[PATCH] ash: support BASH_XTRACEFD when built Bash compatibly

[Denys Vlasenko]

Applied in this form:

        preverrout_fd = 2;
+       if (BASH_XTRACEFD && xflag) {
+               /* NB: bash closes fd == $BASH_XTRACEFD when it is changed.
+                * we do not emulate this. We only use its value.
+                */
+               const char *xtracefd = lookupvar("BASH_XTRACEFD");
+               if (xtracefd && is_number(xtracefd))
+                       preverrout_fd = atoi(xtracefd);
+
+       }
        status = redirectsafe(cmd->ncmd.redirect, REDIR_PUSH | REDIR_SAVEFD2);

On Fri, Aug 4, 2017 at 12:58 PM, Johannes Schindelin
<johannes.schindelin at gmx.de> wrote:
> There is a very useful feature in Bash where you can redirect the trace
> enabled by `set -x` to a file descriptor *different* than 2. This comes
> in particularly handy when validating the error output of commands, say,
> in Git's test suite, while tracing at the same time.
>
> It is such a useful feature, and very easily implemented.
>
> Signed-off-by: Johannes Schindelin <johannes.schindelin at gmx.de>
> ---
>  shell/ash.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/shell/ash.c b/shell/ash.c
> index 8c9f4adc6..1d1596fec 100644
> --- a/shell/ash.c
> +++ b/shell/ash.c
> @@ -182,6 +182,8 @@
>  #define IF_BASH_PATTERN_SUBST       IF_ASH_BASH_COMPAT
>  #define    BASH_SUBSTR          ENABLE_ASH_BASH_COMPAT
>  #define IF_BASH_SUBSTR              IF_ASH_BASH_COMPAT
> +#define    BASH_XTRACEFD        ENABLE_ASH_BASH_COMPAT
> +#define IF_BASH_XTRACEFD            IF_ASH_BASH_COMPAT
>  /* [[ EXPR ]] */
>  #define    BASH_TEST2           (ENABLE_ASH_BASH_COMPAT * ENABLE_ASH_TEST)
>  #define    BASH_SOURCE          ENABLE_ASH_BASH_COMPAT
> @@ -9740,6 +9742,7 @@ evalcommand(union node *cmd, int flags)
>         int status;
>         char **nargv;
>         smallint cmd_is_exec;
> +       IF_BASH_XTRACEFD(const char *xtracefd;)
>
>         /* First expand the arguments. */
>         TRACE(("evalcommand(0x%lx, %d) called\n", (long)cmd, flags));
> @@ -9791,6 +9794,10 @@ evalcommand(union node *cmd, int flags)
>
>         expredir(cmd->ncmd.redirect);
>         redir_stop = pushredir(cmd->ncmd.redirect);
> +#ifdef BASH_XTRACEFD
> +       xtracefd = lookupvar("BASH_XTRACEFD");
> +       if (!xtracefd || (preverrout_fd = atoi(xtracefd)) < 0)
> +#endif
>         preverrout_fd = 2;
>         status = redirectsafe(cmd->ncmd.redirect, REDIR_PUSH | REDIR_SAVEFD2);
>
>
> base-commit: 4dc86699b57ff35c287ca396d562ec206776694a
> --
> 2.13.3.windows.1.1055.g9b9ea5f2d78
>
> Published-As: https://github.com/dscho/busybox-w32/releases/tag/busybox-xtracefd-v1
> Fetch-It-Via: git fetch https://github.com/dscho/busybox-w32 busybox-xtracefd-v1
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