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在shell script 中if 條件式的基本語法如下


if [  CONDITION ] 
then
    ......
else 
    ......
fi


if test CONDITION
the
   .....
else 
   .....
fi

另外在if 條件式中可以利用下面的判斷

[ 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

常用的系統符號變數

$# 參數的數目
$* 代表所有參數
$? Exit status of previous command
$$ PID of this shell's process
$! PID of the most recently started backgroup job
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