sh/TOUR

   1 #       @(#)TOUR        8.1 (Berkeley) 5/31/93
   2 # $FreeBSD$
   3
   4 NOTE -- This is the original TOUR paper distributed with ash and
   5 does not represent the current state of the shell.  It is provided anyway
   6 since it provides helpful information for how the shell is structured,
   7 but be warned that things have changed -- the current shell is
   8 still under development.
   9
  10 ================================================================
  11
  12                        A Tour through Ash
  13
  14                Copyright 1989 by Kenneth Almquist.
  15
  16
  17 DIRECTORIES:  The subdirectory bltin contains commands which can
  18 be compiled stand-alone.  The rest of the source is in the main
  19 ash directory.
  20
  21 SOURCE CODE GENERATORS:  Files whose names begin with "mk" are
  22 programs that generate source code.  A complete list of these
  23 programs is:
  24
  25         program         input files         generates
  26         -------         -----------         ---------
  27         mkbuiltins      builtins            builtins.h builtins.c
  28         mknodes         nodetypes           nodes.h nodes.c
  29         mksyntax            -               syntax.h syntax.c
  30         mktokens            -               token.h
  31
  32 There are undoubtedly too many of these.
  33
  34 EXCEPTIONS:  Code for dealing with exceptions appears in
  35 exceptions.c.  The C language doesn't include exception handling,
  36 so I implement it using setjmp and longjmp.  The global variable
  37 exception contains the type of exception.  EXERROR is raised by
  38 calling error.  EXINT is an interrupt.
  39
  40 INTERRUPTS:  In an interactive shell, an interrupt will cause an
  41 EXINT exception to return to the main command loop.  (Exception:
  42 EXINT is not raised if the user traps interrupts using the trap
  43 command.)  The INTOFF and INTON macros (defined in exception.h)
  44 provide uninterruptible critical sections.  Between the execution
  45 of INTOFF and the execution of INTON, interrupt signals will be
  46 held for later delivery.  INTOFF and INTON can be nested.
  47
  48 MEMALLOC.C:  Memalloc.c defines versions of malloc and realloc
  49 which call error when there is no memory left.  It also defines a
  50 stack oriented memory allocation scheme.  Allocating off a stack
  51 is probably more efficient than allocation using malloc, but the
  52 big advantage is that when an exception occurs all we have to do
  53 to free up the memory in use at the time of the exception is to
  54 restore the stack pointer.  The stack is implemented using a
  55 linked list of blocks.
  56
  57 STPUTC:  If the stack were contiguous, it would be easy to store
  58 strings on the stack without knowing in advance how long the
  59 string was going to be:
  60         p = stackptr;
  61         *p++ = c;       /* repeated as many times as needed */
  62         stackptr = p;
  63 The following three macros (defined in memalloc.h) perform these
  64 operations, but grow the stack if you run off the end:
  65         STARTSTACKSTR(p);
  66         STPUTC(c, p);   /* repeated as many times as needed */
  67         grabstackstr(p);
  68
  69 We now start a top-down look at the code:
  70
  71 MAIN.C:  The main routine performs some initialization, executes
  72 the user's profile if necessary, and calls cmdloop.  Cmdloop
  73 repeatedly parses and executes commands.
  74
  75 OPTIONS.C:  This file contains the option processing code.  It is
  76 called from main to parse the shell arguments when the shell is
  77 invoked, and it also contains the set builtin.  The -i and -m op-
  78 tions (the latter turns on job control) require changes in signal
  79 handling.  The routines setjobctl (in jobs.c) and setinteractive
  80 (in trap.c) are called to handle changes to these options.
  81
  82 PARSING:  The parser code is all in parser.c.  A recursive des-
  83 cent parser is used.  Syntax tables (generated by mksyntax) are
  84 used to classify characters during lexical analysis.  There are
  85 four tables:  one for normal use, one for use when inside single
  86 quotes and dollar single quotes, one for use when inside double
  87 quotes and one for use in arithmetic.  The tables are machine
  88 dependent because they are indexed by character variables and
  89 the range of a char varies from machine to machine.
  90
  91 PARSE OUTPUT:  The output of the parser consists of a tree of
  92 nodes.  The various types of nodes are defined in the file node-
  93 types.
  94
  95 Nodes of type NARG are used to represent both words and the con-
  96 tents of here documents.  An early version of ash kept the con-
  97 tents of here documents in temporary files, but keeping here do-
  98 cuments in memory typically results in significantly better per-
  99 formance.  It would have been nice to make it an option to use
 100 temporary files for here documents, for the benefit of small
 101 machines, but the code to keep track of when to delete the tem-
 102 porary files was complex and I never fixed all the bugs in it.
 103 (AT&T has been maintaining the Bourne shell for more than ten
 104 years, and to the best of my knowledge they still haven't gotten
 105 it to handle temporary files correctly in obscure cases.)
 106
 107 The text field of a NARG structure points to the text of the
 108 word.  The text consists of ordinary characters and a number of
 109 special codes defined in parser.h.  The special codes are:
 110
 111         CTLVAR              Variable substitution
 112         CTLENDVAR           End of variable substitution
 113         CTLBACKQ            Command substitution
 114         CTLBACKQ|CTLQUOTE   Command substitution inside double quotes
 115         CTLESC              Escape next character
 116
 117 A variable substitution contains the following elements:
 118
 119         CTLVAR type name '=' [ alternative-text CTLENDVAR ]
 120
 121 The type field is a single character specifying the type of sub-
 122 stitution.  The possible types are:
 123
 124         VSNORMAL            $var
 125         VSMINUS             ${var-text}
 126         VSMINUS|VSNUL       ${var:-text}
 127         VSPLUS              ${var+text}
 128         VSPLUS|VSNUL        ${var:+text}
 129         VSQUESTION          ${var?text}
 130         VSQUESTION|VSNUL    ${var:?text}
 131         VSASSIGN            ${var=text}
 132         VSASSIGN|VSNUL      ${var:=text}
 133
 134 In addition, the type field will have the VSQUOTE flag set if the
 135 variable is enclosed in double quotes.  The name of the variable
 136 comes next, terminated by an equals sign.  If the type is not
 137 VSNORMAL, then the text field in the substitution follows, ter-
 138 minated by a CTLENDVAR byte.
 139
 140 Commands in back quotes are parsed and stored in a linked list.
 141 The locations of these commands in the string are indicated by
 142 CTLBACKQ and CTLBACKQ+CTLQUOTE characters, depending upon whether
 143 the back quotes were enclosed in double quotes.
 144
 145 The character CTLESC escapes the next character, so that in case
 146 any of the CTL characters mentioned above appear in the input,
 147 they can be passed through transparently.  CTLESC is also used to
 148 escape '*', '?', '[', and '!' characters which were quoted by the
 149 user and thus should not be used for file name generation.
 150
 151 CTLESC characters have proved to be particularly tricky to get
 152 right.  In the case of here documents which are not subject to
 153 variable and command substitution, the parser doesn't insert any
 154 CTLESC characters to begin with (so the contents of the text
 155 field can be written without any processing).  Other here docu-
 156 ments, and words which are not subject to splitting and file name
 157 generation, have the CTLESC characters removed during the vari-
 158 able and command substitution phase.  Words which are subject to
 159 splitting and file name generation have the CTLESC characters re-
 160 moved as part of the file name phase.
 161
 162 EXECUTION:  Command execution is handled by the following files:
 163         eval.c     The top level routines.
 164         redir.c    Code to handle redirection of input and output.
 165         jobs.c     Code to handle forking, waiting, and job control.
 166         exec.c     Code to do path searches and the actual exec sys call.
 167         expand.c   Code to evaluate arguments.
 168         var.c      Maintains the variable symbol table.  Called from expand.c.
 169
 170 EVAL.C:  Evaltree recursively executes a parse tree.  The exit
 171 status is returned in the global variable exitstatus.  The alter-
 172 native entry evalbackcmd is called to evaluate commands in back
 173 quotes.  It saves the result in memory if the command is a buil-
 174 tin; otherwise it forks off a child to execute the command and
 175 connects the standard output of the child to a pipe.
 176
 177 JOBS.C:  To create a process, you call makejob to return a job
 178 structure, and then call forkshell (passing the job structure as
 179 an argument) to create the process.  Waitforjob waits for a job
 180 to complete.  These routines take care of process groups if job
 181 control is defined.
 182
 183 REDIR.C:  Ash allows file descriptors to be redirected and then
 184 restored without forking off a child process.  This is accom-
 185 plished by duplicating the original file descriptors.  The redir-
 186 tab structure records where the file descriptors have been dupli-
 187 cated to.
 188
 189 EXEC.C:  The routine find_command locates a command, and enters
 190 the command in the hash table if it is not already there.  The
 191 third argument specifies whether it is to print an error message
 192 if the command is not found.  (When a pipeline is set up,
 193 find_command is called for all the commands in the pipeline be-
 194 fore any forking is done, so to get the commands into the hash
 195 table of the parent process.  But to make command hashing as
 196 transparent as possible, we silently ignore errors at that point
 197 and only print error messages if the command cannot be found
 198 later.)
 199
 200 The routine shellexec is the interface to the exec system call.
 201
 202 EXPAND.C:  Arguments are processed in three passes.  The first
 203 (performed by the routine argstr) performs variable and command
 204 substitution.  The second (ifsbreakup) performs word splitting
 205 and the third (expandmeta) performs file name generation.
 206
 207 VAR.C:  Variables are stored in a hash table.  Probably we should
 208 switch to extensible hashing.  The variable name is stored in the
 209 same string as the value (using the format "name=value") so that
 210 no string copying is needed to create the environment of a com-
 211 mand.  Variables which the shell references internally are preal-
 212 located so that the shell can reference the values of these vari-
 213 ables without doing a lookup.
 214
 215 When a program is run, the code in eval.c sticks any environment
 216 variables which precede the command (as in "PATH=xxx command") in
 217 the variable table as the simplest way to strip duplicates, and
 218 then calls "environment" to get the value of the environment.
 219
 220 BUILTIN COMMANDS:  The procedures for handling these are scat-
 221 tered throughout the code, depending on which location appears
 222 most appropriate.  They can be recognized because their names al-
 223 ways end in "cmd".  The mapping from names to procedures is
 224 specified in the file builtins, which is processed by the mkbuilt-
 225 ins command.
 226
 227 A builtin command is invoked with argc and argv set up like a
 228 normal program.  A builtin command is allowed to overwrite its
 229 arguments.  Builtin routines can call nextopt to do option pars-
 230 ing.  This is kind of like getopt, but you don't pass argc and
 231 argv to it.  Builtin routines can also call error.  This routine
 232 normally terminates the shell (or returns to the main command
 233 loop if the shell is interactive), but when called from a builtin
 234 command it causes the builtin command to terminate with an exit
 235 status of 2.
 236
 237 The directory bltins contains commands which can be compiled in-
 238 dependently but can also be built into the shell for efficiency
 239 reasons.  The makefile in this directory compiles these programs
 240 in the normal fashion (so that they can be run regardless of
 241 whether the invoker is ash), but also creates a library named
 242 bltinlib.a which can be linked with ash.  The header file bltin.h
 243 takes care of most of the differences between the ash and the
 244 stand-alone environment.  The user should call the main routine
 245 "main", and #define main to be the name of the routine to use
 246 when the program is linked into ash.  This #define should appear
 247 before bltin.h is included; bltin.h will #undef main if the pro-
 248 gram is to be compiled stand-alone.
 249
 250 CD.C:  This file defines the cd and pwd builtins.
 251
 252 SIGNALS:  Trap.c implements the trap command.  The routine set-
 253 signal figures out what action should be taken when a signal is
 254 received and invokes the signal system call to set the signal ac-
 255 tion appropriately.  When a signal that a user has set a trap for
 256 is caught, the routine "onsig" sets a flag.  The routine dotrap
 257 is called at appropriate points to actually handle the signal.
 258 When an interrupt is caught and no trap has been set for that
 259 signal, the routine "onint" in error.c is called.
 260
 261 OUTPUT:  Ash uses it's own output routines.  There are three out-
 262 put structures allocated.  "Output" represents the standard out-
 263 put, "errout" the standard error, and "memout" contains output
 264 which is to be stored in memory.  This last is used when a buil-
 265 tin command appears in backquotes, to allow its output to be col-
 266 lected without doing any I/O through the UNIX operating system.
 267 The variables out1 and out2 normally point to output and errout,
 268 respectively, but they are set to point to memout when appropri-
 269 ate inside backquotes.
 270
 271 INPUT:  The basic input routine is pgetc, which reads from the
 272 current input file.  There is a stack of input files; the current
 273 input file is the top file on this stack.  The code allows the
 274 input to come from a string rather than a file.  (This is for the
 275 -c option and the "." and eval builtin commands.)  The global
 276 variable plinno is saved and restored when files are pushed and
 277 popped from the stack.  The parser routines store the number of
 278 the current line in this variable.
 279
 280 DEBUGGING:  If DEBUG is defined in shell.h, then the shell will
 281 write debugging information to the file $HOME/trace.  Most of
 282 this is done using the TRACE macro, which takes a set of printf
 283 arguments inside two sets of parenthesis.  Example:
 284 "TRACE(("n=%d0, n))".  The double parenthesis are necessary be-
 285 cause the preprocessor can't handle functions with a variable
 286 number of arguments.  Defining DEBUG also causes the shell to
 287 generate a core dump if it is sent a quit signal.  The tracing
 288 code is in show.c.