string - 如何使用 Bash 测试文件中是否存在字符串？

Question

我有一个包含目录名称的文件:

my_list.txt:

/tmp
/var/tmp

如果该名称已存在于文件中，我想在添加目录名称之前检查 Bash。

Answer 1

grep -Fxq "$FILENAME" my_list.txt

如果找到该名称，则退出状态为 0(真)，否则为 1(假)，因此:

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

说明

这里是the man page for grep的相关部分:

grep [options] PATTERN [FILE...]

-F, --fixed-strings

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

-x, --line-regexp

        Select only those matches that exactly match the whole line.

-q, --quiet, --silent

        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

正如评论中正确指出的那样，上述方法默默地对待错误情况，就好像找到了字符串一样。如果您想以不同的方式处理错误，则必须省略 -q 选项，并根据退出状态检测错误:

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.

要抑制 grep 的正常输出，您可以将其重定向到 /dev/null。请注意，标准错误仍然是未定向的，因此 grep 可能打印的任何错误消息最终都会如您所愿地出现在控制台上。

为了处理这三种情况，我们可以使用case语句:

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