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