兩隻小虎,一隻豬

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