shell

MODULE

shell

MODULE SUMMARY

The Erlang Shell

DESCRIPTION

The module shell implements an Erlang shell.

The shell is a user interface program for entering expression sequences. The expressions are evaluated and a value is returned. A history mechanism saves previous commands and their values, which can then be incorporated in later commands.

Variable bindings, and local process dictionary changes which are generated in user expressions, are preserved and the variables can be used in later commands to access their values. The bindings can also be forgotten so the variables can be re-used.

The special shell commands all have the syntax of (local) function calls. They are evaluated as normal function calls and many commands can be used in one expression sequence.

If a command (local function call) is not recognized by the shell, an attempt is first made to find the function in the module user_default, where customized local commands can be placed. If found, then the function is evaluated. Otherwise, an attempt is made to evaluate the function in the module shell_default. The module user_default must be explicitly loaded.

The shell also permits the user to start multiple concurrent jobs. A job can be regarded as a set of processes which can communicate with the shell.

The shell runs in two modes:

• Normal mode, in which commands can be edited and expressions evaluated
• Job Control Mode JCL, in which jobs can be started, killed, detached and connected.

Only the currently connected job can 'talk' to the shell.

Shell Commands

b()

Prints the current variable bindings.


f()

Removes all variable bindings.


f(X)

Removes the binding of variable X.


h()

Prints the history list.


e(N)

Repeats the command N, if N is positive. If it is negative, the Nth previous command is repeated (i.e. e(-1) repeats the previous command).


v(N)

Uses the return value of the command N in the current command.


help()

Evaluates shell_default:help().


c(File)

Evaluates shell_default:c(File). This compiles and loads code in File and purges old versions of code, if necessary. Assumes that the file and module names are the same.

Example

The following example is a long dialogue with the shell. Commands starting with > are inputs to the shell. All other lines are output from the shell. All commands in this example are explained at the end of the dialogue. .

strider 1> erl
Erlang (JAM) emulator version 4.4

Eshell V4.4  (abort with ^G)
1> Str = "abcd".
"abcd"
2> L = length(Str).
4
3> Descriptor = {L, list_to_atom(Str)}.
{4,abcd}
4> L. 
4
5> b().
Descriptor = {4,abcd}
L = 4
Str = "abcd"
ok
6> f(L). 
ok
7> b().
Descriptor = {4,abcd}
Str = "abcd"
ok
8> f(L).
ok
9> {L, _} = Descriptor.
{4,abcd}
10> L.
4
11> {P, Q, R} = Descriptor.
** exited: {{badmatch,{4,abcd}},{erl_eval,expr,3}} **
12> P.
** exited: {{unbound,'P'},{erl_eval,expr,3}} **
13> Descriptor.
{4,abcd}
14> {P, Q} = Descriptor.
{4,abcd}
15> P.
4
16> f().
ok
17> put(aa, hello).
undefined
18> get(aa).
hello
19> Y = test1:demo(1).
11
20> get().
[{aa,worked}]
21> put(aa, hello).
worked
22> Z = test1:demo(2).
** exited: {{badmatch,1},{test1,demo,[2]}} **

=ERROR REPORT==== 24-Jan-1997::07:48:46 ===
!!! Error in process <0.22.0> with exit value: {{badmatch,1}
,{test1,demo,[2]}}
23> Z.
** exited: {{unbound,'Z'},{erl_eval,expr,3}} **
24> get(aa).
hello
25> erase(), put(aa, hello).
undefined
26> spawn(test1, demo, [1]).
<0.25.0>
27> get(aa).
hello
28> io:format("hello hello\n").
hello hello
ok
29> e(28).
hello hello
ok
30> v(28).
ok
31> test1:loop(0).
Hello Number: 0
Hello Number: 1
Hello Number: 2
Hello Number: 3

User switch command
 --> i
 --> c
.
.
.
Hello Number: 3374
Hello Number: 3375
Hello Number: 3376
Hello Number: 3377
Hello Number: 3378
** exited: killed **
32> halt().
strider 2>

Comments

Command 1 sets the variable Str to the string "abcd".

Command 2 sets L to the length of the string evaluating the BIF atom_to_list.

Command 3 builds the tuple Descriptor.

Command 4 prints the value of the variable L.

Command 5 evaluates the internal shell command b(), which is an abbreviation of "bindings". This prints the current shell variables and their bindings. The ok at the end is the return value of the b() function.

Command 6 f(L) evaluates the internal shell command f(L) (abbreviation of "forget"). The value of the variable L is removed.

Command 7 prints the new bindings.

Command 8 shows that the value of L has disappeared from the bindings.

Command 9 performs a pattern matching operation on Descriptor, binding a new value to L.

Command 10 prints the current value of L.

Command 11 tries to match {P, Q, R} against Descriptor which is {4, abc}. The match fails and none of the new variables become bound. The printout starting with "** exited:" is not the value of the expression (the expression had no value because its evaluation failed), but rather a warning printed by the system to inform the user that an error has occurred. The values of the other variables (L, Str, etc.) are unchanged.

Commands 12 and 13 show that P is unbound because the previous command failed, and that Descriptor has not changed.

Commands 14 and 15 show a correct match where P and Q are bound.

Command 16 clears all bindings.

The next few commands assume that test1:demo(X) is defined in the following way:

demo(X) ->
    put(aa, worked),
    X = 1,
    X + 10.

Commands 17 and 18 set and inspect the value of the item aa in the process dictionary.

Command 19 evaluates test1:demo(1). The evaluation succeeds and the changes made in the process dictionary become visible to the shell. The new value of the dictionary item aa can be seen in command 20.

Commands 21 and 22 change the value of the dictionary item aa to hello and call test1:demo(2). Evaluation fails and the changes made to the dictionary in test1:demo(2), before the error occurred, are discarded.

Commands 23 and 24 show that Z was not bound and that the dictionary item aa has retained its original value.

Commands 25, 26 and 27 show the effect of evaluating test1:demo(1) in the background. In this case, the expression is evaluated in a newly spawned process. Any changes made in the process dictionary are local to the newly spawned process and therefore not visible to the shell.

Commands 28, 29 and 30 use the history facilities of the shell.

Command 29 is e(28). This re-evaluates command 28. Command 30 is v(28). This uses the value (result) of command 28. In the cases of a pure function (a function with no side effects), the result is the same. For a function with side effects, the result can be different.

For the next command, it is assumed that test1:loop(N) is defined in the following way:

loop(N) ->
    io:format("Hello Number: ~w~n", [N]), 
    loop(N+1).
    

Command 31 evaluates test1:loop(0), which puts the system into an infinite loop. At this point the user types Control G, which suspends output from the current process, which is stuck in a loop, and activates JCL mode. In JCL mode the user can start and stop jobs.

In this particular case, the i command ("interrupt") is used to terminate the looping program, and the c command is used to connect to the shell again. Since the process was running in the background before we killed it, there will be more printouts before the "** exited: killed **" message is shown.

The halt() command exits the Erlang system.

JCL Mode

When the shell starts, it starts a single evaluator process. This process, together with any local processes which it spawns, is referred to as a job. Only the current job, which is said to be connected, can perform operations with standard IO. All other jobs, which are said to be detached, are blocked if they attempt to use standard IO.

All jobs which do not use standard IO run in the normal way.

^G (Control G) detaches the current job and JCL mode is activated. The JCL mode prompt is "-->". If "?" is entered at the prompt, the following help message is displayed:

          --> ?
          c [nn]   - connect to job 
          i [nn]   - interrupt job
          k [nn]   - kill job
          j        - list all jobs
          s        - start local shell
          r [node] - start remote shell
          q        - quit Erlang
          ?  | h   - this message

The JCL commands have the following meaning:

c [nn]

Connects to job number <nn> or the current job. The standard shell is resumed. Operations which use standard IO by the current job will be interleaved with user inputs to the shell.


i [nn]

Stops the current evaluator process for job number nn or the current job, but does not kill the shell process. Accordingly, any variable bindings and the process dictionary will be preserved and the job can be connected again. This command can be used to interrupt an endless loop.


k [nn]

Kills job number nn or the current job. All spawned processes in the job are killed, provided they have not evaluated the group_leader/1 BIF and are located on the local machine. Processes spawned on remote nodes will not be killed.


j

Lists all jobs. A list of all known jobs is printed. The current job name is prefixed with '*'.


s

Starts a new job. This will be assigned the new index [nn] which can be used in references.


r [node]

Starts a remote job on node. This is used in distributed Erlang to allow a shell running on one node to control a number of applications running on a network of nodes.


q

Quits Erlang.


?

Displays this message.

Bugs

There is no way of changing the length of the history list or saving it between sessions.

AUTHORS

Robert Virding - rv@erix.ericsson.se