
then all other arguments are echoed literally; this is used for
echoing a literal -n.
eval list
Concatenates the elements of list with spaces and feeds the
resulting string to the interpreter for rescanning and execu‐
tion.
exec cmd
Replaces es with the given command. If the exec contains only
redirections, then these redirections apply to the current shell
and the shell does not exit. For example,
exec {>[2] err.out}
places further output to standard error in the file err.out.
Unlike some other shells, es requires that redirections in an
exec be enclosed in a program fragment.
exit [status]
Causes the current shell to exit with the given exit status. If
no argument is given, zero (true) is used. (This is different
from other shells, that often use the status of the last command
executed.)
false Always returns a false (non-zero) return value.
forever cmd
Runs the command repeatedly, until the shell exits or the com‐
mand raises an exception. This is equivalent to a while {true}
{cmd} loop except that forever does not catch any exceptions,
including break.
fork cmd
Runs a command in a subshell. This insulates the parent shell
from the effects of state changing operations such as cd and
variable assignments. For example:
fork {cd ..; make}
runs make(1) in the parent directory (..), but leaves the shell
in the current directory.
if [test then] ... [else]
Evaluates the command test. If the result is true, the command
then is run and if completes. If the result of the test is
false, the next test-then pair is checked, until one where the
test is true is found. If none of the tests are true, the else
command is run.
limit [-h] [resource [value]]
Similar to the csh(1) limit builtin, this command operates upon
the resource limits of a process. With no arguments, limit
prints all the current limits; with one argument, limit prints
the named limit; with two arguments, it sets the named limit to
the given value. The -h flag displays/alters the hard limits.
The resources which can be shown or altered are cputime, file‐
size, datasize, stacksize, coredumpsize and memoryuse. For
example:
limit coredumpsize 0
disables core dumps.
The limit values must either be the word ``unlimited'' or a num‐
ber with an optional suffix indicating units. For size limits,
the suffixes k (kilobytes), m (megabytes), and g (gigabytes) are
recognized. For time limits, s (seconds), m (minutes), and h
(hours) are known; in addition, times of the form hh:mm:ss and
mm:ss are accepted. See getrlimit(2) for details on resource
limit semantics.
newpgrp
Puts es into a new process group. This builtin is useful for
making es behave like a job-control shell in a hostile environ‐
ment. One example is the NeXT Terminal program, which implic‐
itly assumes that each shell it forks will put itself into a new
process group. Note that the controlling tty for the process
must be on standard error (file descriptor 2) when this opera‐
tion is run.
result value ...
Returns its arguments. This is es's identity function.
return value
Causes the current function to exit, returning the named value.
throw exception arg ...
Raise the named exception, passing all of the arguments to throw
to the enclosing exception handler.
time cmd arg ...
Prints, on the shell's standard error, the real, user, and sys‐
tem time consumed by executing the command.
true Always returns a true (zero) return value.
umask [mask]
Sets the current umask (see umask(2)) to the octal mask. If no
argument is present, the current mask value is printed.
unwind-protect body cleanup
Runs body and, when it completes or raises an exception, runs
cleanup.
var var ...
Prints definitions of the named variables, suitable for being
used as input to the shell.
vars [-vfs] [-epi]
Prints all shell variables, functions, and settor functions (in
a form suitable for use as shell input), which match the crite‐
ria specified by the options.
-v variables (that are not functions or settor functions)
-f functions
-s settor functions
-e exported values
-p private (not exported) values
-i internal (predefined and builtin) values
-a all of the above
If none of -v, -f, or -s are specified, -v is used. If none of
-e, -p, or -i are specified, -e is used.
wait [pid]
Waits for the specified pid, which must have been started by es.
If no pid is specified, waits for any child process to exit.
whatis progam ...
For each named program, prints the pathname, primitive, lambda,
or code fragment which would be run if the program appeared as
the first word of a command.
while test body
Evaluates the test and, if it is true, runs the body and
repeats.
%read Reads from standard input and returns either the empty list (in
the case of end-of-file) or a single element string with up to
one line of data, including possible redirections. This func‐
tion reads one character at a time in order to not read more
data out of a pipe than it should. The terminating newline (if
present) is not included in the returned string.
Hook Functions
A subset of the %-named functions are known as ``hook functions.'' The
hook functions are called to implement some internal shell operations,
and are available as functions in order that their values can be
changed. Typically, a call to a hook function is from code generated
by the syntactic sugar rewritings.
%and cmd ...
Runs the commands in order, stopping after the first one that
has a false return value. Returns the result of the last com‐
mand run.
%append fd file cmd
Runs the command with file descriptor fd set up to append to the
file.
%background cmd
Runs the command in the background. The shell variable apid
contains the process ID of the background process, which is
printed if the shell is interactive (according to %is-interac‐
tive).
%backquote separator cmd
Runs the command in a child process and returns its standard
output as a list, separated (with the same rules used in %split)
into elements according to separator.
%batch-loop
Parses commands from the current input source and passes the
commands to the function %dispatch, which is usually a dynami‐
cally bound identifier. This function catches the exception eof
which causes it to return. This function is invoked by the
shell on startup and from the dot (.) and eval commands, when
the input source is not interactive. (See also %interactive-
loop.)
%close fd cmd
Runs the command with the given file descriptor closed.
%count list
Returns the number of arguments to the primitive.
%create fd file cmd
Runs the command with file descriptor fd set up to write to the
file.
%dup newfd oldfd cmd
Runs the command with the file descriptor oldfd copied (via
dup(2)) to file descriptor newfd.
%eval-noprint cmd
Run the command. (Passed as the argument to %batch-loop and
%interactive-loop.)
%eval-print cmd
Print and run the command. (Passed as the argument to %batch-
loop and %interactive-loop when the -x option is used.)
%exec-failure file argv0 args ...
This function, if it exists, is called in the context of a child
process if an executable file was found but execve(2) could not
run it. If the function returns, an error message is printed
and the shell exits, but the function can exec a program if it
thinks it knows what to do. Note that the name of the program
appears twice in the arguments to %exec-failure, once as a file‐
name and once as the first element of the argv array; in some
cases the two will be identical, but in others the former will
be a full pathname and the latter will just be the basename.
Some versions of es may provide a builtin version of this func‐
tion to handle #!-style shell scripts if the kernel does not.
%exit-on-false cmd
Runs the command, and exits if any command (except those execut‐
ing as the tests of conditional statements) returns a non-zero
status. (This function is used as an argument to %batch-loop
and %interactive-loop when the shell is invoked with the -e
option.)
%flatten separator list
Concatenate the elements of list into one string, separated by
the string separator.
%here fd word ... cmd
Runs the command with the words passed as input on file descrip‐
tor fd.
%home [user]
Returns the home directory of the named user, or $home if there
are no arguments.
%interactive-loop
Prompts, parses commands from the current input source and
passes the commands to the function %dispatch, which is usually
a dynamically bound identifier. This function catches the
exception eof which causes it to return. This function is
invoked by the shell on startup and from the dot (.) commands,
when the input source is interactive. (See also %batch-loop.)
%noeval-noprint cmd
Do nothing. (Passed as the argument to %batch-loop and %inter‐
active-loop when the -n option is used.)
%noeval-print cmd
Print but don't run the command. (Passed as the argument to
%batch-loop and %interactive-loop when the -x and -n options are
used.)
%not cmd
Runs the command and returns false if its exit status was true,
otherwise returns true.
%one list
If list is one element long, %one returns its value; otherwise
it raises an exception. %one is used to ensure that redirection
operations get passed exactly one filename.
%open fd file cmd
Runs the command with file open for reading on file descriptor
fd.
%open-append fd file cmd
Runs the command with file open for reading and appending on
file descriptor fd.
%open-create fd file cmd
Runs the command with file open for reading and writing on file
descriptor fd. If the file already exists, it is truncated.
%open-write fd file cmd
Runs the command with file open for reading and writing on file
descriptor fd.
%openfile mode fd file cmd
Runs the command with file opened according to mode on file
descriptor fd. The modes (r, w, a, r+, w+, and a+) have the
same meanings in %openfile as they do in fopen(3). %openfile is
invoked by the redirection hook functions: %append, %create,
%open, %open-append, %open-create, and %open-write.
%or cmd ...
Runs the commands in order, stopping after the first one that
has a true return value. Returns the result of the last command
run.
%parse prompt1 prompt2
Reads input from the current input source, printing prompt1
before reading anything and prompt2 before reading continued
lines. Returns a code fragment suitable for execution. Raises
the exception eof on end of input.
%pathsearch program
Looks for an executable file named program in the directories
listed in $path. If such a file is found, it is returned; if
one is not found, an error exception is raised.
%pipe cmd [outfd infd cmd] ...
Runs the commands, with the file descriptor outfd in the left-
hand process connected by a pipe to the file descriptor infd in
the right-hand process. If there are more than two commands, a
multi-stage pipeline is created.
%prompt
Called by %interactive-loop before every call to %parse. This
function allows the user to provide any actions that he or she
may wish to have executed before being prompted (e.g., updating
the value of the prompt variable to contain all or part of the
current working directory).
%readfrom var input cmd
Runs cmd with the variable var locally bound to the name of a
file which contains the output of running the command input.
%seq cmd ...
Runs the commands, in order.
%whatis program ...
For each named program, returns the pathname, primitive, lambda,
or code fragment which would be run if the program appeared as
the first word of a command.
%writeto var output cmd
Runs cmd with the variable var locally bound to the name of a
file which is used as the input for the command output.
Utility Functions
These functions are useful for people customizing the shell, may be
used by other builtin commands, and probably don't make much sense to
replace, though that is always possible.
%apids Returns the process IDs of all background processes that the
shell has not yet waited for.
%fsplit separator [args ...]
Splits its arguments into separate strings at every occurrence
of any of the characters in the string separator. Repeated
instances of separator characters cause null strings to appear
in the result. (This function is used by some builtin settor
functions.)
%is-interactive
Returns true if the current interpreter context is interactive;
that is, if shell command input is currently coming from an
interactive user. More precisely, this is true if the innermost
enclosing read-eval-print loop is %interactive-loop rather than
%batch-loop.
%newfd Returns a file descriptor that the shell thinks is not currently
in use.
%run program argv0 args ...
Run the named program, which is not searched for in $path, with
the argument vector set to the remaining arguments. This
builtin can be used to set argv[0] (by convention, the name of
the program) to something other than file name.
%split separator [args ...]
Splits its arguments into separate strings at every occurrence
of any of the characters in the string separator. Repeated
instances of separator characters are coalesced. Backquote sub‐
stitution splits with the same rules.
%var var ...
For each named variable, returns a string which, if interpreted
by es would assign to the variable its current value.
PRIMITIVES
Primitives exist in es so that, in the presence of spoofing and redefi‐
nitions, there is a way to refer to built-in behaviors. This ability
is necessary for the shell to be able to unambiguously refer to itself,
but is also useful for users who have otherwise made their environment
unnecessary but don't want to kill the current shell.
Primitives are referenced with the
$&name
notation. In this section, the ``$&'' prefixes will be omitted when
primitive names are mentioned. Note that, by convention, primitive
names follow C identifier names where es variable and function names
often contain ``%'' and ``-'' characters.
The following primitives directly implement the builtin functions with
the same names:
access forever throw
catch fork umask
echo if wait
exec newpgrp
exit result
In addition, the primitive dot implements the ``.'' builtin function.
The cd primitive is used in the implementation of the cd builtin, but
does not understand no arguments to imply $home. The vars and inter‐
nals primitives are used by the implementation of the vars builtin.
The following primitives implement the hook functions of the same
names, with ``%'' prefixes:
apids here read
close home run
count newfd seq
dup openfile split
flatten parse var
fsplit pipe whatis
The following primitives implement the similar named hook functions,
with ``%'' prefixes and internal hyphens:
batchloop exitonfalse isinteractive
The background primitive is used to implement the %background hook
function, but does not print the process ID of the background process
or set $apid. The backquote primitive is used to implement the %back‐
quote hook function, but returns the exit status of the child as the
first value of its result instead of setting $bqstatus to it.
The following primitives implement the similarly named settor func‐
tions:
sethistory setnoexport setsignals
Some primitives are included in es conditionally, based on compile-time
configuration options. Those primitives, and the functions to which
they are bound, are
execfailure %exec-failure
limit limit
readfrom %readfrom
time time
writeto %writeto
The primitive resetterminal is if es is compiled with support for the
readline or editline libraries. It is used in the implementation of
settor functions of the TERM and TERMCAP variables to notify the line
editing packages that the terminal configuration has changed.
Several primitives are not directly associated with other function.
They are:
$&collect
Invokes the garbage collector. The garbage collector in es runs
rather frequently; there should be no reason for a user to issue
this command.
$&noreturn lambda args ...
Call the lambda, but in such a way that it does not catch the
return exception. This primitive exists in order that some con‐
trol-flow operations in es (e.g., while and &&) can be imple‐
mented as lambdas rather than primitives.
$&primitives
Returns a list of the names of es primitives.
$&version
Returns the current version number and release date for es.
OPTIONS-c Run the given command, placing the rest of the arguments to es
in $*.
-s Read commands from standard input; i.e., put the first argument
to es in $* rather than using it as the name of a file to
source.
-i Force es to be an interactive shell. Normally es is only inter‐
active if it is run with commands coming from standard input and
standard input is connected to a terminal.
-l Run $home/.esrc on startup, i.e., be a login shell. -l is
implied if the name the shell was run under (that is, argv[0])
starts with a dash (-).
-e Exit if any command (except those executing as the tests of con‐
ditional statements) returns a non-zero status.
-v Echo all input to standard error.
-x Print commands to standard error before executing them.
-n Turn off execution of commands. This can be used for checking
the syntax of scripts. When combined with -x, es prints the
entered command based on the internal (parsed) representation.
-p Don't initialize functions from the environment. This is used
to help make scripts that don't break unexpectedly when the
environment contains functions that would override commands used
in the script.
-o Don't open /dev/null on file descriptors 0, 1, and 2, if any of
those descriptors are inherited closed.
-d Don't trap SIGQUIT or SIGTERM. This is used for debugging.
FILES
$home/.esrc, /dev/null
BUGS
Lexical scope which is shared by two variables (or closures) in a par‐
ent shell is split in child shells.
The interpreter should be properly tail recursive; that is, tail calls
should not consume stack space.
break and return should have lexical scope.
Woe betide the environment string set by some other program to contain
either the character control-a or the sequence control-b followed by
control-a or control-b.
-x is not nearly as useful as it should be.
Line numbers in error messages refer to the last line parsed, rather
than something more useful.
Too many creatures have fept in.
Please send bug reports to haahr@adobe.com and byron@netapp.com.
SEE ALSOhistory(1), rc(1), sh(1), execve(2), getrlimit(2), fopen(3), getp‐
went(3)
Paul Haahr and Byron Rakitzis, Es — A shell with higher-order func‐
tions, Proceedings of the Winter 1993 Usenix Conference, San Diego, CA.
Tom Duff, Rc — A Shell for Plan 9 and UNIX Systems, Unix Research Sys‐
tem, 10th Edition, Volume 2. (Saunders College Publishing)
5 March 1992 ES(1)
