S shell learning note v1.0

1. what is shell
2. hello world program
3. variables in shell
- 3.1. system variables
- 3.2. user defined variables
  - 3.2.1. define and use
  - 3.2.2. local vs global variable
4. control structure
- 4.1. condition judgement
5. fucntion
6. debug
- 6.1. echo
- 6.2. sh -v ./shell or sh -x ./shell

1 what is shell

shell is the middle man of kernel and user.

to show shell installed in the system. different shell may has differnet grammar
```
cat /etc/shells
```
show current shell used.
```
echo $SHELL
```

2 hello world program

2.1 src

#!/bin/sh
echo "hello shell"

2.2 add run privilege

$ sudo chmod +x ./helloshell.sh

2.3 run shell

./helloshell.sh

3 variables in shell

variables in shell are system variables and user defined variables.

3.1 system variables

use

$ set

to show the system variables.

3.2 user defined variables

3.2.1 define and use

variables in shell have no data type. shell will automatically transfer the type. e.g.

$ msg="hello world"

Attention: there is no blank spaces before and after '='

3.2.2 local vs global variable

global shell variable stored in ~/.bashrc, otherwise, they are all local shell variables.

4 control structure

4.1 condition judgement

use test or [] to judge a condition in shell.

4.1.1 math operation judgement

seq	example	explain
1	a -eq b	a == b
2	a -ne b	a != b
3	a -lt b	a < b
4	a -le b	a <= b
5	a -gt b	a > b
6	a -ge b	a >= b

4.1.2 string comparation

seq	example	explain
1	str1 = str2	str1 == str2
2	str1 != str2	str1 != str2
3	str1	str1 not null or not defined.

4.1.3 logic judgement

seq	example	explain
1	!	not
2	exp1 -a exp2	exp1 && exp2
3	exp1 -o exp2	exp1 or exp2

4.1.4 if-else

#!/bin/sh

if [ $# -ne 1 ]; then
    echo "$0 : you must give one integer"
    exit 1
fi

if [ $1 -gt 0 ]; then
    echo "$1 is positive"
else
    echo "$1 is negative"
fi

4.1.5 for

#!/bin/sh

for i in 1 2 3 4 5
do
    echo "welcome $i"
done

4.1.6 while

#!/bin/sh

# test while loop

if [ $# -ne 1 ]; then
    echo "usage: $0 integer"
    exit 1
fi

n=$1
i=0
while [ $i -le 10 ]
do
    echo "$n * $i = `expr $i * $n`"
    i=`expr $i + 1`
done

4.1.7 case

#!/bin/sh

# test case statement

if [ $# -ne 1 ]; then
    echo "usage: $0 [a|b|c]"
    exit 1
fi

case $1 in
    "a" )
        echo "a select"
        ;;
    "b" )
        echo "b select"
        ;;
    "c" )
        echo "c select"
        ;;
    * )
        echo "no action"
        ;;
esac

5 fucntion

#!/bin/sh

demo()
{
    echo "all function args: $*"
    echo "the first arg : $1"
    echo "the second arg : $2"
    echo "the third arg : $3"
}

# call the function
demo -f foo bar

exit 0

6 debug

6.1 echo

use echo to info you.

6.2 sh -v ./shell or sh -x ./shell

$ sh -v ./shell print the content shell get

$ sh -x ./shell show the command then print it.

Shell 学习笔记 (org-mode制作)