The linux command line 31. Flow control: branching with case

31. Flow control: branching with case

28장에서 다뤘던 flow control을 이어서 다룸.

shell에서는 단순히 if 명령어 이외에 case 명령어도 제공해줌.

The case command

case word in
        [pattern [| pattern]...) commands ;;]...
esac

28장에서 작성했던 명령어는 다음과 같음.

#!/bin/bash

# read-menu: a menu driven system information program

clear
echo "
Please Select:

1. Display System Information
2. Display Disk Space
3. Display Home Space Utilization
0. Quit
"

read -p "Enter selection [0-3] > "

if [[ "$REPLY" =~ ^[0-3]$ ]]; then
      if [[ "$REPLY" == 0 ]]; then
            echo "Program terminated."
            exit
      fi
      if [[ "$REPLY" == 1 ]]; then
            echo "Hostname: $HOSTNAME"
            uptime
            exit
      fi
      if [[ "$REPLY" == 2 ]]; then
            df -h
            exit
      fi
      if [[ "$REPLY" == 3 ]]; then
            if [[ "$(id -u)" -eq 0 ]]; then
                  echo "Home Space Utilization (All Users)"
                  du -sh /home/*
            else
                  echo "Home Space Utilization ($USER)"
                  du -sh "$HOME"
            fi
            exit
      fi
else
      echo "Invalid entry." >&2
      exit 1
fi

if/else문 대신에 case를 사용하면 다음과 같음.

#!/bin/bash

# case-menu: a menu driven system information program

clear
echo "
Please Select:

1. Display System Information
2. Display Disk Space
3. Display Home Space Utilization
0. Quit
"

read -p "Enter selection [0-3] > "

case "$REPLY" in
    0)      echo "Program terminated."
            exit
            ;;
    1)      echo "Hostname: $HOSTNAME"
            uptime
            ;;
    2)      df -h
            ;;
    3)      if [[ "$(id -u)" -eq 0 ]]; then
                echo "Home Space Utilization (All Users)"
                du -sh /home/*
            else
                echo "Home Space Utilization ($USER)"
                du -sh "$HOME"
            fi
            ;;
    *)      echo "Invalid entry" >&2
            exit 1
            ;;
esac

patterns

case에서 사용하는 패턴들은 다음과 같음.

| a)            | matches if word equals a.                         |
| [[:alpha::]]) | matches if word is a single alphabetic character. |
| ???)          | matches if word is exactly three characters long. |
| *.txt)        | matches if word ends with the char .txt.          |
| *)            | matchs any value of word.                         |

위 패턴의 예시.

#!/bin/bash

read -p "enter word > "

case "$REPLY" in
    [[:alpha:]])   echo "is a single alphabetic character." ;;
    [ABC][0-9])    echo "is A, B, or C followed by a digit." ;;
    ???)           echo "is three characters long." ;;
    *.txt)         echo "is a word ending in '.txt'" ;;
    *)             echo "is something else." ;;
esac

vertical bar operator로 여러 패턴을 섞어서 사용하는것도 가능하다.

#!/bin/bash

# case-menu: a menu driven system information program

clear
echo "
Please Select:

A. Display System Information
B. Display Disk Space
C. Display Home Space Utilization
Q. Quit
"

read -p "Enter selection [A, B, C or Q] > "

case "$REPLY" in
    q|Q)    echo "Program terminated."
            exit
            ;;
    a|A)    echo "Hostname: $HOSTNAME"
            uptime
            ;;
    b|B)    df -h
            ;;
    c|C)    if [[ "$(id -u)" -eq 0 ]]; then
                echo "Home Space Utilization (All Users)"
                du -sh /home/*
            else
                echo "Home Space Utilization ($USER)"
                du -sh "$HOME"
            fi
            ;;
    *)      echo "Invalid entry" >&2
            exit 1
            ;;
esac

performing multiple actions

base 4.0 이전에서는 한번 성공한 match 이후에 매칭되지 않는다.

#!/bin/bash

# case4-1: test a character

read -n 1 -p "Type a character > "
echo
case "$REPLY" in
    [[:upper:]])    echo "'$REPLY' is upper case." ;;
    [[:lower:]])    echo "'$REPLY' is lower case." ;;
    [[:alpha:]])    echo "'$REPLY' is alphabetic." ;;
    [[:digit:]])    echo "'$REPLY' is a digit." ;;
    [[:graph:]])    echo "'$REPLY' is a visible character." ;;
    [[:punct:]])    echo "'$REPLY' is a punctuation symbol." ;;
    [[:space:]])    echo "'$REPLY' is a whitespace character." ;;
    [[:xdigit:]])   echo "'$REPLY' is a hexadecimal digit." ;;
esac

위 스크립트를 실행시키면 하나만 결과가 나온다.

[me@linuxbox ~]$ case4-1
Type a character > a
'a' is lower case.

최근의 bash 버전에서는 ;;& operator를 사용해 이후 매칭까지 실행시킬 수 있다.

#!/bin/bash

# case4-2: test a character

read -n 1 -p "Type a character > "
echo
case "$REPLY" in
    [[:upper:]])    echo "'$REPLY' is upper case." ;;&
    [[:lower:]])    echo "'$REPLY' is lower case." ;;&
    [[:alpha:]])    echo "'$REPLY' is alphabetic." ;;&
    [[:digit:]])    echo "'$REPLY' is a digit." ;;&
    [[:graph:]])    echo "'$REPLY' is a visible character." ;;&
    [[:punct:]])    echo "'$REPLY' is a punctuation symbol." ;;&
    [[:space:]])    echo "'$REPLY' is a whitespace character." ;;&
    [[:xdigit:]])   echo "'$REPLY' is a hexadecimal digit." ;;&
esac

bash 4.0 이후의 쉘에서는 이후까지 매칭되는 것을 볼 수 있다.

[me@linuxbox ~]$ case4-2
Type a character > a
'a' is lower case.
'a' is alphabetic.
'a' is a visible character.
'a' is a hexadecimal digit.