-x使shell输出正在运行的每个命令.这对于调试脚本很有用.

从set下的bash手册页：

-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 com‐
mand 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 applies to the shell environment and each subshell envi‐
ronment 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 context where -e is ignored,that setting will
not have any effect until the compound command or the command containing the function call
completes.

…

-x After expanding each simple command,for command,case
command,select command,or arithmetic
for command,display the expanded value of PS4,followed by the command and its expanded
arguments or associated word list.