bash shell test条件测试[[ ]]和[ ]异同小结

来源：互联网发布：linux怎么看内存大小编辑：程序博客网时间：2024/05/19 13:17

test 命令非常强大，但是很难满足其转义需求以及字符串和算术比较之间的区别。

大于小于等于不等于if [ "$a" -gt "$b" ]if [ "$a" -lt "$b" ]if [ "$a" -eq "$b" ]if [ "$a" -ne "$b" ]if [[ "$a" -gt "$b" ]]if [[ "$a" -lt "$b" ]]if [[ "$a" -eq "$b" ]]if [[ "$a" -ne "$b" ]]if ((“$a” > “$b”))if ((“$a” < “$b”))if ((“$a” == “$b”))if ((“$a” != “$b”))

字符串比较

大于小于等于不等于if [ "$a" \> "$b" ]if [ "$a" \< "$b" ]if [ "$a" = "$b" ]
if [ "$a" == "$b" ]if [ "$a" != "$b" ]if [[ "$a" > "$b" ]]if [[ "$a" < "$b" ]]if [[ "$a" = "$b" ]]
if [[ "$a" == "$b" ]]if [[ "$a" != "$b" ]]

[[ ]] and []
功能[[ ]][ ]例子字符串比较>\> -=(or ==)= -!=!= -表达式组合&&-a[[ -n $var && -f $var ]] && echo “$var is a file”||-o -

模式匹配
(Pattern matching)

=(or ==)木有[[ $name = "a*" ]] -> the string “a*”正则匹配(RegularExpression matching)=~木有[[ $(date) =~ ^Fri\ ...\ 13 ]] && echo “It’s Friday the 13th!”

特性

例子

文件或文件夹存在

-e

[[ -e $config ]] && echo ”config file exists: $config”

文件新旧比较

-nt/-ot

[[ $file0 -nt $file1 ]] && echo ”$file0 is newer than $file1″

同一个文件

-ef

[[ $input -ef $output ]] \ && { echo ”will not overwrite input file: $input”; exit 1; }

否定

逻辑组合判断的一些例子：
if [ $condition1 ] && [ $condition2 ]
if [ $condition1 -a $condition2 ]
if [[ $condition1 && $condition2 ]]
if [ $condition1 ] || [ $condition2 ]
if [ $condition1 -o $condition2 ]
if [[ $condition1 || $condition2 ]] # Also works.
#The &&, ||, operators work within a [[ ]] test, despite giving an error within a [ ] construct.