Shell Structured Commands: Control Flow (part 1)

if - then - elsestatements

If-then-else statements are similar to if-else statements.

if command 
then
commands
else
commands
fi
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Different from if-else, if-else depends on true or false to determine whether the condition is true. If-then statements depend on whether shell command is successfully executed after if. That is, the exit status code of the command is 0.

If 0, then the command is executed. If not 0, the else command is executed. Fi indicates the end of the if-then-else statement.

If-then-else statements can also be nested like if-else statements, with the basic format:

if command1 
then
command set 1 
elif command2
then
command set 2 
elif command3 
then
command set 3 
elif command4 
then
command set 4
fi
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testThe command

What if if-then-else statements are required to determine conditions other than exit status codes, such as true and false? To resolve this, use the test command: test condition.

testCommand description:

Test evaluates the expression, and if it evaluates true, it returns the exit status code :0 (true); Otherwise, return the exit status code :1 (false); If there is no expression, the test command also returns the exit status code :1 (false).

Test with if-then-else:

if test command 
then
commands
else
commands
fi
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testA synonym of the command, square brackets:[]

Basic usage format:

if [ condition ] then
commands
fi
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Note: You must add a space after the first square bracket and before the second square bracket, otherwise an error will be reported.

Boolean logic support:

[condition1] && [condition2] # and computing [condition1] | | [condition2] # or operationCopy the code

testSupporting judgment:

1. The numerical comparison

#! /bin/bash
# control flow
var1=133
var2=145

if [ $var1 -eq $var2 ] 
then
  echo "$var1Is equal to the$var2"
elif [ $var1 -ge $var2 ]
then
  echo "$var1Greater than or equal to$var2"
elif [ $var1 -le $var2 ]
then
  echo "$var1Less than or equal to$var2" 133 is less than or equal to 145
elif [ $var1 -lt $var2 ]
then
  echo "$var1Less than$var2"
elif [ $var1 -gt $var2 ]
then
  echo "$var1Is greater than$var2"
elif [ $var1 -ne $var2 ]
then
  echo "$var1Is not equal to$var2"
else
  echo "Do not recognize"
fi

exit 0
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Note that using floating-point numbers in the test command returns an error: 145.5: Integer expression expected, the bash shell can only handle integers.

2. String comparison

#! /bin/bash
# string comparison
var1="abc"
var2="acb"
var3=""
var4="abc"

if [ -n $var1 ];then
 echo "The string var1 has a length greater than zero" Execute script output
fi

if [ $var2 ];then
 echo "The string var2 has a value"Execute script output
fi

if [ -z $var3 ];then
 echo "The string var3 has a length of 0"# execute script output, var3 will be executed even if it is not initialized
fi

if test $var4 = $var1 ; then
  echo "String var4=var1"Execute script output
  if [ $var1\ >$var2 ]; then
   echo "String var1 > var2"Execute script output
  elif [ $var1! =$var2 ]
  then
   echo "The string var1 does not equal var2."
  else 
   echo "The string var1 is less than var2"
  fi
else
  echo "String var4! =var1"
fi

exit 0
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Note: Greater-than and less-than signs must be escaped, otherwise the shell will treat them as redirection symbols and string values as file names; The greater than and less than order is different from that used by the sort command.

3. The file is

Example: Create a new directory, new file, write script data, change file permissions, and execute the script.

#! /bin/bash
# file comparison
var1="./compare"
var2="wonderful"
if [ -d $var1 ]; then
   echo "${var1}Directory exists,"
   if [ -f $var1/$var2 ] && [ -s $var1/$var2 ]
   then
     echo "${var2}File exists and is not empty. Switch workspace."
     cd. /${var1}
     if [ -x $var2 ]; then
       echo "The file exists and is executable with the following content:"
       cat -n $var2. /$var2
     else
       echo "File exists, not executable, write script"
       ( echo "#! /bin/bash" ) > $var2
       ( echo "echo $(date)perform$var1/${var2}Script successful 🔥") > >$var2
       echo -n "Modify permissions and execute results"
       chmod u+x $var2. /$var2
     fi
   else
   echo "${var2}File does not exist or file is empty"
   cd. /${var1}
      if [ -e $var2 ]; then
          echo "File exists"
      else
          touch $var2
      fi
   echo "Write 'script' to the file$var2 "
   ( echo "#! /bin/bash" ) > $var2
   ( echo "echo $(date)perform$var1/${var2}Script successful 🔥") > >$var2
   echo "Change$var2Is executable."
   chmod u+x $var2
   echo "Perform${var2}The script". /${var2}

   fi
else
   echo "${var1}Directory does not exist"
   echo "To create$var1Directory"
   mkdir $var1
   echo "Change directory"
   cd $var1
   echo "Current working directory:$PWD"
   echo "Create an empty file:$var2"
   touch $var2
   echo "Write 'script' to the file$var2 "
   ( echo "#! /bin/bash" ) > $var2
   ( echo "echo $(date)perform$var1/${var2}Script successful 🔥") > >$var2
   echo "Change$var2Is executable."
   chmod u+x $var2
   echo "Perform${var2}The script". /${var2}
fi
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if - then - elseAdvanced features of

• Double parentheses for mathematical expressions
• Double-sided parentheses for advanced string handling

Double brace

Double parentheses: Allows advanced mathematical expressions to be used during comparisons, whereas test only supports simple arithmetic comparisons: GT, GE… .

Note: You can use the double parenthesis command in an if statement or in a normal command in a script to assign values.

Basic format:

(( expression ))
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Advanced mathematical expressions supported by double parentheses:

#! /bin/bash
# control flow
var=5

if (( $var** 3 > 100));then
   echo "Double parenthesis assignment :"
   var1=$var
   (( var = $var ** 3 ))
   echo -n "The output$var1 ** 3的值为: $var" Output 5 ** 3:125
   echo ""
fi
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The two sides brackets

Double-sided parentheses: This command provides an advanced feature for string comparisons. Basic format:

[[ expression ]]
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Expression in parentheses can be compared using the standard string used in the test command, but it also uses a feature that the test command does not: pattern matching. Pattern matching uses ==, and the string to the right of the double equals sign is treated as a pattern. Example:

#! /bin/bash
# control flow
if [[ $USER == q* ]]
then
echo "Hello $USER"
else
   echo "Match failed"
fi

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caseThe command

The case command matches variables in the mode of pattern matching.

The format of the case command is as follows:

case variable in
pattern1 | pattern2) The vertical line operator separates multiple patterns on a single line
commands1;;
pattern3)
commands2;;# Note that double semicolons are required as part of the syntax
*) 
default commands;
esac # Closing sign

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Example:

#! /bin/bash
# control flow
case $USER in 
*li|*n*)
echo "hello $USER";;
li)
echo "hi li";;
*)
echo "by";;
esac
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References:

Linux command line and shell scripting