系统调用,shell命令和程序

When I use a C compiler on the command line,when I type cc [filename],I suppose the shell uses a fork() system call to duplicate its process and then an exec() system call will load the cc compiler executable into the child process’ core image. Then the child process containing the cc executable will do its thing while the parent process executing the shell waits,or not. Is it right?

那就对了.父进程(shell)在子进程的PID上调用wait()并等待它退出.

What about shell commands like cp,ls,and others. What are they? are they executable programs that will also be executed in a new child process forked by the shell?

一样.这些是二进制文件,就像编译器一样,shell也为它们做同样的事情.

现在有一些命令不是外部二进制文件,称为“内置函数”.这些是shell识别自身的命令,不需要为外部二进制文件调用.为什么？

>有些语法具有特殊的语法,如if和while,因此必须在shell中构建.
>有些像cd和read一样,改变了shell进程的状态,因此必须是内置函数. (外部二进制文件不可能更改shell的当前目录,因为分叉进程只能更改自己的PWD,而不能更改它们的父进程.)
>其他的,比如echo和printf,可能是单独的二进制文件,恰好是由shell实现的.

这是我从输入帮助中获得的bash builtins的完整列表：

job_spec [&]                                                          history [-c] [-d offset] [n] or history -anrw [filename] or histor>
 (( expression ))                                                      if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [>
 . filename [arguments]                                                jobs [-lnprs] [jobspec ...] or jobs -x command [args]
 :                                                                     kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill>
 [ arg... ]                                                            let arg [arg ...]
 [[ expression ]]                                                      local [option] name[=value] ...
 alias [-p] [name[=value] ... ]                                        logout [n]
 bg [job_spec ...]                                                     mapfile [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callbac>
 bind [-lpvsPVS] [-m keymap] [-f filename] [-q name] [-u name] [-r k>  popd [-n] [+N | -N]
 break [n]                                                             printf [-v var] format [arguments]
 builtin [shell-builtin [arg ...]]                                     pushd [-n] [+N | -N | dir]
 caller [expr]                                                         pwd [-LP]
 case WORD in [PATTERN [| PATTERN]...) COMMANDS ;;]... esac            read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars>
 cd [-L|[-P [-e]]] [dir]                                               readarray [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callb>
 command [-pVv] command [arg ...]                                      readonly [-aAf] [name[=value] ...] or readonly -p
 compgen [-abcdefgjksuv] [-o option]  [-A action] [-G globpat] [-W w>  return [n]
 complete [-abcdefgjksuv] [-pr] [-DE] [-o option] [-A action] [-G gl>  select NAME [in WORDS ... ;] do COMMANDS; done
 compopt [-o|+o option] [-DE] [name ...]                               set [-abefhkmnptuvxBCHP] [-o option-name] [--] [arg ...]
 continue [n]                                                          shift [n]
 coproc [NAME] command [redirections]                                  shopt [-pqsu] [-o] [optname ...]
 declare [-aAfFgilrtux] [-p] [name[=value] ...]                        source filename [arguments]
 dirs [-clpv] [+N] [-N]                                                suspend [-f]
 disown [-h] [-ar] [jobspec ...]                                       test [expr]
 echo [-neE] [arg ...]                                                 time [-p] pipeline
 enable [-a] [-dnps] [-f filename] [name ...]                          times
 eval [arg ...]                                                        trap [-lp] [[arg] signal_spec ...]
 exec [-cl] [-a name] [command [arguments ...]] [redirection ...]      true
 exit [n]                                                              type [-afptP] name [name ...]
 export [-fn] [name[=value] ...] or export -p                          typeset [-aAfFgilrtux] [-p] name[=value] ...
 false                                                                 ulimit [-SHacdefilmnpqrstuvx] [limit]
 fc [-e ename] [-lnr] [first] [last] or fc -s [pat=rep] [command]      umask [-p] [-S] [mode]
 fg [job_spec]                                                         unalias [-a] name [name ...]
 for NAME [in WORDS ... ] ; do COMMANDS; done                          unset [-f] [-v] [name ...]
 for (( exp1; exp2; exp3 )); do COMMANDS; done                         until COMMANDS; do COMMANDS; done
 function name { COMMANDS ; } or name () { COMMANDS ; }                variables - Names and meanings of some shell variables
 getopts optstring name [arg]                                          wait [id]
 hash [-lr] [-p pathname] [-dt] [name ...]                             while COMMANDS; do COMMANDS; done
 help [-dms] [pattern ...]                                             { COMMANDS ; }

除了内置函数,还有函数和别名.这些是定义新功能的方法,无需创建单独的脚本/二进制文件.

uppercase() {
    tr '[:lower:]' '[:upper:]' <<< "$*"
}

alias ls='ls --color=auto -F'

函数和别名通常是为了方便或添加补充功能.

What about shell scripts? … Will the command line fork() a new process and exec() this script in the new process? And then will this new process containing the script fork() other processes to execute date,cc compiler,etc ??

是的,完全正确.运行shell脚本时,父shell会分叉子进程,并且脚本在那里运行.脚本中的命令因此分离出这个子进程;他们是原壳的孙子.