bash 檔中的 if else判斷式

[Reference]
sites.google.com/site/tiger2000/home 
第二十四章 Shell Script 

Testing 判斷式

Testing 判斷式通常與if-then-else一起互用, 以便發揮其效果. 先從一個最簡單的例子看起 :

$ cat myscript7
#!/bin/sh
if [ -f /etc/passwd ]; then
   echo "/etc/passwd is a file"
else
   echo "PANIC : /etc/passwd is not a file!!"
fi

先說明一下 [ -f /etc/passwd ] 這個最重要的部份, [ ] 裡面就是判斷式, 而這裡是判斷 /etc/passwd 是不是一個 file ?
由結果來看看是執行 then 下面的敘述, 或者執行 else 的敘述.

同時很重要一點是 底下這些敘述都是不正確的敘述, 會有錯誤產生喔 :
[-f /etc/passwd ]  [-f 連在一起 
[ -f /etc/passwd]  passwd] 連在一起
[ -f/etc/passwd ]  -f與/etc/passwd連在一起

這裡是一個雙重條件判斷的範例:

CONFIG=config.ini
if [ -e $CONFIG -a "`grep ^Mailer $CONFIG`" ]; then
   SERVER_LIST=(`grep ^Mailer $CONFIG | cut -d= -f2 | sed 's/,/ /g'`)
   SERVER_NUM=${#SERVER_LIST[@]}
else
   echo "$CONFIG not found"
   exit 2
fi

上述的範例有些複雜, 簡單的說它透過 -a (代表 and) 判斷是否同時滿足  -e $CONFIG 與 `grep ^Mailer $CONFIG?
所以另外一個 -o (寄是代表 or) 就會類似這樣了:

if [ -e $CONFIG -o "`grep ^Mailer $CONFIG`" ]; then
   XXXXX (自己發揮)
fi


所以利用 [ ] 我們可以做出以下這些判斷, 這些也都很常用喔 !!

test     true if ....



[ string1 = string2 ]                   string1 and string2 are equal
[ string1 != string2 ]                  string1 and string2 are not equal
[ string1 \< string2 ]   string1 is lexically less than string2
                                        (e.g. 'a' is less than 'b')
[ string1 \> string2 ]   string1 is lexically greater than string2
                                        (e.g. 'b' is greater than 'a')
[ -z string ]    string is zero (e.g. a empty string)
[ -n string ]    string is nonzero (e.g. a VAR string)
[ -e file ]    file exists
[ -f file ]    file is a file
[ -d file ]    file is a directory
[ -c file ]    file is a character device
[ -b file ]    file is a block device
[ -p file ]    file is a named pipe
[ -s file ]    file is not empty
[ -k file ]    file's sticky bit is set
[ -S file ]    file is a socket
[ -L file ]    file is a symbolic link
[ -r file ]    file is readable by user
[ -w file ]    file is writeable by user
[ -x file ]    file is executeable by user
[ -O file ]    file is owner by user
[ -G file ]    file is group owned by a greoup 
[ -u file ]    file has its set user ID bit set
[ -g file ]    file has its group user ID bit set
[ file1 -nt file2 ]   file1 is newer than file2
[ file1 -ot file2 ]   file1 is older than file2
[ file -ef file2 ]   file1 is another name for file2
[ n1 -eq n2 ]    true if integer n1 = integer n2
[ n1 -ne n2 ]    true if integer n1 <> n2     
[ n1 -gt n2 ]    true if n1 > n2                 
[ n1 -ge n2 ]    true if n1 >= n2
[ n1 -lt n2 ]    true if n1 < n2
[ n1 -le n2 ]    true if n1 <= n2