shell

tony2099 included in linux

2021-04-11 768 words 4 minutes

what’s shell

a program which interactive with linux ; list:
1. sh(Bourne shell)
2. bash
3. zsh
4. dash
5. …

/bin/sh a linker to a actual shell language

1

!# /bin/sh   // a program will run the script

1

echo $0 // the name of the shell language

shell basic

IFS: input field seperator, default value: space, tab newline

define variables

export vs not export

1
2


export a=1;  # can access in child process
a=1; # only access in current process

shell type

login vs nonlogin
1. login: first login into pc，run once,need password
  1. ssh root@…
  2. bash –login
2. non-login: no need password
  1. everytime open a new termial in linux system
  2. bash
interactive vs noninteractive
1. interactive: run from user’s input
2. noninteractive: run from a prcoess;

source file

what

what? global config file for shell script;
1. alias
2. global variables.;
profile vs ..rc;
1. profile:run once, global variables,
2. run every time
  1. alias: alias ls=‘ls -la’
  2. function: pyn init;

profile vs bashrc

1. bash

in b stage, will read b1, b2,b3 by order, only read the first one

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24


+----------------+-----------+-----------+------+
|                |Interactive|Interactive|Script|
|                |login      |non-login  |      |
+----------------+-----------+-----------+------+
|/etc/profile    |   A       |           |      |
+----------------+-----------+----------vi -+------+
|/etc/bashrc     |           |    A      |      |
+----------------+-----------+-----------+------+
|~/.bashrc       |           |    B      |      |
+----------------+-----------+-----------+------+
|~/.bash_profile |   B1      |           |      |
+----------------+-----------+-----------+------+
|~/.bash_login   |   B2      |           |      |
+----------------+-----------+-----------+------+
|~/.profile      |   B3      |           |      |
+----------------+-----------+-----------+------+
|BASH_ENV        |           |           |  A   |
+----------------+-----------+-----------+------+
|                |           |           |      |
+----------------+-----------+-----------+------+
|                |           |           |      |
+----------------+-----------+-----------+------+
|~/.bash_logout  |    C      |           |      |
+----------------+-----------+-----------+------

why sperate two kind of kind history reason
profile vs rc profile: login; load one time; 1. global enviroment: export PATH: 2. bashrc: load every time; alias and functions 1. alias rm=‘rm -i’

zsh

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28


+----------------+-----------+-----------+------+
|                |Interactive|Interactive|Script|
|                |login      |non-login  |      |
+----------------+-----------+-----------+------+
|/etc/zshenv     |    A      |    A      |  A   |
+----------------+-----------+-----------+------+
|~/.zshenv       |    B      |    B      |  B   |
+----------------+-----------+-----------+------+
|/etc/zprofile   |    C      |           |      |
+----------------+-----------+-----------+------+
|~/.zprofile     |    D      |           |      |
+----------------+-----------+-----------+------+
|/etc/zshrc      |    E      |    C      |      |
+----------------+-----------+-----------+------+
|~/.zshrc        |    F      |    D      |      |
+----------------+-----/------+-----------+------+
|/etc/zlogin     |    G      |           |      |
+----------------+-----------+-----------+------+
|~/.zlogin       |    H      |           |      |
+----------------+-----------+-----------+------+
|                |           |           |      |
+----------------+-----------+-----------+------+
|                |           |           |      |
+----------------+-----------+-----------+------+
|~/.zlogout      |    I      |           |      |
+----------------+-----------+-----------+------+
|/etc/zlogout    |    J      |           |      |
+----------------+-----------+-----------+------+

some variables

PPID: parent id
$$ currrent id

syntax

1. evaluating expressions

1. expansion

1. variable

have no dataType explictly;

1
2
3
4
5
6
7


# integer or string
a=100
a=tang 
# array
a=(1 2 3)

readonly a=100 # readonly 

declare: weak form of typing a variables

1
2
3


declare -i a; # integer
declare -a a; # array;
decare  -r a; #  readonly

unset: remove variable or function

2.function

get return value
1. $?: last return or exit code
define a function

1
2
3
4
5
6


function name() {
    echo $1 $2
    return 123
}
name tang jim
echo $?

using return or exit to stop the excution

3. control flow(if else)

1. if

1
2
3
4
5


if [ 1=1 ]; then
    echo true
else
    echo false
fi

Operator	Description
! EXPRESSION	The EXPRESSION is false.
-n STRING	The length of STRING is greater than zero.
-z STRING	The lengh of STRING is zero (ie it is empty).
STRING1 = STRING2	STRING1 is equal to STRING2
STRING1 != STRING2	STRING1 is not equal to STRING2
INTEGER1 -eq INTEGER2	INTEGER1 is numerically equal to INTEGER2
INTEGER1 -gt INTEGER2	INTEGER1 is numerically greater than INTEGER2
INTEGER1 -lt INTEGER2	INTEGER1 is numerically less than INTEGER2
-d FILE	FILE exists and is a directory.
-e FILE	FILE exists.
-r FILE	FILE exists and the read permission is granted.
-s FILE	FILE exists and it’s size is greater than zero (ie. it is not empty).
-w FILE	FILE exists and the write permission is granted.
-x FILE	FILE exists and the execute permission is granted.

2. for…in

1
2
3
4


a=(1 2 3)
for v in "${a[@]}"; do
    echo "$v"
done

create a custom command

lint

without double quote, the value will be splited by IFS, the glob will be expanded, and the result will join by space
1 2

a="a b*" echo $a #a b.sh

math arithmetic

expr
let or declare

1
2


    declare -i a; a+=100;
    let  a=0 a+=100;

(());
1

a=$((100+2))

Contents

shell

what’s shell

shell basic

define variables

shell type

source file

what

profile vs bashrc

1. bash

zsh

some variables

syntax

1. evaluating expressions

1. expansion

1. variable

2.function

3. control flow(if else)

1. if

2. for…in

create a custom command

lint

math arithmetic