What does `set -x` do?

set -x enables a mode of the shell where all executed commands are printed to the terminal. In your case it's clearly used for debugging, which is a typical use case for set -x: printing every command as it is executed may help you to visualize the control flow of the script if it is not functioning as expected.

set +x disables it.

###set -x

Prints a trace of simple commands, for commands, case commands, selectcommands, and arithmetic for commands and their arguments orassociated word lists after they are expanded and before they areexecuted. The value of the PS4 variable is expanded and the resultantvalue is printed before the command and its expanded arguments.

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###Example

set -xecho `expr 10 + 20 `+ expr 10 + 20+ echo 3030

set +xecho `expr 10 + 20 `30

Above example illustrates the usage of set -x. When it is used, above arithmetic expression has been expanded. We could see how a single line has been evaluated step by step.

• First step expr has been evaluated.
• Second step echo has been evaluated.

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when it comes to your shell script,

[ "$DEBUG" == 'true' ] && set -x

Your script might have been printing some additional lines of information when the execution mode selected as DEBUG. Traditionally people used to enable debug mode when a script called with optional argument such as -d

-u: disabled by default. When activated, an error message is displayed when using an unconfigured variable.

-v: inactive by default. After activation, the original content of the information will be displayed (without variable resolution) before theinformation is output.

-x: inactive by default. If activated, the command content will be displayed before the command is run (after variable resolution, thereis a ++ symbol).

Compare the following differences:

/ # set -v && echo $HOME/root/ # set +v && echo $HOMEset +v && echo $HOME/root/ # set -x && echo $HOME+ echo /root/root/ # set +x && echo $HOME+ set +x/root/ # set -u && echo $NOSET/bin/sh: NOSET: parameter not set/ # set +u && echo $NOSET