What does 'set -x' do in Dockerfile?

Question

I am learning from existing Dockerfile's to start creating my own Dockerfile. I notice that most of the times, commands are started with either

RUN set -x ... 

or

RUN set -ex; ... 

Why does this do and why is this necessary? Is it necessary?

Answer 1

The -e causes the command to stop on any errors. A more typical syntax is to separate commands with && to stop on any error.

The -x causes the shell to output each command being run. This is useful for debugging scripts.

From the bash man page under set:

-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.