1# @(#)TOUR 8.1 (Berkeley) 5/31/93 2# $FreeBSD$ 3 4NOTE -- This is the original TOUR paper distributed with ash and 5does not represent the current state of the shell. It is provided anyway 6since it provides helpful information for how the shell is structured, 7but be warned that things have changed -- the current shell is 8still under development. 9 10================================================================ 11 12 A Tour through Ash 13 14 Copyright 1989 by Kenneth Almquist. 15 16 17DIRECTORIES: The subdirectory bltin contains commands which can 18be compiled stand-alone. The rest of the source is in the main 19ash directory. 20 21SOURCE CODE GENERATORS: Files whose names begin with "mk" are 22programs that generate source code. A complete list of these 23programs is: 24 25 program intput files generates 26 ------- ------------ --------- 27 mkbuiltins builtins builtins.h builtins.c 28 mkinit *.c init.c 29 mknodes nodetypes nodes.h nodes.c 30 mksignames - signames.h signames.c 31 mksyntax - syntax.h syntax.c 32 mktokens - token.h 33 bltin/mkexpr unary_op binary_op operators.h operators.c 34 35There are undoubtedly too many of these. Mkinit searches all the 36C source files for entries looking like: 37 38 INIT { 39 x = 1; /* executed during initialization */ 40 } 41 42 RESET { 43 x = 2; /* executed when the shell does a longjmp 44 back to the main command loop */ 45 } 46 47 SHELLPROC { 48 x = 3; /* executed when the shell runs a shell procedure */ 49 } 50 51It pulls this code out into routines which are when particular 52events occur. The intent is to improve modularity by isolating 53the information about which modules need to be explicitly 54initialized/reset within the modules themselves. 55 56Mkinit recognizes several constructs for placing declarations in 57the init.c file. 58 INCLUDE "file.h" 59includes a file. The storage class MKINIT makes a declaration 60available in the init.c file, for example: 61 MKINIT int funcnest; /* depth of function calls */ 62MKINIT alone on a line introduces a structure or union declara- 63tion: 64 MKINIT 65 struct redirtab { 66 short renamed[10]; 67 }; 68Preprocessor #define statements are copied to init.c without any 69special action to request this. 70 71INDENTATION: The ash source is indented in multiples of six 72spaces. The only study that I have heard of on the subject con- 73cluded that the optimal amount to indent is in the range of four 74to six spaces. I use six spaces since it is not too big a jump 75from the widely used eight spaces. If you really hate six space 76indentation, use the adjind (source included) program to change 77it to something else. 78 79EXCEPTIONS: Code for dealing with exceptions appears in 80exceptions.c. The C language doesn't include exception handling, 81so I implement it using setjmp and longjmp. The global variable 82exception contains the type of exception. EXERROR is raised by 83calling error. EXINT is an interrupt. EXSHELLPROC is an excep- 84tion which is raised when a shell procedure is invoked. The pur- 85pose of EXSHELLPROC is to perform the cleanup actions associated 86with other exceptions. After these cleanup actions, the shell 87can interpret a shell procedure itself without exec'ing a new 88copy of the shell. 89 90INTERRUPTS: In an interactive shell, an interrupt will cause an 91EXINT exception to return to the main command loop. (Exception: 92EXINT is not raised if the user traps interrupts using the trap 93command.) The INTOFF and INTON macros (defined in exception.h) 94provide uninterruptable critical sections. Between the execution 95of INTOFF and the execution of INTON, interrupt signals will be 96held for later delivery. INTOFF and INTON can be nested. 97 98MEMALLOC.C: Memalloc.c defines versions of malloc and realloc 99which call error when there is no memory left. It also defines a 100stack oriented memory allocation scheme. Allocating off a stack 101is probably more efficient than allocation using malloc, but the 102big advantage is that when an exception occurs all we have to do 103to free up the memory in use at the time of the exception is to 104restore the stack pointer. The stack is implemented using a 105linked list of blocks. 106 107STPUTC: If the stack were contiguous, it would be easy to store 108strings on the stack without knowing in advance how long the 109string was going to be: 110 p = stackptr; 111 *p++ = c; /* repeated as many times as needed */ 112 stackptr = p; 113The folloing three macros (defined in memalloc.h) perform these 114operations, but grow the stack if you run off the end: 115 STARTSTACKSTR(p); 116 STPUTC(c, p); /* repeated as many times as needed */ 117 grabstackstr(p); 118 119We now start a top-down look at the code: 120 121MAIN.C: The main routine performs some initialization, executes 122the user's profile if necessary, and calls cmdloop. Cmdloop is 123repeatedly parses and executes commands. 124 125OPTIONS.C: This file contains the option processing code. It is 126called from main to parse the shell arguments when the shell is 127invoked, and it also contains the set builtin. The -i and -j op- 128tions (the latter turns on job control) require changes in signal 129handling. The routines setjobctl (in jobs.c) and setinteractive 130(in trap.c) are called to handle changes to these options. 131 132PARSING: The parser code is all in parser.c. A recursive des- 133cent parser is used. Syntax tables (generated by mksyntax) are 134used to classify characters during lexical analysis. There are 135three tables: one for normal use, one for use when inside single 136quotes, and one for use when inside double quotes. The tables 137are machine dependent because they are indexed by character vari- 138ables and the range of a char varies from machine to machine. 139 140PARSE OUTPUT: The output of the parser consists of a tree of 141nodes. The various types of nodes are defined in the file node- 142types. 143 144Nodes of type NARG are used to represent both words and the con- 145tents of here documents. An early version of ash kept the con- 146tents of here documents in temporary files, but keeping here do- 147cuments in memory typically results in significantly better per- 148formance. It would have been nice to make it an option to use 149temporary files for here documents, for the benefit of small 150machines, but the code to keep track of when to delete the tem- 151porary files was complex and I never fixed all the bugs in it. 152(AT&T has been maintaining the Bourne shell for more than ten 153years, and to the best of my knowledge they still haven't gotten 154it to handle temporary files correctly in obscure cases.) 155 156The text field of a NARG structure points to the text of the 157word. The text consists of ordinary characters and a number of 158special codes defined in parser.h. The special codes are: 159 160 CTLVAR Variable substitution 161 CTLENDVAR End of variable substitution 162 CTLBACKQ Command substitution 163 CTLBACKQ|CTLQUOTE Command substitution inside double quotes 164 CTLESC Escape next character 165 166A variable substitution contains the following elements: 167 168 CTLVAR type name '=' [ alternative-text CTLENDVAR ] 169 170The type field is a single character specifying the type of sub- 171stitution. The possible types are: 172 173 VSNORMAL $var 174 VSMINUS ${var-text} 175 VSMINUS|VSNUL ${var:-text} 176 VSPLUS ${var+text} 177 VSPLUS|VSNUL ${var:+text} 178 VSQUESTION ${var?text} 179 VSQUESTION|VSNUL ${var:?text} 180 VSASSIGN ${var=text} 181 VSASSIGN|VSNUL ${var=text} 182 183In addition, the type field will have the VSQUOTE flag set if the 184variable is enclosed in double quotes. The name of the variable 185comes next, terminated by an equals sign. If the type is not 186VSNORMAL, then the text field in the substitution follows, ter- 187minated by a CTLENDVAR byte. 188 189Commands in back quotes are parsed and stored in a linked list. 190The locations of these commands in the string are indicated by 191CTLBACKQ and CTLBACKQ+CTLQUOTE characters, depending upon whether 192the back quotes were enclosed in double quotes. 193 194The character CTLESC escapes the next character, so that in case 195any of the CTL characters mentioned above appear in the input, 196they can be passed through transparently. CTLESC is also used to 197escape '*', '?', '[', and '!' characters which were quoted by the 198user and thus should not be used for file name generation. 199 200CTLESC characters have proved to be particularly tricky to get 201right. In the case of here documents which are not subject to 202variable and command substitution, the parser doesn't insert any 203CTLESC characters to begin with (so the contents of the text 204field can be written without any processing). Other here docu- 205ments, and words which are not subject to splitting and file name 206generation, have the CTLESC characters removed during the vari- 207able and command substitution phase. Words which are subject 208splitting and file name generation have the CTLESC characters re- 209moved as part of the file name phase. 210 211EXECUTION: Command execution is handled by the following files: 212 eval.c The top level routines. 213 redir.c Code to handle redirection of input and output. 214 jobs.c Code to handle forking, waiting, and job control. 215 exec.c Code to to path searches and the actual exec sys call. 216 expand.c Code to evaluate arguments. 217 var.c Maintains the variable symbol table. Called from expand.c. 218 219EVAL.C: Evaltree recursively executes a parse tree. The exit 220status is returned in the global variable exitstatus. The alter- 221native entry evalbackcmd is called to evaluate commands in back 222quotes. It saves the result in memory if the command is a buil- 223tin; otherwise it forks off a child to execute the command and 224connects the standard output of the child to a pipe. 225 226JOBS.C: To create a process, you call makejob to return a job 227structure, and then call forkshell (passing the job structure as 228an argument) to create the process. Waitforjob waits for a job 229to complete. These routines take care of process groups if job 230control is defined. 231 232REDIR.C: Ash allows file descriptors to be redirected and then 233restored without forking off a child process. This is accom- 234plished by duplicating the original file descriptors. The redir- 235tab structure records where the file descriptors have be dupli- 236cated to. 237 238EXEC.C: The routine find_command locates a command, and enters 239the command in the hash table if it is not already there. The 240third argument specifies whether it is to print an error message 241if the command is not found. (When a pipeline is set up, 242find_command is called for all the commands in the pipeline be- 243fore any forking is done, so to get the commands into the hash 244table of the parent process. But to make command hashing as 245transparent as possible, we silently ignore errors at that point 246and only print error messages if the command cannot be found 247later.) 248 249The routine shellexec is the interface to the exec system call. 250 251EXPAND.C: Arguments are processed in three passes. The first 252(performed by the routine argstr) performs variable and command 253substitution. The second (ifsbreakup) performs word splitting 254and the third (expandmeta) performs file name generation. If the 255"/u" directory is simulated, then when "/u/username" is replaced 256by the user's home directory, the flag "didudir" is set. This 257tells the cd command that it should print out the directory name, 258just as it would if the "/u" directory were implemented using 259symbolic links. 260 261VAR.C: Variables are stored in a hash table. Probably we should 262switch to extensible hashing. The variable name is stored in the 263same string as the value (using the format "name=value") so that 264no string copying is needed to create the environment of a com- 265mand. Variables which the shell references internally are preal- 266located so that the shell can reference the values of these vari- 267ables without doing a lookup. 268 269When a program is run, the code in eval.c sticks any environment 270variables which precede the command (as in "PATH=xxx command") in 271the variable table as the simplest way to strip duplicates, and 272then calls "environment" to get the value of the environment. 273There are two consequences of this. First, if an assignment to 274PATH precedes the command, the value of PATH before the assign- 275ment must be remembered and passed to shellexec. Second, if the 276program turns out to be a shell procedure, the strings from the 277environment variables which preceded the command must be pulled 278out of the table and replaced with strings obtained from malloc, 279since the former will automatically be freed when the stack (see 280the entry on memalloc.c) is emptied. 281 282BUILTIN COMMANDS: The procedures for handling these are scat- 283tered throughout the code, depending on which location appears 284most appropriate. They can be recognized because their names al- 285ways end in "cmd". The mapping from names to procedures is 286specified in the file builtins, which is processed by the mkbuil- 287tins command. 288 289A builtin command is invoked with argc and argv set up like a 290normal program. A builtin command is allowed to overwrite its 291arguments. Builtin routines can call nextopt to do option pars- 292ing. This is kind of like getopt, but you don't pass argc and 293argv to it. Builtin routines can also call error. This routine 294normally terminates the shell (or returns to the main command 295loop if the shell is interactive), but when called from a builtin 296command it causes the builtin command to terminate with an exit 297status of 2. 298 299The directory bltins contains commands which can be compiled in- 300dependently but can also be built into the shell for efficiency 301reasons. The makefile in this directory compiles these programs 302in the normal fashion (so that they can be run regardless of 303whether the invoker is ash), but also creates a library named 304bltinlib.a which can be linked with ash. The header file bltin.h 305takes care of most of the differences between the ash and the 306stand-alone environment. The user should call the main routine 307"main", and #define main to be the name of the routine to use 308when the program is linked into ash. This #define should appear 309before bltin.h is included; bltin.h will #undef main if the pro- 310gram is to be compiled stand-alone. 311 312CD.C: This file defines the cd and pwd builtins. The pwd com- 313mand runs /bin/pwd the first time it is invoked (unless the user 314has already done a cd to an absolute pathname), but then 315remembers the current directory and updates it when the cd com- 316mand is run, so subsequent pwd commands run very fast. The main 317complication in the cd command is in the docd command, which 318resolves symbolic links into actual names and informs the user 319where the user ended up if he crossed a symbolic link. 320 321SIGNALS: Trap.c implements the trap command. The routine set- 322signal figures out what action should be taken when a signal is 323received and invokes the signal system call to set the signal ac- 324tion appropriately. When a signal that a user has set a trap for 325is caught, the routine "onsig" sets a flag. The routine dotrap 326is called at appropriate points to actually handle the signal. 327When an interrupt is caught and no trap has been set for that 328signal, the routine "onint" in error.c is called. 329 330OUTPUT: Ash uses it's own output routines. There are three out- 331put structures allocated. "Output" represents the standard out- 332put, "errout" the standard error, and "memout" contains output 333which is to be stored in memory. This last is used when a buil- 334tin command appears in backquotes, to allow its output to be col- 335lected without doing any I/O through the UNIX operating system. 336The variables out1 and out2 normally point to output and errout, 337respectively, but they are set to point to memout when appropri- 338ate inside backquotes. 339 340INPUT: The basic input routine is pgetc, which reads from the 341current input file. There is a stack of input files; the current 342input file is the top file on this stack. The code allows the 343input to come from a string rather than a file. (This is for the 344-c option and the "." and eval builtin commands.) The global 345variable plinno is saved and restored when files are pushed and 346popped from the stack. The parser routines store the number of 347the current line in this variable. 348 349DEBUGGING: If DEBUG is defined in shell.h, then the shell will 350write debugging information to the file $HOME/trace. Most of 351this is done using the TRACE macro, which takes a set of printf 352arguments inside two sets of parenthesis. Example: 353"TRACE(("n=%d0, n))". The double parenthesis are necessary be- 354cause the preprocessor can't handle functions with a variable 355number of arguments. Defining DEBUG also causes the shell to 356generate a core dump if it is sent a quit signal. The tracing 357code is in show.c. 358