diff options
Diffstat (limited to '')
| -rw-r--r-- | bin/1sh/TOUR | 301 |
1 files changed, 0 insertions, 301 deletions
diff --git a/bin/1sh/TOUR b/bin/1sh/TOUR deleted file mode 100644 index 3f196c9b..00000000 --- a/bin/1sh/TOUR +++ /dev/null @@ -1,301 +0,0 @@ -# @(#)TOUR 8.1 (Berkeley) 5/31/93 -# $FreeBSD: releng/12.1/bin/sh/TOUR 317882 2017-05-06 13:28:42Z jilles $ - -NOTE -- This is the original TOUR paper distributed with ash and -does not represent the current state of the shell. It is provided anyway -since it provides helpful information for how the shell is structured, -but be warned that things have changed -- the current shell is -still under development. - -================================================================ - - A Tour through Ash - - Copyright 1989 by Kenneth Almquist. - - -DIRECTORIES: The subdirectory bltin contains commands which can -be compiled stand-alone. The rest of the source is in the main -ash directory. - -SOURCE CODE GENERATORS: Files whose names begin with "mk" are -programs that generate source code. A complete list of these -programs is: - - program input files generates - ------- ----------- --------- - mkbuiltins builtins.def builtins.h builtins.c - mknodes nodetypes nodes.h nodes.c - mksyntax - syntax.h syntax.c - mktokens - token.h - -There are undoubtedly too many of these. - -EXCEPTIONS: Code for dealing with exceptions appears in -exceptions.c. The C language doesn't include exception handling, -so I implement it using setjmp and longjmp. The global variable -exception contains the type of exception. EXERROR is raised by -calling error. EXINT is an interrupt. - -INTERRUPTS: In an interactive shell, an interrupt will cause an -EXINT exception to return to the main command loop. (Exception: -EXINT is not raised if the user traps interrupts using the trap -command.) The INTOFF and INTON macros (defined in exception.h) -provide uninterruptible critical sections. Between the execution -of INTOFF and the execution of INTON, interrupt signals will be -held for later delivery. INTOFF and INTON can be nested. - -MEMALLOC.C: Memalloc.c defines versions of malloc and realloc -which call error when there is no memory left. It also defines a -stack oriented memory allocation scheme. Allocating off a stack -is probably more efficient than allocation using malloc, but the -big advantage is that when an exception occurs all we have to do -to free up the memory in use at the time of the exception is to -restore the stack pointer. The stack is implemented using a -linked list of blocks. - -STPUTC: If the stack were contiguous, it would be easy to store -strings on the stack without knowing in advance how long the -string was going to be: - p = stackptr; - *p++ = c; /* repeated as many times as needed */ - stackptr = p; -The following three macros (defined in memalloc.h) perform these -operations, but grow the stack if you run off the end: - STARTSTACKSTR(p); - STPUTC(c, p); /* repeated as many times as needed */ - grabstackstr(p); - -We now start a top-down look at the code: - -MAIN.C: The main routine performs some initialization, executes -the user's profile if necessary, and calls cmdloop. Cmdloop -repeatedly parses and executes commands. - -OPTIONS.C: This file contains the option processing code. It is -called from main to parse the shell arguments when the shell is -invoked, and it also contains the set builtin. The -i and -m op- -tions (the latter turns on job control) require changes in signal -handling. The routines setjobctl (in jobs.c) and setinteractive -(in trap.c) are called to handle changes to these options. - -PARSING: The parser code is all in parser.c. A recursive des- -cent parser is used. Syntax tables (generated by mksyntax) are -used to classify characters during lexical analysis. There are -four tables: one for normal use, one for use when inside single -quotes and dollar single quotes, one for use when inside double -quotes and one for use in arithmetic. The tables are machine -dependent because they are indexed by character variables and -the range of a char varies from machine to machine. - -PARSE OUTPUT: The output of the parser consists of a tree of -nodes. The various types of nodes are defined in the file node- -types. - -Nodes of type NARG are used to represent both words and the con- -tents of here documents. An early version of ash kept the con- -tents of here documents in temporary files, but keeping here do- -cuments in memory typically results in significantly better per- -formance. It would have been nice to make it an option to use -temporary files for here documents, for the benefit of small -machines, but the code to keep track of when to delete the tem- -porary files was complex and I never fixed all the bugs in it. -(AT&T has been maintaining the Bourne shell for more than ten -years, and to the best of my knowledge they still haven't gotten -it to handle temporary files correctly in obscure cases.) - -The text field of a NARG structure points to the text of the -word. The text consists of ordinary characters and a number of -special codes defined in parser.h. The special codes are: - - CTLVAR Parameter expansion - CTLENDVAR End of parameter expansion - CTLBACKQ Command substitution - CTLBACKQ|CTLQUOTE Command substitution inside double quotes - CTLARI Arithmetic expansion - CTLENDARI End of arithmetic expansion - CTLESC Escape next character - -A variable substitution contains the following elements: - - CTLVAR type name '=' [ alternative-text CTLENDVAR ] - -The type field is a single character specifying the type of sub- -stitution. The possible types are: - - VSNORMAL $var - VSMINUS ${var-text} - VSMINUS|VSNUL ${var:-text} - VSPLUS ${var+text} - VSPLUS|VSNUL ${var:+text} - VSQUESTION ${var?text} - VSQUESTION|VSNUL ${var:?text} - VSASSIGN ${var=text} - VSASSIGN|VSNUL ${var:=text} - VSTRIMLEFT ${var#text} - VSTRIMLEFTMAX ${var##text} - VSTRIMRIGHT ${var%text} - VSTRIMRIGHTMAX ${var%%text} - VSLENGTH ${#var} - VSERROR delayed error - -In addition, the type field will have the VSQUOTE flag set if the -variable is enclosed in double quotes and the VSLINENO flag if -LINENO is being expanded (the parameter name is the decimal line -number). The parameter's name comes next, terminated by an equals -sign. If the type is not VSNORMAL (including when it is VSLENGTH), -then the text field in the substitution follows, terminated by a -CTLENDVAR byte. - -The type VSERROR is used to allow parsing bad substitutions like -${var[7]} and generate an error when they are expanded. - -Commands in back quotes are parsed and stored in a linked list. -The locations of these commands in the string are indicated by -CTLBACKQ and CTLBACKQ+CTLQUOTE characters, depending upon whether -the back quotes were enclosed in double quotes. - -Arithmetic expansion starts with CTLARI and ends with CTLENDARI. - -The character CTLESC escapes the next character, so that in case -any of the CTL characters mentioned above appear in the input, -they can be passed through transparently. CTLESC is also used to -escape '*', '?', '[', and '!' characters which were quoted by the -user and thus should not be used for file name generation. - -CTLESC characters have proved to be particularly tricky to get -right. In the case of here documents which are not subject to -variable and command substitution, the parser doesn't insert any -CTLESC characters to begin with (so the contents of the text -field can be written without any processing). Other here docu- -ments, and words which are not subject to file name generation, -have the CTLESC characters removed during the variable and command -substitution phase. Words which are subject to file name -generation have the CTLESC characters removed as part of the file -name phase. - -EXECUTION: Command execution is handled by the following files: - eval.c The top level routines. - redir.c Code to handle redirection of input and output. - jobs.c Code to handle forking, waiting, and job control. - exec.c Code to do path searches and the actual exec sys call. - expand.c Code to evaluate arguments. - var.c Maintains the variable symbol table. Called from expand.c. - -EVAL.C: Evaltree recursively executes a parse tree. The exit -status is returned in the global variable exitstatus. The alter- -native entry evalbackcmd is called to evaluate commands in back -quotes. It saves the result in memory if the command is a buil- -tin; otherwise it forks off a child to execute the command and -connects the standard output of the child to a pipe. - -JOBS.C: To create a process, you call makejob to return a job -structure, and then call forkshell (passing the job structure as -an argument) to create the process. Waitforjob waits for a job -to complete. These routines take care of process groups if job -control is defined. - -REDIR.C: Ash allows file descriptors to be redirected and then -restored without forking off a child process. This is accom- -plished by duplicating the original file descriptors. The redir- -tab structure records where the file descriptors have been dupli- -cated to. - -EXEC.C: The routine find_command locates a command, and enters -the command in the hash table if it is not already there. The -third argument specifies whether it is to print an error message -if the command is not found. (When a pipeline is set up, -find_command is called for all the commands in the pipeline be- -fore any forking is done, so to get the commands into the hash -table of the parent process. But to make command hashing as -transparent as possible, we silently ignore errors at that point -and only print error messages if the command cannot be found -later.) - -The routine shellexec is the interface to the exec system call. - -EXPAND.C: As the routine argstr generates words by parameter -expansion, command substitution and arithmetic expansion, it -performs word splitting on the result. As each word is output, -the routine expandmeta performs file name generation (if enabled). - -VAR.C: Variables are stored in a hash table. Probably we should -switch to extensible hashing. The variable name is stored in the -same string as the value (using the format "name=value") so that -no string copying is needed to create the environment of a com- -mand. Variables which the shell references internally are preal- -located so that the shell can reference the values of these vari- -ables without doing a lookup. - -When a program is run, the code in eval.c sticks any environment -variables which precede the command (as in "PATH=xxx command") in -the variable table as the simplest way to strip duplicates, and -then calls "environment" to get the value of the environment. - -BUILTIN COMMANDS: The procedures for handling these are scat- -tered throughout the code, depending on which location appears -most appropriate. They can be recognized because their names al- -ways end in "cmd". The mapping from names to procedures is -specified in the file builtins.def, which is processed by the -mkbuiltins command. - -A builtin command is invoked with argc and argv set up like a -normal program. A builtin command is allowed to overwrite its -arguments. Builtin routines can call nextopt to do option pars- -ing. This is kind of like getopt, but you don't pass argc and -argv to it. Builtin routines can also call error. This routine -normally terminates the shell (or returns to the main command -loop if the shell is interactive), but when called from a non- -special builtin command it causes the builtin command to -terminate with an exit status of 2. - -The directory bltins contains commands which can be compiled in- -dependently but can also be built into the shell for efficiency -reasons. The header file bltin.h takes care of most of the -differences between the ash and the stand-alone environment. -The user should call the main routine "main", and #define main to -be the name of the routine to use when the program is linked into -ash. This #define should appear before bltin.h is included; -bltin.h will #undef main if the program is to be compiled -stand-alone. A similar approach is used for a few utilities from -bin and usr.bin. - -CD.C: This file defines the cd and pwd builtins. - -SIGNALS: Trap.c implements the trap command. The routine set- -signal figures out what action should be taken when a signal is -received and invokes the signal system call to set the signal ac- -tion appropriately. When a signal that a user has set a trap for -is caught, the routine "onsig" sets a flag. The routine dotrap -is called at appropriate points to actually handle the signal. -When an interrupt is caught and no trap has been set for that -signal, the routine "onint" in error.c is called. - -OUTPUT: Ash uses its own output routines. There are three out- -put structures allocated. "Output" represents the standard out- -put, "errout" the standard error, and "memout" contains output -which is to be stored in memory. This last is used when a buil- -tin command appears in backquotes, to allow its output to be col- -lected without doing any I/O through the UNIX operating system. -The variables out1 and out2 normally point to output and errout, -respectively, but they are set to point to memout when appropri- -ate inside backquotes. - -INPUT: The basic input routine is pgetc, which reads from the -current input file. There is a stack of input files; the current -input file is the top file on this stack. The code allows the -input to come from a string rather than a file. (This is for the --c option and the "." and eval builtin commands.) The global -variable plinno is saved and restored when files are pushed and -popped from the stack. The parser routines store the number of -the current line in this variable. - -DEBUGGING: If DEBUG is defined in shell.h, then the shell will -write debugging information to the file $HOME/trace. Most of -this is done using the TRACE macro, which takes a set of printf -arguments inside two sets of parenthesis. Example: -"TRACE(("n=%d0, n))". The double parenthesis are necessary be- -cause the preprocessor can't handle functions with a variable -number of arguments. Defining DEBUG also causes the shell to -generate a core dump if it is sent a quit signal. The tracing -code is in show.c. |