bash grep 后 echo 不起作用 returns 没有

Question

我知道人们通常在 Linux 命令提示符（而不是脚本）中使用 grep。我碰巧将 grep 放在脚本中，并将运行放入一个奇怪的案例中。如果 grep 命令 returns 什么都没有，则下一行 echo 不起作用。下面是脚本。

grep "abc" /home/testuser/myapp.log
echo "abc"

这是 grep 的正常行为吗？如果是，为什么？

Answer 1

您可以使用以下命令将 echo 与 grep:

printf "%s\n" "$(grep -o "abc" /home/testuser/myapp.log)"

参考资料

grep man page

Answer 2

您的脚本中似乎启用了 set -e。

通常，如果选择了一行，grep 的退出状态为 0，如果没有选择任何行，则为 1，如果发生错误，则为 2。但是请注意，如果使用 -q 或 --quiet 或 --silent 并选择了一行，即使发生错误，退出状态也是 0。在您的情况下，如果 abc 未在 /home/testuser/myapp.log 中找到，grep 将 return 1.

bash shell 通常会执行脚本中的下一行，即 echo "abc" 即使 grep return 的退出状态为 1 . 但是，如果 set -e 启用，bash 将不会在您的脚本中执行更多行。

来自 bash 联机帮助页：

-e      Exit immediately if a pipeline (which may consist of a single simple command), a list, or  a  compound  command
        (see  SHELL GRAMMAR above), exits with a non-zero status.  The shell does not exit if the command that fails is
        part of the command list immediately following a while or until keyword, part of the test following the  if  or
        elif  reserved words, part of any command executed in a && or || list except the command following the final &&
        or ||, any command in a pipeline but the last, or if the command's return value is being inverted with !.  If a
        compound  command  other  than a subshell returns a non-zero status because a command failed while -e was being
        ignored, the shell does not exit.  A trap on ERR, if set, is executed before the shell exits.  This option  ap‐
        plies  to  the  shell  environment  and each subshell environment separately (see COMMAND EXECUTION ENVIRONMENT
        above), and may cause subshells to exit before executing all the commands in the subshell.
   
        If a compound command or shell function executes in a context where -e is being ignored, none of  the  commands
        executed within the compound command or function body will be affected by the -e setting, even if -e is set and
        a command returns a failure status.  If a compound command or shell function sets -e while executing in a  con‐
        text  where -e is ignored, that setting will not have any effect until the compound command or the command con‐
        taining the function call completes.

Answer 3

“下一行回显不起作用”是什么意思？ echo 是不是根本没有达到？它是否运行但产生非零退出代码？或者你在输出中看到 abc 了吗？知道你的脚本 return 对 grep 没有任何影响，并且对脚本的其余部分一无所知，我可以说脚本不会在 stdout 上为 [=12= 输出任何内容] 命令和 stdout 上的 abc 用于 echo。

bash grep 后 echo 不起作用 returns 没有

bash echo does not work after grep returns nothing

linux

bash

grep

echo

参考资料