Shell basic syntax -- process control

if else sentence

The basic structure
Be careful :

1. if and elif The statement should be followed by then To start executing the statement block
2. then You can wrap , Write on the same line with a semicolon , Suggest writing on the same line , The structure will be clearer
3. No, else if This kind of sentence , Can only write elif

if [  ]; then
   # if  Sentence block 
elif [  ]; then
   #  Sentence block 
else
   #  Sentence block 
fi

Conditional operator [] And [[]] The difference

1. [[]] Operators only have bash Only in the middle , other shell Such as ash, dash There is no such grammar in
2. [[]] Yes [] Features not available in , For example, compare whether two strings are equal , And there are spaces in the string , Use [] You must use double quotation marks

   if [ "$str" = "hello man" ]; then
   

   While using [[]] There is no need to worry about this problem

   if [[ $str = "hello man" ]]; then
   

3. Logical operations . Use [] Condition operator , &&, ||, <, > These symbols are used as part of the command , Not as a logical operator , Use [[]] This is not a problem with conditional operators , Such as :

   #  Below  >  The symbol will be used as a redirection command , &&  As part of the command ,  Will create 200  and  100 Two documents ,  And the statement will report an error : [: missing `]'
   if [ 300 > 200 && 200 > 100 ]; then
   
   #  The following statements can normally perform logical operations 
   if [[ 300 > 200 && 200 > 100 ]]; then
   

4. Fuzzy matching . Use [[]] You can use =~ Operators and regular syntax to achieve fuzzy matching, such as matching yes :

   #  Use  []  Only the following expressions can be used when writing 
   if [ $str = "y" -o $str = "yes" ];then
   
   #  Use  [[]]  You can write it as follows 
   if [[ $str =~ ^y(es)$ ]];then
   	echo ${
          BASH_REMATCH[1]}  #  The matching value exists in this system variable 
   	...
   
   #  You can also use the following wildcard writing 
   if [[ $str = y* ]];then
   

5. Experience , Generally, when making judgments, you will use [] Conditional operator , But when comparing strings , It is best to [[]] Will reduce the probability of error

Condition identifier

Judge the numbers

if [ $a -eq $b ]; then #  be equal to  equal
if [ $a -ne $b ]; then #  It's not equal to  not equal
if [ $a -gt $b ]; then #  Greater than  greater than
if [ $a -ge $b ]; then #  Greater than or equal to  greater equal
if [ $a -lt $b ]; then #  Less than  little than
if [ $a -le $b ]; then #  Less than or equal to  little equal

Judgment string

#  Use  []  Operator 
if [ -z "$str" ]; then           #  The judgment is empty ( String variables are quoted )
if [ -n "$str" ]; then           #  Judgment is not empty ( String variables are quoted )
if [ "$str" = "abc" ]; then      #  Judge equal ( String variables are quoted )
if [ "$str" == "abc" ]; then     #  Judge equal ( String variables are quoted )
if [ "$str" != "abc" ]; then     #  Judgment is not equal ( String variables are quoted )
if [ "$str1" \> "$str2" ]; then  #  The judgment is greater than ( String variables are quoted )
if [ "$str1" \< "$str2" ]; then  #  Judgment is less than 

#  Use  [[]]  Operator 
if [[ -z $str ]]; then           #  The judgment is empty 
if [[ -n $str ]]; then           #  Judgment is not empty 
if [[ $str = "ab c" ]]; then     #  Judge equal 
if [[ $str == "ab c" ]]; then    #  Judge equal 
if [[ $str != "ab c" ]]; then    #  Judgment is not equal 
if [[ $str1 > $str2 ]]; then     #  The judgment is greater than 
if [[ $str1 < $str2 ]]; then     #  Judgment is less than 

File to judge

if [ -e $file ]; then  #  Judge that the file exists 
if [ -L $file ]; then  #  Determine whether the file exists and is a linked file ( Be the first to judge ,  Otherwise, you will first judge other properties of the file pointed to by the soft link )
if [ -s $file ]; then  #  Determine whether the file exists and is a character device 
if [ -b $file ]; then  #  Determine whether the file exists and is a block device file 
if [ -d $file ]; then  #  Determine whether the file exists and is a folder / Catalog 
if [ -f $file ]; then  #  Judge whether the file exists and is a normal file 
if [ -S $file ]; then  #  Determine whether the file exists and is a socket file 
if [ -p $file ]; then  #  Determine whether the file exists and is a pipeline file 
#  Judgment of file permissions 
if [ -r $file ]; then #  Judge whether the file has readable permission 
if [ -w $file ]; then #  Judge whether the file has writable permission 
if [ -x $file ]; then #  Judge whether the file has executable permission  

Logical operations

#  Use  []  when 
if [ $a -o $b ];then 	#  or  or
if [ $a -a $b ];then 	#  And  and 
if [ ! $a ];then		#  Not 

if [ expression1 ] || [ expression2 ];then	#  or 
if [ expression1 ] && [ expression2 ];then	#  And 
if ! [ expression ]							#  Not 

#  Use  [[]] when 
if [[ $a && $b ]];then	#  or 
if [[ $a && $b ]];then  #  And 
if [[ ! $a ]];then		#  Not 

if [[ expression1 ]] || [[ expression2 ]];then	#  or 
if [[ expression1 ]] && [[ expression2 ]];then	#  And 
if ! [[ expression ]]							#  Not 

switch case choice

case ${var} in
    [1-9]) #  Range 
        # do something
        ;;
    [^a-zA-Z]) #  Not in this range 
        # do something
        ;;
    *)
        #do something
        ;;
esac

select in choice

select var in ch_a ch_b ch_c ch_d
do
    # Often combine case in  Statements use 
done

while loop

while test
do
    #  The loop body 
done

while true #  The way to write a dead circle 
do
done

for loop

for ((i=0; i<=100; i++))
do
    #  The loop body 
done


for var #  Without any parameters, all position parameters will be cycled 
do

done

for in loop

for var in a b c d e
do 
    #
done

for var in {
    1..100}
do
    #
done

break And continue

shell Medium break and continue And ordinary C Keywords with the same name in a series of programs have different behaviors
break n : sign out n Layer cycle
continue n : skip n Layer cycle , perform