Bash中运算符"="和"=="之间有什么区别？

Question

似乎这两个运营商几乎相同 - 有区别吗？我何时应该使用=何时==？

Answer 1

您必须==在以下数字比较中使用(( ... )):

$ if (( 3 == 3 )); then echo "yes"; fi
yes
$ if (( 3 = 3 ));  then echo "yes"; fi
bash: ((: 3 = 3 : attempted assignment to non-variable (error token is "= 3 ")

您可使用在字符串比较[[ ... ]]或[ ... ]或test:

$ if [[ 3 == 3 ]]; then echo "yes"; fi
yes
$ if [[ 3 = 3 ]]; then echo "yes"; fi
yes
$ if [ 3 == 3 ]; then echo "yes"; fi
yes
$ if [ 3 = 3 ]; then echo "yes"; fi
yes
$ if test 3 == 3; then echo "yes"; fi
yes
$ if test 3 = 3; then echo "yes"; fi
yes

"字符串比较？",你说？

$ if [[ 10 < 2 ]]; then echo "yes"; fi    # string comparison
yes
$ if (( 10 < 2 )); then echo "yes"; else echo "no"; fi    # numeric comparison
no
$ if [[ 10 -lt 2 ]]; then echo "yes"; else echo "no"; fi  # numeric comparison
no

Answer 2

关于POSIX有一个细微的差别.摘自Bash参考:

string1 == string2
如果字符串相等则为True. =可用于代替==严格的POSIX合规性.