1.shell

Shell command line interpreter, Linux scripting language

1.1 variables

1. Common system variables: $HOME $PWD $SHELL $USER
2. Strict space rules

• There must be no spaces on either side of the equal sign
• There are spaces in the variable. You can wrap it with "" or ()
• There must be a space between expr operators
• There should be spaces on the left and right sides of conditional judgment []
• There should be spaces after if, while and for

1. The default types in bash are string types
2. export promotes the variable to a global variable
3. $0 script name, 1 − 9 ginseng number ， 1-9 parameters, 1 − 9 parameters, # get the number of parameters
4. ∗ Obtain take place have ginseng number ， can straight meet use to f o r Follow ring ， use " *Get all parameters, which can be directly used in the for loop“ * get all parameters, which can be directly used in the for loop. If "*", the will be spliced
5. @ Obtain take place have ginseng number ， turn become column surface ， use " @Get all parameters, turn them into a list, and use“ @Get all parameters, turn them into a list, and use "@" to have the same effect
6. $? Get the return value of the previous command

1.2 operators

1. Numerical operation method: $(()), $[], expr
2. Multiplication operator:

stay[]Internal*  stay expr Zhongwei\*

1.3 condition judgment

1. Value comparison judgment: - lt -le -eq
2. File permission judgment: - r -w -x
3. File type judgment: - f -e -d
4. String comparison judgment:=
5. Multi conditional judgment: & & and | or

1.4 process control

1. if select statement:

if [ xxx ];then
	xxx
elif [ xxx ];then
fi

1. case selection statement:

case $var in 
xxx)
	xxx
;;
xxx)
	xxx
;;
*)
	xxx
;;
easc

1. for loop statement:

for ((xxx;xxx;xxx))
do
	xxx
done

for item in x y z
do
	xxx
done

1. while loop statement:

while [ xxx ]
do
	xxx
done

1.5 console input

1. read xxx

   -p "prompt"

   -t wait time s

1.6 function

1. System function

• basename file name interception

  basename /home/aaa/example.txt   # out:expample.txt
  basename /home/aaa/example.txt .txt #out:example
  

• dirname file path interception

  dirname /home/aaa/example.txt # out:/home/aaa
  

1. Custom function

   function funname(){
   	xxx
   }
   # Since the shell script runs line by line, it must be declared before calling the function
   # The formal parameter of the function is specified by $n. The function is called in the following way: funname $x $y
   

1.7 tools

1. cut: crop data, default "\ t"

• -f: Column number, multiple columns are connected with "," and range with "-"

• -d: Separator

  ifconfig eth0 |grep inet|cut -d " " -f 10   # Cut out the local IP address of linux. The version is different and the display is different
  

1. sed: the stream editor processes one line at a time and does not change the content of the source file

• -Short for e edit, a short option before multiple commands

  • a add

     "na xxx" # Means to insert xxx under line n
    

  • d delete

    "/xxx/d" # Indicates the line where xxx is deleted
    

  • s replace

    "s/old/new/g" # Means to replace old with new
    

1. awk: text analysis tool, read in line by line, cut with separator, default separator ""

• -F: specify separator

• -v: define variables

  awk -F "x" '/pattern/ {action}' filename #Find the row that matches the pattern, split it with "x", and execute the action action
  
  BEGIN END # Matches the beginning and end of the line
  -v i=1 # When using variable i later, you don't need to use $i, just use i
  

  • Built in variable
    • FILENAME: file name
    • NR: line number
    • NF: number of columns after cutting each row

  ifconfig eth0|grep inet|awk -F " " '{print $2}' # # Like cut, cut out the local IP address of linux. The version is different and the display is different
  

1. Sort: sort files

• -n: sort number by value size
• -r: reverse sort
• -t: Specifies the separator used for sorting
• -k: Specifies the column to sort

sort -t 'x' -nk y filename # Split by x, sort by y column

1.8 question bank

1. Query the line number of the empty line in file.txt

   awk '/^$/ {print NR}' file.txt
   

2. Calculate the sum of the second column of file.txt and output [format of each row: name score]

   cat file.txt |awk -F " " '{sum+=$2} END{print sum}'
   

3. shell checks if the file exists

   if [ -f file.txt ];then
   	echo "exists"
   else
   	echo "not exsits"
   fi
   

4. Sort digital files

   sort -n file.txt
   

5. Find all the text in the / home folder. The text content contains the keyword "xxx"

   grep -r "xxx" /home |cut -d ":" -f 1