psrelay(1)

psrelay/Makefile

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+CFLAGS += -I../libpsargs
+
+psrelay: psrelay.o ../libpsargs/libpsargs.o

psrelay/psrelay.1

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+.TH PSRELAY 1
+
+.SH NAME
+
+psrelay \(en conditionally execute a process
+
+.SH SYNOPSIS
+
+psrelay
+"["
+.RB [ command
+.RB ( argument... )]
+"]"
+"["
+.RB [ command
+.RB ( argument... )]
+"]"
+
+.SH DESCRIPTION
+
+Psrelay runs the first given command and, if it exits successfully, runs the
+second given command, exiting with the status of the second command.
+.PP
+Standard input given to psrelay is not directed to the first given command,
+only to the second given command.
+
+.SH DIAGNOSTICS
+
+Psrelay will print an error message and exit with the appropriate status from
+sysexits(3) in the event of improper invocation or operating system error.
+Psrelay will exit with the status of the second given command if run without
+error.
+
+.SH BUGS
+
+Unintuitively, "psrelay [ wc -c <file | xargs test 4 = ] [ cat ]" will
+redirect the contents of "file" into "psrelay [ wc -c" and pipe that output to
+"xargs test 4 = ] [ cat ]".
+
+.SH COPYRIGHT
+
+Public domain.

psrelay/psrelay.c

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+#include <errno.h> /* errno */
+#include <stdio.h> /* fprintf(3) */
+#include <string.h> /* strerror(3) */
+#if !defined EX_OK || !defined EX_OSERR || !defined EX_USAGE
+#	include <sysexits.h>
+#endif
+#include <unistd.h> /* fork(2) */
+#include <sys/types.h> /* <sys/wait.h> */
+#include <sys/wait.h> /* wait(2), WIFEXITED, WEXITSTATUS */
+#include "libpsargs.h"
+
+int oserr(char *argv0, char *s){
+
+	fprintf(stderr, "%s: %s\n", argv0, s);
+
+	return EX_OSERR;
+}
+
+int usage(char *s){
+
+	fprintf(stderr,
+		"Usage: %s \"[\" [command (arguments...)] \"]\""
+		" \"[\" [command (arguments...)] \"]\"\n",
+		s);
+
+	return EX_USAGE;
+}
+
+int main(int argc, char *argv[]){
+	int child;
+	char **corr;
+	char **curr;
+	int i;
+	int r;
+
+	if(argc < 2 || check_arg(curr = &argv[1]) != 2)
+		return usage(argc == 0 ? "<no argv[0]>" : argv[0]);
+
+	do{
+		*(corr = corresponding_arg(curr++)) = NULL;
+
+		if(i == 1 || (r = fork()) == 0)
+			execvp(*curr, curr);
+		if(i == 1 || r == -1 || r == 0)
+			return oserr(argv[0], strerror(errno));
+
+		wait(&child);
+		if(!WIFEXITED(child) || WEXITSTATUS(child) != 0)
+			return 1;
+
+		curr = corr + 1;
+	}while(++i < 2);
+	/* UNREACHABLE */
+
+}