2- command line entry

2- command line entry

2019.9.14

Five command-line tool

• Binary executable file
• shell built-in commands
• Interpreted scripting
• shell functions
• Aliases

1. shell functions

$ fac() { (echo 1; seq $1) | paste -s -d\* | bc}

$ fac 5
120

• shell function is performed by the function own shell, with us is the function performed by Bash
• We define a FAC () function to generate a string of numbers using seq, paste function with these numbers into line with * and spaced apart, this equation is then passed to BC, which it evaluates and displays the result
• File ~/.bashrcis Bash configuration file, all of the shell functions can be made at this definition, such benefits are shell functions readily available

1. Aliases

$ alias l = 'ls -1 --group-directories-first'

• Alias ​​no parameters, so FAC () function can not be defined with such aliases.
• Aliases can reduce the number of keystrokes;
• Aliases are often defined in the '.bashrc' or '.bash_aliases' profile

• Run with no arguments aliasto see all the parameters of the alias. The default alias on ubuntu have the following general, have to say, I do not look or really do not know

  $alias
  alias egrep='egrep --color=auto'
  alias fgrep='fgrep --color=auto'
  alias grep='grep --color=auto'
  alias l='ls -CF'
  alias la='ls -A'
  alias ll='ls -alF'
  alias ls='ls --color=auto'

1. type -a usage

$ type -a cd
cd is a shell builtin

$type -a fac
fac is a function
fac () 
{ 
    ( echo 1;
    seq $1 ) | paste -s -d\* | bc
}

A combination of command-line tool - a combination of pipeline

• The output of tool a tool passed to the next, which can continue to pass virtually without restriction

$ seq 30 | grep 3
3
13
23
30
# 生成1-30的序列传给 grep，筛选出含有3的元素显示
# grep: 筛选元素

$ seq 30 | grep 3 | wc -l
4

# 在上一步的基础上统计行数
# wc: 计数功能；参数 -l 只输出行数量

Input and output redirection

• Pipeline in the last command-line tool is output to the terminal, we can also save it to a file;

# 我们在当下目录下新建文件 test
$ seq 1 > test
$ cat test
1
# 用序列1 覆盖test文件

$ seq 3 >> test
$ cat test
1
1
2
3
# 在test文件末尾加上序列3

$ echo -n "hello" > test
$ cat test
hello$...
# echo -n：就像上面的效果，hello 之后紧接着就是下一个 bash 语句

$ echo "hello" > test
$ cat test
hello
$ ...
# 没有参数 -n，hello后面相当于有个 \n；
# 同样的，> 和 >> 的区别在于覆盖还是在末尾添加

$ echo "hello, world" >> test
$ cat test
hello
hello, world

$ cat test | wc -w
3
# 使用管道组合，wc -w 这个参数表示只统计单词数量
# 这条语句也有很多的等价写法：
$ < test wc -w
3
$ wc -w test
3 test
# 这些等价写法的好处在于不进行额外的进程

With files

• mv
• cp

• mkdir

• All of the above commands can add parameters -v (verbose detail), so that the tool output operation in progress

• In addition to mkdir can add -i (interactive interaction), the tool allows you to confirm the request

For help

• man (manual)
• help
• tool -h / --help

$ man cat
# 输出不止一页，所以我们可以控制页面宽度

$ man cat | head -n 20
# 只输出前20行，也可以用fold；将较长的行变为80字符长度
$ man cat | fold

# 使用 help 可以查看 shell内置命令
$ help cat | head -n 20

# 第三方工具可以使用 -h 查看自带的帮助信息
$ java -h