1. Execution of orders
Enter the command and return
The shell program is asked to find the executable program or code corresponding to the typing command and submit it to the kernel to allocate resources to run it in stages.
There are two types of commands that can be executed in the shell:
Internal commands: Internal integration commands that come with the shell
help can view the list of internal commands
[root@centos7 ~]# help
GNU bash, version 4.2.46(2)-release (x86_64-redhat-linux-gnu)
These shell commands are defined internally. Type `help' to see this list.
Type `help name' to find out more about the function `name'.
Use `info bash' to find out more about the shell in general.
Use `man -k' or `info' to find out more about commands not in this list.
A star (*) next to a name means that the command is disabled.
job_spec [&] history [-c] [-d offset] [n] or history -anrw [filename] or >
(( 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 ... o>
[ 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 c>
bind [-lpvsPVS] [-m keymap] [-f filename] [-q name] [-u name]> 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 >
cd [-L|[-P [-e]]] [dir] readarray [-n count] [-O origin] [-s count] [-t] [-u fd] [-C>
command [-pVv] command [arg ...] readonly [-aAf] [name[=value] ...] or readonly -p
compgen [-abcdefgjksuv] [-o option] [-A action] [-G globpat]> return [n]
complete [-abcdefgjksuv] [-pr] [-DE] [-o option] [-A action] > 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] [comma> 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 ; }
enable can also view a list of commands
[root@centos7 ~]# enable enable . enable : enable [ enable alias enable bg enable bind enable break enable builtin enable caller enable cd enable command enable compgen enable complete enable compopt enable continue enable declare enable dirs enable disown enable echo enable enable enable eval enable exec enable exit enable export enable false enable fc enable fg enable getopts enable hash enable help enable history enable jobs enable kill enable let enable local enable logout enable mapfile enable popd enable printf enable pushd enable pwd enable read enable readarray enable readonly enable return enable set enable shift
External commands: There are corresponding executable files in the file system path
View Path Command
I. wheris
[root@centos7 ~]# whereis ls #More Help Documents, More Details ls: /usr/bin/ls /usr/share/man/man1/ls.1.gz /usr/share/man/man1p/ls.1p.gz
Which-a
[root@centos7 ~]# which -a ls
alias ls='ls --color=auto'
/usr/bin/lsIf you decide whether the command is internal or external
Command:type
Example:[root@centos7 ~]# type pwd pwd is a shell builtin #Internal orders [root@centos7 ~]# type hostname hostname is /usr/bin/hostname #External commands are represented as a file
It is also possible that a command is both an external and an internal command, which can be viewed in this way.
[root@centos7 ~]# type -a pwd pwd is a shell builtin pwd is /usr/bin/pwd
So the question arises, is the input command internal or external?
The process of command execution is as follows:
See if internal commands are faster to execute and higher priority to internal commands. Then check the hash table (record the path of external commands). If it is found, it will be executed later. If you can't find the path of this command under disk, you'll find it in the following way.
External command path finding process (first execution)
[root@centos7 bin]# echo $PATH /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin
When we execute another command, when we confirm that it is not an internal command, we will look for it as an external command in the above directory file and execute it when we find it, such as this:
[root@centos7 bin]# type hostname hostname is /usr/bin/hostname #Find it under / usr/bin
If you can't find it, you will make an error.
[root@centos7 bin]# lsls bash: lsls: command not found... #Error in command input
Once the command is found, the path of the command is recorded in the hash table. When it is used again, it can be executed quickly and the command invocation rate can be increased.
Example:
root@centos7 bin]# hash hits command 2 /usr/bin/whereis 2 /usr/bin/man 3 /usr/bin/ls
Look at the command again:
[root@centos7 bin]# type whereis whereis is hashed (/usr/bin/whereis) [root@centos7 bin]# type ifconfig ifconfig is hashed (/usr/sbin/ifconfig)
Clear hash cache
[root@centos7 etc]# hash hits command 6 /usr/sbin/ifconfig 1 /usr/bin/rm 1 /usr/bin/cat 9 /usr/bin/whereis 3 /usr/bin/cp 2 /usr/bin/man 4 /usr/bin/ls 1 /usr/bin/clear [root@centos7 etc]# hash -r [root@centos7 etc]# hash hash: hash table empty