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1 # @(#)TOUR 8.1 (Berkeley) 5/31/93
2 # $FreeBSD: src/bin/sh/TOUR,v 1.7 2006/04/16 11:54:01 schweikh Exp $
3 # $DragonFly: src/bin/sh/TOUR,v 1.3 2007/01/04 06:24:11 pavalos Exp $
5 NOTE -- This is the original TOUR paper distributed with ash and
6 does not represent the current state of the shell. It is provided anyway
7 since it provides helpful information for how the shell is structured,
8 but be warned that things have changed -- the current shell is
9 still under development.
11 ================================================================
13 A Tour through Ash
15 Copyright 1989 by Kenneth Almquist.
18 DIRECTORIES: The subdirectory bltin contains commands which can
19 be compiled stand-alone. The rest of the source is in the main
20 ash directory.
22 SOURCE CODE GENERATORS: Files whose names begin with "mk" are
23 programs that generate source code. A complete list of these
24 programs is:
26 program input files generates
27 ------- ----------- ---------
28 mkbuiltins builtins builtins.h builtins.c
29 mkinit *.c init.c
30 mknodes nodetypes nodes.h nodes.c
31 mksignames - signames.h signames.c
32 mksyntax - syntax.h syntax.c
33 mktokens - token.h
34 bltin/mkexpr unary_op binary_op operators.h operators.c
36 There are undoubtedly too many of these. Mkinit searches all the
37 C source files for entries looking like:
39 INIT {
40 x = 1; /* executed during initialization */
41 }
43 RESET {
44 x = 2; /* executed when the shell does a longjmp
45 back to the main command loop */
46 }
48 SHELLPROC {
49 x = 3; /* executed when the shell runs a shell procedure */
50 }
52 It pulls this code out into routines which are when particular
53 events occur. The intent is to improve modularity by isolating
54 the information about which modules need to be explicitly
55 initialized/reset within the modules themselves.
57 Mkinit recognizes several constructs for placing declarations in
58 the init.c file.
59 INCLUDE "file.h"
60 includes a file. The storage class MKINIT makes a declaration
61 available in the init.c file, for example:
62 MKINIT int funcnest; /* depth of function calls */
63 MKINIT alone on a line introduces a structure or union declara-
64 tion:
65 MKINIT
66 struct redirtab {
67 short renamed[10];
68 };
69 Preprocessor #define statements are copied to init.c without any
70 special action to request this.
72 INDENTATION: The ash source is indented in multiples of six
73 spaces. The only study that I have heard of on the subject con-
74 cluded that the optimal amount to indent is in the range of four
75 to six spaces. I use six spaces since it is not too big a jump
76 from the widely used eight spaces. If you really hate six space
77 indentation, use the adjind (source included) program to change
78 it to something else.
80 EXCEPTIONS: Code for dealing with exceptions appears in
81 exceptions.c. The C language doesn't include exception handling,
82 so I implement it using setjmp and longjmp. The global variable
83 exception contains the type of exception. EXERROR is raised by
84 calling error. EXINT is an interrupt. EXSHELLPROC is an excep-
85 tion which is raised when a shell procedure is invoked. The pur-
86 pose of EXSHELLPROC is to perform the cleanup actions associated
87 with other exceptions. After these cleanup actions, the shell
88 can interpret a shell procedure itself without exec'ing a new
89 copy of the shell.
91 INTERRUPTS: In an interactive shell, an interrupt will cause an
92 EXINT exception to return to the main command loop. (Exception:
93 EXINT is not raised if the user traps interrupts using the trap
94 command.) The INTOFF and INTON macros (defined in exception.h)
95 provide uninterruptible critical sections. Between the execution
96 of INTOFF and the execution of INTON, interrupt signals will be
97 held for later delivery. INTOFF and INTON can be nested.
99 MEMALLOC.C: Memalloc.c defines versions of malloc and realloc
100 which call error when there is no memory left. It also defines a
101 stack oriented memory allocation scheme. Allocating off a stack
102 is probably more efficient than allocation using malloc, but the
103 big advantage is that when an exception occurs all we have to do
104 to free up the memory in use at the time of the exception is to
105 restore the stack pointer. The stack is implemented using a
106 linked list of blocks.
108 STPUTC: If the stack were contiguous, it would be easy to store
109 strings on the stack without knowing in advance how long the
110 string was going to be:
111 p = stackptr;
112 *p++ = c; /* repeated as many times as needed */
113 stackptr = p;
114 The following three macros (defined in memalloc.h) perform these
115 operations, but grow the stack if you run off the end:
116 STARTSTACKSTR(p);
117 STPUTC(c, p); /* repeated as many times as needed */
118 grabstackstr(p);
120 We now start a top-down look at the code:
122 MAIN.C: The main routine performs some initialization, executes
123 the user's profile if necessary, and calls cmdloop. Cmdloop
124 repeatedly parses and executes commands.
126 OPTIONS.C: This file contains the option processing code. It is
127 called from main to parse the shell arguments when the shell is
128 invoked, and it also contains the set builtin. The -i and -j op-
129 tions (the latter turns on job control) require changes in signal
130 handling. The routines setjobctl (in jobs.c) and setinteractive
131 (in trap.c) are called to handle changes to these options.
133 PARSING: The parser code is all in parser.c. A recursive des-
134 cent parser is used. Syntax tables (generated by mksyntax) are
135 used to classify characters during lexical analysis. There are
136 three tables: one for normal use, one for use when inside single
137 quotes, and one for use when inside double quotes. The tables
138 are machine dependent because they are indexed by character vari-
139 ables and the range of a char varies from machine to machine.
141 PARSE OUTPUT: The output of the parser consists of a tree of
142 nodes. The various types of nodes are defined in the file node-
143 types.
145 Nodes of type NARG are used to represent both words and the con-
146 tents of here documents. An early version of ash kept the con-
147 tents of here documents in temporary files, but keeping here do-
148 cuments in memory typically results in significantly better per-
149 formance. It would have been nice to make it an option to use
150 temporary files for here documents, for the benefit of small
151 machines, but the code to keep track of when to delete the tem-
152 porary files was complex and I never fixed all the bugs in it.
153 (AT&T has been maintaining the Bourne shell for more than ten
154 years, and to the best of my knowledge they still haven't gotten
155 it to handle temporary files correctly in obscure cases.)
157 The text field of a NARG structure points to the text of the
158 word. The text consists of ordinary characters and a number of
159 special codes defined in parser.h. The special codes are:
161 CTLVAR Variable substitution
162 CTLENDVAR End of variable substitution
163 CTLBACKQ Command substitution
164 CTLBACKQ|CTLQUOTE Command substitution inside double quotes
165 CTLESC Escape next character
167 A variable substitution contains the following elements:
169 CTLVAR type name '=' [ alternative-text CTLENDVAR ]
171 The type field is a single character specifying the type of sub-
172 stitution. The possible types are:
174 VSNORMAL $var
175 VSMINUS ${var-text}
176 VSMINUS|VSNUL ${var:-text}
177 VSPLUS ${var+text}
178 VSPLUS|VSNUL ${var:+text}
179 VSQUESTION ${var?text}
180 VSQUESTION|VSNUL ${var:?text}
181 VSASSIGN ${var=text}
182 VSASSIGN|VSNUL ${var:=text}
184 In addition, the type field will have the VSQUOTE flag set if the
185 variable is enclosed in double quotes. The name of the variable
186 comes next, terminated by an equals sign. If the type is not
187 VSNORMAL, then the text field in the substitution follows, ter-
188 minated by a CTLENDVAR byte.
190 Commands in back quotes are parsed and stored in a linked list.
191 The locations of these commands in the string are indicated by
192 CTLBACKQ and CTLBACKQ+CTLQUOTE characters, depending upon whether
193 the back quotes were enclosed in double quotes.
195 The character CTLESC escapes the next character, so that in case
196 any of the CTL characters mentioned above appear in the input,
197 they can be passed through transparently. CTLESC is also used to
198 escape '*', '?', '[', and '!' characters which were quoted by the
199 user and thus should not be used for file name generation.
201 CTLESC characters have proved to be particularly tricky to get
202 right. In the case of here documents which are not subject to
203 variable and command substitution, the parser doesn't insert any
204 CTLESC characters to begin with (so the contents of the text
205 field can be written without any processing). Other here docu-
206 ments, and words which are not subject to splitting and file name
207 generation, have the CTLESC characters removed during the vari-
208 able and command substitution phase. Words which are subject to
209 splitting and file name generation have the CTLESC characters re-
210 moved as part of the file name phase.
212 EXECUTION: Command execution is handled by the following files:
213 eval.c The top level routines.
214 redir.c Code to handle redirection of input and output.
215 jobs.c Code to handle forking, waiting, and job control.
216 exec.c Code to do path searches and the actual exec sys call.
217 expand.c Code to evaluate arguments.
218 var.c Maintains the variable symbol table. Called from expand.c.
220 EVAL.C: Evaltree recursively executes a parse tree. The exit
221 status is returned in the global variable exitstatus. The alter-
222 native entry evalbackcmd is called to evaluate commands in back
223 quotes. It saves the result in memory if the command is a buil-
224 tin; otherwise it forks off a child to execute the command and
225 connects the standard output of the child to a pipe.
227 JOBS.C: To create a process, you call makejob to return a job
228 structure, and then call forkshell (passing the job structure as
229 an argument) to create the process. Waitforjob waits for a job
230 to complete. These routines take care of process groups if job
231 control is defined.
233 REDIR.C: Ash allows file descriptors to be redirected and then
234 restored without forking off a child process. This is accom-
235 plished by duplicating the original file descriptors. The redir-
236 tab structure records where the file descriptors have been dupli-
237 cated to.
239 EXEC.C: The routine find_command locates a command, and enters
240 the command in the hash table if it is not already there. The
241 third argument specifies whether it is to print an error message
242 if the command is not found. (When a pipeline is set up,
243 find_command is called for all the commands in the pipeline be-
244 fore any forking is done, so to get the commands into the hash
245 table of the parent process. But to make command hashing as
246 transparent as possible, we silently ignore errors at that point
247 and only print error messages if the command cannot be found
248 later.)
250 The routine shellexec is the interface to the exec system call.
252 EXPAND.C: Arguments are processed in three passes. The first
253 (performed by the routine argstr) performs variable and command
254 substitution. The second (ifsbreakup) performs word splitting
255 and the third (expandmeta) performs file name generation. If the
256 "/u" directory is simulated, then when "/u/username" is replaced
257 by the user's home directory, the flag "didudir" is set. This
258 tells the cd command that it should print out the directory name,
259 just as it would if the "/u" directory were implemented using
260 symbolic links.
262 VAR.C: Variables are stored in a hash table. Probably we should
263 switch to extensible hashing. The variable name is stored in the
264 same string as the value (using the format "name=value") so that
265 no string copying is needed to create the environment of a com-
266 mand. Variables which the shell references internally are preal-
267 located so that the shell can reference the values of these vari-
268 ables without doing a lookup.
270 When a program is run, the code in eval.c sticks any environment
271 variables which precede the command (as in "PATH=xxx command") in
272 the variable table as the simplest way to strip duplicates, and
273 then calls "environment" to get the value of the environment.
274 There are two consequences of this. First, if an assignment to
275 PATH precedes the command, the value of PATH before the assign-
276 ment must be remembered and passed to shellexec. Second, if the
277 program turns out to be a shell procedure, the strings from the
278 environment variables which preceded the command must be pulled
279 out of the table and replaced with strings obtained from malloc,
280 since the former will automatically be freed when the stack (see
281 the entry on memalloc.c) is emptied.
283 BUILTIN COMMANDS: The procedures for handling these are scat-
284 tered throughout the code, depending on which location appears
285 most appropriate. They can be recognized because their names al-
286 ways end in "cmd". The mapping from names to procedures is
287 specified in the file builtins, which is processed by the mkbuilt-
288 ins command.
290 A builtin command is invoked with argc and argv set up like a
291 normal program. A builtin command is allowed to overwrite its
292 arguments. Builtin routines can call nextopt to do option pars-
293 ing. This is kind of like getopt, but you don't pass argc and
294 argv to it. Builtin routines can also call error. This routine
295 normally terminates the shell (or returns to the main command
296 loop if the shell is interactive), but when called from a builtin
297 command it causes the builtin command to terminate with an exit
298 status of 2.
300 The directory bltins contains commands which can be compiled in-
301 dependently but can also be built into the shell for efficiency
302 reasons. The makefile in this directory compiles these programs
303 in the normal fashion (so that they can be run regardless of
304 whether the invoker is ash), but also creates a library named
305 bltinlib.a which can be linked with ash. The header file bltin.h
306 takes care of most of the differences between the ash and the
307 stand-alone environment. The user should call the main routine
308 "main", and #define main to be the name of the routine to use
309 when the program is linked into ash. This #define should appear
310 before bltin.h is included; bltin.h will #undef main if the pro-
311 gram is to be compiled stand-alone.
313 CD.C: This file defines the cd and pwd builtins. The pwd com-
314 mand runs /bin/pwd the first time it is invoked (unless the user
315 has already done a cd to an absolute pathname), but then
316 remembers the current directory and updates it when the cd com-
317 mand is run, so subsequent pwd commands run very fast. The main
318 complication in the cd command is in the docd command, which
319 resolves symbolic links into actual names and informs the user
320 where the user ended up if he crossed a symbolic link.
322 SIGNALS: Trap.c implements the trap command. The routine set-
323 signal figures out what action should be taken when a signal is
324 received and invokes the signal system call to set the signal ac-
325 tion appropriately. When a signal that a user has set a trap for
326 is caught, the routine "onsig" sets a flag. The routine dotrap
327 is called at appropriate points to actually handle the signal.
328 When an interrupt is caught and no trap has been set for that
329 signal, the routine "onint" in error.c is called.
331 OUTPUT: Ash uses it's own output routines. There are three out-
332 put structures allocated. "Output" represents the standard out-
333 put, "errout" the standard error, and "memout" contains output
334 which is to be stored in memory. This last is used when a buil-
335 tin command appears in backquotes, to allow its output to be col-
336 lected without doing any I/O through the UNIX operating system.
337 The variables out1 and out2 normally point to output and errout,
338 respectively, but they are set to point to memout when appropri-
339 ate inside backquotes.
341 INPUT: The basic input routine is pgetc, which reads from the
342 current input file. There is a stack of input files; the current
343 input file is the top file on this stack. The code allows the
344 input to come from a string rather than a file. (This is for the
345 -c option and the "." and eval builtin commands.) The global
346 variable plinno is saved and restored when files are pushed and
347 popped from the stack. The parser routines store the number of
348 the current line in this variable.
350 DEBUGGING: If DEBUG is defined in shell.h, then the shell will
351 write debugging information to the file $HOME/trace. Most of
352 this is done using the TRACE macro, which takes a set of printf
353 arguments inside two sets of parenthesis. Example:
354 "TRACE(("n=%d0, n))". The double parenthesis are necessary be-
355 cause the preprocessor can't handle functions with a variable
356 number of arguments. Defining DEBUG also causes the shell to
357 generate a core dump if it is sent a quit signal. The tracing
358 code is in show.c.