测试 Bash 函数返回值的正确方法是什么？

Question

我想在 if 语句中测试 Bash 函数的返回值，如下所示：

if [[ func arg ]] ; then …

但我收到如下错误消息：conditional binary operator expected.

这样做的正确方法是什么？

是不是下面这个？

 if [[ $(func arg) ]] ; then ...

Answer 1

如果是退出代码而不是结果，你可以使用

if func arg; then ...

如果您不能使函数返回正确的退出代码（使用return N），并且您必须使用字符串结果，请使用Alex Gitelman's answer。

$ help if:

if: if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else COMMANDS; ] fi

根据条件执行命令。

if COMMANDS 列表被执行。如果其退出状态为零，则 then COMMANDS 列表被执行。否则，每个elif COMMANDS 列表是 依次执行，如果退出状态为零，则对应的 then COMMANDS list 被执行并且 if 命令完成。否则， else COMMANDS 列表被执行，如果存在的话。退出状态 整个构造是执行的最后一个命令的退出状态，或者为零 如果没有条件测试为真。

退出状态： 返回最后执行的命令的状态。

Answer 2

如果您需要测试两个条件，一个是函数/命令的退出状态，另一个是例如变量的值，使用这个：

if func arg && [[ $foo -eq 1 ]]; then echo TRUE; else echo FALSE; fi

Answer 3

如果函数返回多个单词，似乎会产生此错误。

例如，1 2。

只是引用它：

"$(func arg)"

示例：

$ if [[ 1 2 ]] ; then echo 1 ; fi
-bash: conditional binary operator expected
-bash: syntax error near `2'
$ if [[ "1 2" ]] ; then echo 1 ; fi
1

---

如果你比较 0 和非 0，只需使用

if [[ "$(func arg)" != "0" ]]

Answer 4

在相关说明中，如果函数返回各种退出代码而不是真/假，那么：

func args; ec=$?      # call function and grab the exit code
                      # it is better to have them on the same line so that a future addition of a command
                      # before the case statement wouldn't break the logic
case $ec in
  value1) # commands
          ;;
  value2) # commands
          ;;
  *)      # commands
          ;;
esac

Answer 5

select 在这里提供了很多帮助。

PS3="What's your choice? (^D to stop choosing): "
select mainmenuinput in updatesystem installsamba installvsftpd installwebmin configuresambaforactivedirectory quitprogram; do
    case "$mainmenuinput" in

    "updatesystem")
        echo "Update System..."
    ;;

    "installsamba")
        echo "Installing Samba..."
    ;;

    #echo And so forth...
    esac
done

echo Done

有关select 的帮助，请咨询man bash 并搜索“选择”。不提供任何输入将重复菜单。

select name [ in word ] ; do list ; done
       The  list  of words following in is expanded, generating a list of items.  The set of expanded words is printed on the standard error, each preceded by a number.  If the in word is omitted, the
       positional parameters are printed (see PARAMETERS below).  The PS3 prompt is then displayed and a line read from the standard input.  If the line consists of a number corresponding  to  one  of
       the  displayed  words, then the value of name is set to that word.  If the line is empty, the words and prompt are displayed again.  If EOF is read, the command completes.  Any other value read
       causes name to be set to null.  The line read is saved in the variable REPLY.  The list is executed after each selection until a break command is executed.  The exit status  of  select  is  the
       exit status of the last command executed in list, or zero if no commands were executed.

样本输出：

[rinzler ~] $ ./test.sh
1) updatesystem                      4) installwebmin
2) installsamba                      5) configuresambaforactivedirectory
3) installvsftpd                     6) quitprogram
What's your choice? (^D to stop choosing): 1
Update System...
What's your choice? (^D to stop choosing): 2
Installing Samba...
What's your choice? (^D to stop choosing):
1) updatesystem                      4) installwebmin
2) installsamba                      5) configuresambaforactivedirectory
3) installvsftpd                     6) quitprogram
What's your choice? (^D to stop choosing):
Done