Control judgment statements in shell programming

If

• Single-format and nested

if　conditional expression; then #当条件为真时执行以下语句  command list else  #为假时执行以下语句    command List  fiif  Statements can also be nested using   an if  condition expression 1;then      if  condition expression 2;then          command list       Else          if  condition expression 3;then              command list           else               command list          fi      fi  else   command list  

• Multi-branch format

if "  $ "   ; then    LPR $1  "$"#elif with Else If    (CD $1; LPR $1)  else    " $ not a file or directory "   fi

• Case

 Case inch 1 )          1;;      #表示第一个分支结束 2)           22;;      #第二个分支结束*)      #表示其他可能值 must be at the end, otherwise the statement behind him cannot execute the echo Unkownesac    #Case statement ends

• While/until

 whileAn expression Docommand List done Sum=0I=0   while true#true是系统的关键词 Express the True DoI= ' Expr $i +1' Sum= ' Expr $Sum +$i 'if[$i =" -" ]; Then Break; Fi done echo $i $Sum Last this program shows - 5050  

Let's change the program a little bit later.

sum=0I=0   while[$i! =" -" ]   DoI= ' Expr $i +1' Sum= ' Expr $Sum +$i ' done echo $i $Sum The result of the program operation is the same as above, but the program is more concise than the above in this cycle can also be until as a test condition it is exactly the same as the condition of the while test, that is, when the condition is false will continue to execute the statement in the loop body, otherwise  Exit the loop body and use this example below. Sum=0I=0until [$i=" -" ]   DoI= ' Expr $i +1' Sum= ' Expr $Sum +$i ' done echo $i $Sum when I is not equal to 100 the loop is when the condition is a false loop, otherwise it exits, and the first example is when I is not equal to 100 cycles, that is, the test condition is true. 

For

• Enumeration usage

 for inch list  of names  Do     Command List   for in {1..  Ten }do  echo $ndonefor in  a b c d e;  Do   Echo $letterdone

• File usage

 for  in *does       echo $filedonefor in  A1 A2 A3 A4 A5  do     diff AA/$File bb/$File  

• Cumulative format

 for ((i=1; $i <i++ ))  Do  for ((i= 1; $i < 1 '))     Do   Echo $idone

• Other loop control statements

The break command does not perform the current loop within the body of a break, and the following statement exits from the current loop.
The Continue command is a program that ignores the following statement in this body and executes from the loop header.  

• Determine the properties of a file

Format:- operator filename  -e file exists return 1, otherwise return 0  -R file readable return 1, otherwise  return 0-w file can write return 1, otherwise return 0   -x file can be returned 1, otherwise 0  -o file belongs to user return 1, otherwise return 0  -z file length is 0 return 1, otherwise return 0.   -F File returns 1 for normal file, otherwise returns 0  -d file as directory file when 1 is returned, otherwise 0  

• Test string

String 1 = string 2   is true when two strings are equal = string 2 True when     two   strings are unequal-n string      True if the length of the string is greater than 0   -Z string   is a true  string     when the length of the string is 0

• Test two integer relationships

Number 1-eq number 22 equals True   -NE number 22 number is true   -GT number 2 digit 1 greater than the number 2 true   -GE Digital 2 
     number 1 is greater than or equal to  the number 2 is true -lt Digit 2  number 1  is less than the number 2 is true -le digit 2  digit 1 is less than or equal to the number 2 is true  

• Logic test

-A and   -o or   ! Non -

Control judgment statements in shell programming