如何使用Bash测试文件中是否存在字符串? [英] How to test if string exists in file with Bash?

问题描述

我有一个包含目录名称的文件:

I have a file that contains directory names:

my_list.txt:

/tmp
/var/tmp

如果文件中已经存在目录名称，我想先登录Bash.

I'd like to check in Bash before I'll add a directory name if that name already exists in the file.

推荐答案

grep -Fxq "$FILENAME" my_list.txt

如果找到该名称，则退出状态为0(true)，否则为1(false)，因此:

The exit status is 0 (true) if the name was found, 1 (false) if not, so:

if grep -Fxq "$FILENAME" my_list.txt
then
    # code if found
else
    # code if not found
fi

说明

以下是 grep 的手册页的相关部分:

grep [options] PATTERN [FILE...]

-F，--fixed-strings

将PATTERN解释为固定字符串列表，用换行符分隔，其中任何一个都将被匹配.

        Interpret PATTERN as a list of fixed strings, separated by newlines, any of which is to be matched.

-x，--line-regexp

仅选择与整行完全匹配的匹配项.

        Select only those matches that exactly match the whole line.

-q，--quiet，--silent

安静;不要在标准输出中写任何东西.如果发现任何匹配项，则立即以零状态退出，即使检测到错误也是如此.另请参见-s或--no-messages选项.

        Quiet; do not write anything to standard output. Exit immediately with zero status if any match is found, even if an error was detected. Also see the -s or --no-messages option.

错误处理

正如注释中正确指出的那样，以上方法默默地将错误情况视为已找到字符串.如果要以其他方式处理错误，则必须省略-q选项，并根据退出状态检测错误:

Error handling

As rightfully pointed out in the comments, the above approach silently treats error cases as if the string was found. If you want to handle errors in a different way, you'll have to omit the -q option, and detect errors based on the exit status:

通常，如果找到选定的行，则退出状态为0，否则为1.但是，如果发生错误，则退出状态为2，除非使用-q或--quiet或--silent选项并且找到选定的行.但是请注意，对于grep，cmp和diff之类的程序，POSIX仅要求在出现错误的情况下退出状态大于1.因此，出于可移植性考虑，建议使用针对此一般条件进行测试的逻辑，而不是与2严格相等.

Normally, the exit status is 0 if selected lines are found and 1 otherwise. But the exit status is 2 if an error occurred, unless the -q or --quiet or --silent option is used and a selected line is found. Note, however, that POSIX only mandates, for programs such as grep, cmp, and diff, that the exit status in case of error be greater than 1; it is therefore advisable, for the sake of portability, to use logic that tests for this general condition instead of strict equality with 2.

要禁止来自grep的正常输出，可以将其重定向到/dev/null.请注意，标准错误仍然是无向的，因此grep可能会打印的任何错误消息都将最终出现在控制台上.

To suppress the normal output from grep, you can redirect it to /dev/null. Note that standard error remains undirected, so any error messages that grep might print will end up on the console as you'd probably want.

要处理这三种情况，我们可以使用case语句:

To handle the three cases, we can use a case statement:

case `grep -Fx "$FILENAME" "$LIST" >/dev/null; echo $?` in
  0)
    # code if found
    ;;
  1)
    # code if not found
    ;;
  *)
    # code if an error occurred
    ;;
esac

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