Bash 中的运算符“=”和“==”有什么区别？
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What is the difference between operator "=" and "==" in Bash?
提问by Debugger
It seems that these two operators are pretty much the same - is there a difference? When should I use =and when ==?
似乎这两个运营商几乎相同 - 有区别吗？我应该什么时候使用=，什么时候使用==？
回答by Paused until further notice.
You must use ==in numeric comparisons in (( ... )):
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 ")
You may use either for string comparisons in [[ ... ]]or [ ... ]or test:
您可以在[[ ... ]]or[ ... ]或 中用于字符串比较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
"String comparisons?", you say?
“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
回答by ghostdog74
There's a subtle difference with regards to POSIX. Excerpt from the Bash reference:
POSIX 有细微的差别。摘自Bash 参考：
string1 == string2
True if the strings are equal.=may be used in place of==for strict POSIX compliance.
string1 == string2
如果字符串相等，则为真。=可用于代替==严格的 POSIX 合规性。
