Re: Document -x and -vx give the same results

[Greg Wooledge]

On Mon, Nov 15, 2021 at 12:40:22AM +0800, 積丹尼 Dan Jacobson wrote:
> Man page says:
>        -v        Print shell input lines as they are read.
>        -x        Print commands and their arguments as they are executed.
> Perhaps mention that -x and -vx give the same results, often or always.
> GNU bash, version 5.1.8

They're extremely different.

-v only prints lines when bash reads them from its script.  NOT when
bash executes them.  It prints the line exactly as it's read, without
any expansions.

-x prints commands that bash EXECUTES.  Arguments are expanded, and
redirections are not shown.

You almost never want -v.  It's pretty useless.

unicorn:~$ cat foo
#!/bin/bash
for i in {1..10}; do
  : stuff
done
unicorn:~$ bash -v foo
#!/bin/bash
for i in {1..10}; do
  : stuff
done
unicorn:~$ bash -x foo
+ for i in {1..10}
+ : stuff
+ for i in {1..10}
+ : stuff
+ for i in {1..10}
+ : stuff
+ for i in {1..10}
+ : stuff
+ for i in {1..10}
+ : stuff
+ for i in {1..10}
+ : stuff
+ for i in {1..10}
+ : stuff
+ for i in {1..10}
+ : stuff
+ for i in {1..10}
+ : stuff
+ for i in {1..10}
+ : stuff

In the "bash -v foo" output, you can see each line of the script as it
gets read.  Bash reads the entire for loop at once, then parses it, then
runs it.  The -v output shows you NOTHING after the loop has been read.

The -x output shows you each iteration of the loop.