连载中

常用命令

ls -al #查看目录 
pwd #返回当前目录
cal #查看日历
ps -elf #查看进程
more #查看更多 可以组合 ps -elf|more

bash # start a subshell
exit # exit the subshell

组合命令

(ls; cal 2022) | wc

echo

作用: 打印

echo -n "You must be "
# -n keeps output on the same line

Variable

注意事项

• 等号两边不能有空格
• local variables 小写
• global variables / environment variables 通常大写

保留符

$$ # evaluates to the PID of current shell (not the current command/process)

Usage

hello=world # 不建议，应当加上引号
name="Mickey Mouse"

echo $name # >>> name

PERSON=Mary # you may also use export again
export $PERSON
echo $PERSON

name1=John name2=”Mary Anne” 
$ a=1 b=3 c=5 d=10
# var1=str1 var2=str2 ... varN=strN
echo $name1 and $name2 # >>> John and Mary Anne

Common Built-In Variables

• HOME: Store the full pathname of the home directory: where you will go to (home) when you just type cd.
• PATH: Store a list of directories to be searched when running programs or shell scripts, separated by “:”.
• PS1: Store primary prompt string, with a default value of '\s-\v\$ ', meaning system-version then $.
• PS2: Store secondary prompt string, with a default value of '> '. It is used for continuation of input lines.
• PWD: Store current working directory set by cd command.
• HOSTNAME
• UID
• PPID: Store process id of parent.
• HISTSIZE

Special Option Variables

• history: Setting it will enable command history to be stored, useful for future, default to on.

• noclobber: Setting it will prevent overwriting of files when using I/O redirection, default to off.

• ignoreeof: Setting it will prevent accidental logging out with <Ctrl-D> (end of input), often used when entering data from keyboard, default to off.

• allexport: Setting it will automatically export all modified variables, default to off.

To turn on/off, use set –o/+o variable

• set –o noclobber # turn on noclobber feature
• set +o history # turn off history storage

$(str)

将返回bash 执行 str 的结果

$ p2=”($PWD) $(whoami)- ”
$ echo $p2
# >>> (/home/12345678d) 12345678d-

Array

declare

var=(val1 val2 ... valN) # initialize an array of size N
var[i]=val # assign val to element i in array var

access

${var[i]} # access element i in array var
${#var[i]} # return length of element i in array var
${#var[*]} # return number of non-null elements in array var

Quotation

Single quote (’str’)

• The strong quote.
• Enclosed string looks like literal.
• No substitution and no execution is done.

相当于忽略所有转义

Double quote (”str”)

• The weak quote.
• Enclosed string is almost like literal.
• Substitution is done for variable contents (prefixed with "$").
• Execution is done for back-quoted commands.
• Watch out for "!!", which is trying to match a past command.
• This is a bug in showing the current command.

Backquote

(`str`)

A special backquote enables a command to be executed

In bash, it is more common to put it as $(str) instead.

Shell Scripts

• We use bash, so the first line is "#!/bin/bash".
• Warning: never create a shell script called test.
• chmod 700 file or chmod u+x file

Guidelines

• A shell script does not do unnecessary work.
• A shell script should perform the task for which it is intended.
• A shell script should run without errors.
• Shell script program logic is clearly defined and apparent.
• Shell scripts should be reusable.

Read input

echo –n ’Please enter your name: ’
read name

read –p ’Please enter your name: ’ name
echo name is $name

Command Line Argument

• The script name is assigned to $0 (inclusive of the ./ part of the path).
• All words following the script name are assigned to $1, $2, $3 … ${10}, ${11}, and so on.