Es gives several characters special meaning; special characters automatically 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:

prints out

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:

alert (bell)

backspace

escape

form-feed

newline

carriage return

tab

hexadecimal character nn

octal character nnn

The number sign begins a comment in es. All characters up to but not including the next newline are ignored.

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 continuation 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.

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 represented by Lists have no hierarchical structure; a list inside another list is expanded so that the outer list contains all the elements of the inner list. Thus, the following are all equivalent:

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.

Two lists may be joined by the concatenation operator A single word is a list of length one, so

produces the output

For lists of more than one element, concatenation produces the cross (Cartesian) product of the elements in both lists:

produces the output

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:

Any variable which has not been assigned a value returns the empty list when referenced. In addition, multiple references are allowed:

prints

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 statement. 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,

has the same effect as

and

is the same as

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:

A variable name produced by some complex operation, such as concatenation, should be enclosed in parentheses:

Thus:

prints

Each element of the list in parentheses is treated as an independent variable and expanded separately. Thus, given the above definitions,

prints

To count the number of elements in a variable, use

This returns a single-element list with the number of elements in

Variables may be indexed with the notation

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

then

prints

Subscript indices which refer to nonexistent elements expand to the empty list. Thus, given the definition above

prints

Subscript ranges are of the form lo ... hi and refer to all the elements between lo and hi. If lo is omitted, then is used as a default value; if hi is omitted, the length of the list is used. Thus

removes the first element of similar to the effect of in rc(1) or sh(1).

The notation where n is an integer, is a shorthand for Thus, es's arguments may be referred to as and so on.

Note that the list of subscripts may be given by any es expression, so

returns the first 10 elements of

Es inserts carets (concatenation operators) for free in certain situations, in order to save some typing on the user's behalf. For example, the following are all equivalent:

Es inserts a free-caret between the and as well as between and 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.

To create a single-element list from a multi-element list, with the components space-separated, use

Flattening is useful when the normal list concatenation rules need to be bypassed. For example, to append a single period at the end of use:

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,

will always echo just a star. In order for non-literal metacharacters to be expanded, an 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 character 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 patterns. 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 which is usually the home directory of the current user. A tilde followed by a username is replaced with the home directory of that user, according to getpwent(3).

The tilde operator is used in es for matching strings against wildcard patterns. The command

returns a true value if and only if the subject matches any of the patterns. 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

returns zero (true), while

returns one (false). The null list is matched by the null list, so

checks to see whether is empty or not. This may also be achieved by the test

Note that inside a command es does not match patterns against file names, so it is not necessary to quote the characters and However, 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 single-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:

is true.

The double-tilde operator is used in es for extracting the parts of strings that match patterns. The command

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 subject. If the subject does not match, the next pattern is tried.

For example, the result of the extraction operation

is the list

A list may be formed from the output of a command by using backquote substitution:

returns a list formed from the standard output of the command in braces. The characters stored in the variable (for ``input field separator'') are used to split the output into list elements. By default, has the value space-tab-newline. The braces may be omitted if the command is a single word. Thus may be used instead of This last feature is useful when defining functions that expand to useful argument lists. A frequent use is:

followed by

(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 for a single command substitution, use:

will be temporarily ignored and the command's output will be split as specified by the list following the double backquote. For example:

splits up into fields.

The return value of a command is obtained with the construct

The return value of an external program is its exit status (which in other shells can be found in special variables such as or as either a small integer or the name of signal. Thus

might produce the output

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 (or the empty string), that list is considered false.

The return value of an assignment operation is the assigned value.

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.

inverts the truth value of the exit status of a command.

The standard output of a command may be redirected to a file with

and the standard input may be taken from a file with

File descriptors other than 0 and 1 may be specified also. For example, to redirect standard error to a file, use:

In order to duplicate a file descriptor, use Thus to redirect both standard output and standard error to the same file, use

To close a file descriptor that may be open, use For example, to close file descriptor 7:

In order to place the output of a command at the end of an already existing file, use:

If the file does not exist, then it is created.

To open a file for reading and writing, use the redirection operator; 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

If the end-of-file marker is quoted, then no variable substitution occurs inside the here document. Otherwise, every variable is substituted 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

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 documents, except that input is taken directly from a string on the command line. Its use is illustrated here:

(This feature enables es to export functions that use here documents.)

Two or more commands may be combined in a pipeline by placing the vertical 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 indicates that file descriptor n of the left process is connected to file descriptor m of the right process. is a shorthand for As an example, to pipe the standard error of a command to wc(1), use:

The exit status of a pipeline is considered true if and only if every command in the pipeline exits true.

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 convenient 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:

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:

Es allows the intermixing of code with strings. A program fragment, which is a group of commands enclosed in braces 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 arguments are ignored. Thus the following all produce the same output:

Since program fragments in the first position in a command are executed, braces may be used as a grouping mechanism for commands. For example, to run several commands, with output from all of them redirected to the same file, one can do

In addition, program fragments can continue across multiple physical lines without explicit line continuations, so the above command could also be written:

A lambda is a variant on a program fragment which takes arguments. A lambda has the form

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 parameters 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,

is a complicated way of producing the output The first word is a function which echoes its arguments, and the second word is the argument to the function, the word

Lambdas, like other program fragments, can appear anywhere in a list. A more complicated example in the same spirit:

This command executes a lambda which runs its first argument, named using its second argument, named as the argument for the first. The first argument of this function is another lambda, seen previously, and the second argument is the word

These lambda expressions

produce this output:

A function in es is introduced with the syntax

If the function name appears as the first word of a command, the commands 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 If name for which the appropriate 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

Functions may be deleted with the syntax

which is equivalent to the assignment

If, as the most common case, a function variable is bound to a lambda, when the function is invoked, the variable 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.

Variable assignments may be made local to a set of commands with the construct:

The command may be a program fragment, so for example:

sets to a minimal useful path and removes 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).

In addition to local variables, es supports a different form of temporary variable binding, using let-bound, or ``lexically scoped,'' variables. (Lexical scoping is the form of binding used by most compiled programming languages, such as C or Scheme.) A lexically scoped variable is introduced with a statement:

All references to any of the variables defined in a statement by any code located lexically (that is, textually) within the command portion of the statement will refer to the let-bound variable rather than any environment or local-bound variable; the immediate text of the 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 and (also known as ``dynamic'') binding: (note that is es's default prompt.)

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.

The 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 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

produces the output

A settor function is a variable of the form 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 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 and synchronized.

This settor function is called when any assignment is made to the variable It assigns the new value to the variable but disables any settor function for to prevent an infinite recursion. Then it returns its argument unchanged for use in the actual assignment to

Settor functions do not apply to lexically bound variables.

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

A primitive can be used anywhere a lambda is expected. The list of primitives is returned as the result of running the primitive

For details on specific primitives, see the section entitled PRIMITIVES below.

Exceptions in es are used for many forms of non-structured control flow, notably error reporting, signals, and flow of control constructs such as and

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:

Exit from a loop. The return value of the loop is the argument to the exception.

Raised by when the end of input is reached.

A run-time error. Almost all shell errors are reported with the exception. The default interactive loop and the outermost level of the interpreter catch this exception and print the message. Source is the name of the routine (typically a primitive) which raised the error.

When raised from a signal catcher, causes the body of the clause to be run again.

Causes the current function to exit, with value as the return value (exit status).

Raised when the shell itself receives a signal, and the signal is listed in the variable Signame is the name of the signal that was raised.

See the builtin commands and for details on how to manipulate exceptions.

! 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}

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}

$#var <={%count $var} $^var <={%flatten ' ' $var} `{cmd args} <={%backquote <={%flatten '' $ifs} {cmd args}} ``ifs {cmd args} <={%backquote <={%flatten '' ifs} {cmd 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).

Tests if the named paths are accessible according to the options presented. Normally, 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 accessible. If the 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 option was used, raises an exception. If the 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 is used for path searching). The default test is whether a file exists. These options change the test:

Is the file readable (by the current user)?

Is the file writable?

Is the file executable?

Is the file a plain file?

Is the file a directory?

Is the file a character device?

Is the file a block device?

Is the file a symbolic link?

Is the file a socket?

Is the file a named pipe (FIFO)?

Exits the current loop. Value is used as the return value for the loop command.

Runs body. If it raises an exception, catcher is run and passed the exception as an argument.

Changes the current directory to directory. With no argument, changes the current directory to

Prints its arguments to standard output, terminated by a newline. Arguments are separated by spaces. If the first argument is no final newline is printed. If the first argument is then all other arguments are echoed literally; this is used for echoing a literal

Concatenates the elements of list with spaces and feeds the resulting string to the interpreter for rescanning and execution.

Replaces es with the given command. If the contains only redirections, then these redirections apply to the current shell and the shell does not exit. For example,

places further output to standard error in the file err.out. Unlike some other shells, es requires that redirections in an be enclosed in a program fragment.

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.)

Always returns a false (non-zero) return value.

Runs the command repeatedly, until the shell exits or the command raises an exception. This is equivalent to a loop except that does not catch any exceptions, including

Runs a command in a subshell. This insulates the parent shell from the effects of state changing operations such as and variable assignments. For example:

runs make(1) in the parent directory but leaves the shell in the current directory.

Evaluates the command test. If the result is true, the command then is run and 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.

Similar to the csh(1) builtin, this command operates upon the resource limits of a process. With no arguments, prints all the current limits; with one argument, prints the named limit; with two arguments, it sets the named limit to the given value. The flag displays/alters the hard limits. The resources which can be shown or altered are and For example:

disables core dumps.

The limit values must either be the word or a number with an optional suffix indicating units. For size limits, the suffixes (kilobytes), (megabytes), and (gigabytes) are recognized. For time limits, (seconds), (minutes), and (hours) are known; in addition, times of the form and are accepted. See getrlimit(2) for details on resource limit semantics.

Puts es into a new process group. This builtin is useful for making es behave like a job-control shell in a hostile environment. One example is the NeXT Terminal program, which implicitly 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 operation is run.

Returns its arguments. This is es's identity function.

Causes the current function to exit, returning the named value.

Raise the named exception, passing all of the arguments to to the enclosing exception handler.

Prints, on the shell's standard error, the real, user, and system time consumed by executing the command.

Always returns a true (zero) return value.

Sets the current umask (see umask(2)) to the octal mask. If no argument is present, the current mask value is printed.

Runs body and, when it completes or raises an exception, runs cleanup.

Prints definitions of the named variables, suitable for being used as input to the shell.

Prints all shell variables, functions, and settor functions (in a form suitable for use as shell input), which match the criteria specified by the options.

variables (that are not functions or settor functions)

functions

settor functions

exported values

private (not exported) values

internal (predefined and builtin) values

all of the above

If none of are specified, is used. If none of are specified, is used.

Waits for the specified pid, which must have been started by es. If no pid is specified, waits for any child process to exit.

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.

Evaluates the test and, if it is true, runs the body and repeats.

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 function 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.

A subset of the 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.

Runs the commands in order, stopping after the first one that has a false return value. Returns the result of the last command run.

Runs the command with file descriptor fd set up to append to the file.

Runs the command in the background. The shell variable contains the process ID of the background process, which is printed if the shell is interactive (according to

Runs the command in a child process and returns its standard output as a list, separated (with the same rules used in into elements according to separator.

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 which causes it to return. This function is invoked by the shell on startup and from the dot and commands, when the input source is not interactive. (See also

Runs the command with the given file descriptor closed.

Returns the number of arguments to the primitive.

Runs the command with file descriptor fd set up to write to the file.

Runs the command with the file descriptor oldfd copied (via dup(2)) to file descriptor newfd.

Run the command. (Passed as the argument to and

Print and run the command. (Passed as the argument to and when the option is used.)

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 a program if it thinks it knows what to do. Note that the name of the program appears twice in the arguments to once as a filename and once as the first element of the 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 function to handle shell scripts if the kernel does not.

Runs the command, and exits if any command (except those executing as the tests of conditional statements) returns a non-zero status. (This function is used as an argument to and when the shell is invoked with the option.)

Concatenate the elements of list into one string, separated by the string separator.

Runs the command with the words passed as input on file descriptor fd.

Returns the home directory of the named user, or if there are no arguments.

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 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

Do nothing. (Passed as the argument to and when the option is used.)

Print but don't run the command. (Passed as the argument to and when the and options are used.)

Runs the command and returns false if its exit status was true, otherwise returns true.

If list is one element long, returns its value; otherwise it raises an exception. is used to ensure that redirection operations get passed exactly one filename.

Runs the command with file open for reading on file descriptor fd.

Runs the command with file open for reading and appending on file descriptor fd.

Runs the command with file open for reading and writing on file descriptor fd. If the file already exists, it is truncated.

Runs the command with file open for reading and writing on file descriptor fd.

Runs the command with file opened according to mode on file descriptor fd. The modes and have the same meanings in as they do in fopen(3). is invoked by the redirection hook functions: and

Runs the commands in order, stopping after the first one that has a true return value. Returns the result of the last command run.

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 on end of input.

Looks for an executable file named program in the directories listed in If such a file is found, it is returned; if one is not found, an exception is raised.

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.

Called by before every call to 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 variable to contain all or part of the current working directory).

Runs cmd with the variable var locally bound to the name of a file which contains the output of running the command input.

Runs the commands, in order.

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.

Runs cmd with the variable var locally bound to the name of a file which is used as the input for the command output.

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.

Returns the process IDs of all background processes that the shell has not yet waited for.

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.)

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 rather than

Returns a file descriptor that the shell thinks is not currently in use.

Run the named program, which is not searched for in with the argument vector set to the remaining arguments. This builtin can be used to set (by convention, the name of the program) to something other than file name.

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 substitution splits with the same rules.

For each named variable, returns a string which, if interpreted by es would assign to the variable its current value.