Linux shell review knowledge points - Chapter 2

1. Parameters (P17 44 48)

1. Positional parameters: use ${10} for more than 9
2. $*: more than 9 parameters
3. $@：
4. $$: process ID number
5. $?: Displays the last exit status of the command

• $* and $@:

#直接使用echo  两者无区别
#输入变量“dog cat” apple
for i in "$@"; do
	echo  $i	
done
for i in "$*"
do
	echo $i
done

#输出：
===============$@=================
dog cat
apple
================$*================
dog cat apple

• If there are more than 9 parameters:
  1. ${10}
  2. Use the shift command, such as:

  
  
  If shift is 3:
  

2. > < redirection

cat < a.sh > b.sh
or: cat a.sh> b.sh
standard output > or (1>)
standard error 2>
standard input 0 <
&> standard output and standard error

3. Special files

/dev/null : Empty the file and determine whether the id who exists

/dev/zero: create an empty temporary swap file

/dev/tty: similar to entering the password after sudo, not displayed on the terminal

4、grep

1. Obtain process information (ps -eLf): ps -eLf | grep firefox

2. Type
grep: BRE
↓ Extended
egrep: ERE
fgrep: interpret all characters literally, no special meaning

3. Parameters
-c display the total number of matching lines
-o only display the matching content
-A num If the match is successful, print the matching line and the next n lines together
-B num If the match is successful, print the matching line and the first n lines together
-C num If the match is successful, print the matching line and n lines before and after it
-v display unmatched lines
-w match word

5. Linux file type

normal file	-
Table of contents	d
character device file	c
block device file	b
socket file	s
symbolic link file	l

5. ls command