I want to run Emacs after logging into bash as user.
But also I want to be able to jump back into the bash prompt if I press CTRL-Z.
I have tried a couple of settings of .bashrc and .profile:
emacs
eval 'emacs'
bash -c emacs
exec emacs -nw
The problem is that all of this variants make CTRL-Z drop me not to bash prompt , but to empty stdin, like bash prompt was not loaded yet.
Any ideas? Thanks.
Thanks to Mark Plotnick, who answered below in comments. Using ioctl you can write to own tty.
c program:
#include "unistd.h"
#include "stdlib.h"
#include "stdio.h"
#include "sys/stat.h"
#include "sys/types.h"
#include "fcntl.h"
#include "termios.h"
#include "sys/ioctl.h"
int main(int argc, char ** argv)
{
if (argc >= 3)
{
int fd = open (argv[1], O_RDWR);
if (fd)
{
char * cmd = argv[2];
while(*cmd)
ioctl(fd, TIOCSTI, cmd++);
if (argc >= 4)
ioctl(fd, TIOCSTI, "\r");
return 0;
}
else
printf("could'n open file\n");
}
else
printf("wrong args\n");
return -1;
}
compile:
gcc my_ioctl.c -o my_ioctl
very end of .profile:
~/my_ioctl $(tty) emacs rr
(my c program does not care about what 3rd arg's actually is).
Hi I have a script that run two program
#Script file
./prog1
./prog2
prog1 is a C program
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv){
printf("prog1 running\n");
int tmp;
scanf("%d", &tmp);
printf("%d\n", tmp+10);
printf("prog1 ended\n");
return 0;
}
prog 2 is a C program as well
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv){
printf("prog2 running\n");
int tmp;
scanf("%d\n", &tmp);
printf("%d\n", tmp+10);
printf("prog2 ended\n");
return 0;
}
I run the command
./script < file
where file is
123
456
The output is
prog1 running
133
prog1 ended
prog2 running
10
prog2 ended
It seems like prog2 did not get the input from file, what is happening under the hood?
Will it be possible that prog2 took "\n" instead of a number?
Your script should be this:
#!/bin/bash
exec 3<&1
tee >(./prog2 >&3) | ./prog1
This use the tee command to duplicate stdin and the recent >() bash feature to open a temporary filedescriptor. (the use of filedesriptor 3 is done to split the stdout without parallelism).
See this answer to read the whole story.
scanf reads buffered input. So when your first program reads from stdin, it speculatively reads ahead all the available input to make future reads from stdin faster (through avoiding having to make so many system calls). When the second program runs, there's no input left, and (since you failed to check the result of scanf()) you end up with 0 in tmp.
You should be able to modify the buffering strategy in your application (at the expense of speed) using the setvbuf() standard function.
I have a Linux process (let's call it the main process) whose standard output is piped to another process (called the downstream process) by means of the shell's pipe operator (|). The main process is set up to receive SIGPIPE signals if the downstream process crashes. Unfortunately, SIGPIPE is not raised until the main process writes to stdout. Is there a way to tell sooner that the downstream process has terminated?
One approach is to write continuously to the downstream process, but that seems wasteful. Another approach is to have a separate watchdog process that monitors all relevant processes, but that is complex. Or perhaps there is some way to use select() to trigger the signal. I am hoping that the main process can do all this itself.
It appears the stdout file descriptor becomes "ready for reading" when the receiver crashes:
$ gcc -Wall select-downstream-crash.c -o select-downstream-crash
$ gcc -Wall crash-in-five-seconds.c -o crash-in-five-seconds
$ ./select-downstream-crash | ./crash-in-five-seconds
... five seconds pass ...
stdout is ready for reading
Segmentation fault
select-downstream-crash.c
#include <err.h>
#include <stdio.h>
#include <sys/select.h>
#include <unistd.h>
int main(void)
{
fd_set readfds;
int rc;
FD_ZERO(&readfds);
FD_SET(STDOUT_FILENO, &readfds);
rc = select(STDOUT_FILENO + 1, &readfds, NULL, NULL, NULL);
if (rc < 0)
err(1, "select");
if (FD_ISSET(STDOUT_FILENO, &readfds))
fprintf(stderr, "stdout is ready for reading\n");
return 0;
}
crash-in-five-seconds.c
#include <stdio.h>
#include <unistd.h>
int main(void)
{
sleep(5);
putchar(*(char*)NULL);
return 0;
}
I tried this on Linux, but don't know if it'll work elsewhere. It would be nice to find some documentation explaining this observation.
If the main process forks the other processes, then it will get SIGCHLD notifications when they exit.
I like to log a programs output 'on demand'. Eg. the output is logged to the terminal, but another process can hook on the current output at any time.
The classic way would be:
myprogram 2>&1 | tee /tmp/mylog
and on demand
tail /tmp/mylog
However, this would create a ever growing log file even if not used until the drive runs out of space. So my attempt was:
mkfifo /tmp/mylog
myprogram 2>&1 | tee /tmp/mylog
and on demand
cat /tmp/mylog
Now I can read /tmp/mylog at any time. However, any output blocks the program until the /tmp/mylog is read. I like the fifo to flush any incoming data not read back. How to do that?
Inspired by your question I've written a simple program that will let you do this:
$ myprogram 2>&1 | ftee /tmp/mylog
It behaves similarly to tee but clones the stdin to stdout and to a named pipe (a requirement for now) without blocking. This means that if you want to log this way it may happen that you're gonna lose your log data, but I guess it's acceptable in your scenario.
The trick is to block SIGPIPE signal and to ignore error on writing to a broken fifo. This sample may be optimized in various ways of course, but so far, it does the job I guess.
/* ftee - clone stdin to stdout and to a named pipe
(c) racic#stackoverflow
WTFPL Licence */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <signal.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
int readfd, writefd;
struct stat status;
char *fifonam;
char buffer[BUFSIZ];
ssize_t bytes;
signal(SIGPIPE, SIG_IGN);
if(2!=argc)
{
printf("Usage:\n someprog 2>&1 | %s FIFO\n FIFO - path to a"
" named pipe, required argument\n", argv[0]);
exit(EXIT_FAILURE);
}
fifonam = argv[1];
readfd = open(fifonam, O_RDONLY | O_NONBLOCK);
if(-1==readfd)
{
perror("ftee: readfd: open()");
exit(EXIT_FAILURE);
}
if(-1==fstat(readfd, &status))
{
perror("ftee: fstat");
close(readfd);
exit(EXIT_FAILURE);
}
if(!S_ISFIFO(status.st_mode))
{
printf("ftee: %s in not a fifo!\n", fifonam);
close(readfd);
exit(EXIT_FAILURE);
}
writefd = open(fifonam, O_WRONLY | O_NONBLOCK);
if(-1==writefd)
{
perror("ftee: writefd: open()");
close(readfd);
exit(EXIT_FAILURE);
}
close(readfd);
while(1)
{
bytes = read(STDIN_FILENO, buffer, sizeof(buffer));
if (bytes < 0 && errno == EINTR)
continue;
if (bytes <= 0)
break;
bytes = write(STDOUT_FILENO, buffer, bytes);
if(-1==bytes)
perror("ftee: writing to stdout");
bytes = write(writefd, buffer, bytes);
if(-1==bytes);//Ignoring the errors
}
close(writefd);
return(0);
}
You can compile it with this standard command:
$ gcc ftee.c -o ftee
You can quickly verify it by running e.g.:
$ ping www.google.com | ftee /tmp/mylog
$ cat /tmp/mylog
Also note - this is no multiplexer. You can only have one process doing $ cat /tmp/mylog at a time.
This is a (very) old thread, but I've run into a similar problem of late. In fact, what I needed is a cloning of stdin to stdout with a copy to a pipe that is non blocking. the proposed ftee in the first answer really helped there, but was (for my use case) too volatile. Meaning I lost data I could have processed if I had gotten to it in time.
The scenario I was faced with is that I have a process (some_process) that aggregates some data and writes its results every three seconds to stdout. The (simplified) setup looked like this (in the real setup I am using a named pipe):
some_process | ftee >(onlineAnalysis.pl > results) | gzip > raw_data.gz
Now, raw_data.gz has to be compressed and has to be complete. ftee does this job very well. But the pipe I am using in the middle was too slow to grab the data flushed out - but it was fast enough to process everything if it could get to it, which was tested with a normal tee. However, a normal tee blocks if anything happens to the unnamed pipe, and as I want to be able to hook in on demand, tee is not an option. Back to the topic: It got better when I put a buffer in between, resulting in:
some_process | ftee >(mbuffer -m 32M| onlineAnalysis.pl > results) | gzip > raw_data.gz
But that was still losing data I could have processed. So I went ahead and extended the ftee proposed before to a buffered version (bftee). It still has all the same properties, but uses an (inefficient ?) internal buffer in case a write fails. It still loses data if the buffer runs full, but it works beautifully for my case. As always there is a lot of room for improvement, but as I copied the code off of here I'd like to share it back to people that might have a use for it.
/* bftee - clone stdin to stdout and to a buffered, non-blocking pipe
(c) racic#stackoverflow
(c) fabraxias#stackoverflow
WTFPL Licence */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <signal.h>
#include <unistd.h>
// the number of sBuffers that are being held at a maximum
#define BUFFER_SIZE 4096
#define BLOCK_SIZE 2048
typedef struct {
char data[BLOCK_SIZE];
int bytes;
} sBuffer;
typedef struct {
sBuffer *data; //array of buffers
int bufferSize; // number of buffer in data
int start; // index of the current start buffer
int end; // index of the current end buffer
int active; // number of active buffer (currently in use)
int maxUse; // maximum number of buffers ever used
int drops; // number of discarded buffer due to overflow
int sWrites; // number of buffer written to stdout
int pWrites; // number of buffers written to pipe
} sQueue;
void InitQueue(sQueue*, int); // initialized the Queue
void PushToQueue(sQueue*, sBuffer*, int); // pushes a buffer into Queue at the end
sBuffer *RetrieveFromQueue(sQueue*); // returns the first entry of the buffer and removes it or NULL is buffer is empty
sBuffer *PeakAtQueue(sQueue*); // returns the first entry of the buffer but does not remove it. Returns NULL on an empty buffer
void ShrinkInQueue(sQueue *queue, int); // shrinks the first entry of the buffer by n-bytes. Buffer is removed if it is empty
void DelFromQueue(sQueue *queue); // removes the first entry of the queue
static void sigUSR1(int); // signal handled for SUGUSR1 - used for stats output to stderr
static void sigINT(int); // signla handler for SIGKILL/SIGTERM - allows for a graceful stop ?
sQueue queue; // Buffer storing the overflow
volatile int quit; // for quiting the main loop
int main(int argc, char *argv[])
{
int readfd, writefd;
struct stat status;
char *fifonam;
sBuffer buffer;
ssize_t bytes;
int bufferSize = BUFFER_SIZE;
signal(SIGPIPE, SIG_IGN);
signal(SIGUSR1, sigUSR1);
signal(SIGTERM, sigINT);
signal(SIGINT, sigINT);
/** Handle commandline args and open the pipe for non blocking writing **/
if(argc < 2 || argc > 3)
{
printf("Usage:\n someprog 2>&1 | %s FIFO [BufferSize]\n"
"FIFO - path to a named pipe, required argument\n"
"BufferSize - temporary Internal buffer size in case write to FIFO fails\n", argv[0]);
exit(EXIT_FAILURE);
}
fifonam = argv[1];
if (argc == 3) {
bufferSize = atoi(argv[2]);
if (bufferSize == 0) bufferSize = BUFFER_SIZE;
}
readfd = open(fifonam, O_RDONLY | O_NONBLOCK);
if(-1==readfd)
{
perror("bftee: readfd: open()");
exit(EXIT_FAILURE);
}
if(-1==fstat(readfd, &status))
{
perror("bftee: fstat");
close(readfd);
exit(EXIT_FAILURE);
}
if(!S_ISFIFO(status.st_mode))
{
printf("bftee: %s in not a fifo!\n", fifonam);
close(readfd);
exit(EXIT_FAILURE);
}
writefd = open(fifonam, O_WRONLY | O_NONBLOCK);
if(-1==writefd)
{
perror("bftee: writefd: open()");
close(readfd);
exit(EXIT_FAILURE);
}
close(readfd);
InitQueue(&queue, bufferSize);
quit = 0;
while(!quit)
{
// read from STDIN
bytes = read(STDIN_FILENO, buffer.data, sizeof(buffer.data));
// if read failed due to interrupt, then retry, otherwise STDIN has closed and we should stop reading
if (bytes < 0 && errno == EINTR) continue;
if (bytes <= 0) break;
// save the number if read bytes in the current buffer to be processed
buffer.bytes = bytes;
// this is a blocking write. As long as buffer is smaller than 4096 Bytes, the write is atomic to a pipe in Linux
// thus, this cannot be interrupted. however, to be save this should handle the error cases of partial or interrupted write none the less.
bytes = write(STDOUT_FILENO, buffer.data, buffer.bytes);
queue.sWrites++;
if(-1==bytes) {
perror("ftee: writing to stdout");
break;
}
sBuffer *tmpBuffer = NULL;
// if the queue is empty (tmpBuffer gets set to NULL) the this does nothing - otherwise it tries to write
// the buffered data to the pipe. This continues until the Buffer is empty or the write fails.
// NOTE: bytes cannot be -1 (that would have failed just before) when the loop is entered.
while ((bytes != -1) && (tmpBuffer = PeakAtQueue(&queue)) != NULL) {
// write the oldest buffer to the pipe
bytes = write(writefd, tmpBuffer->data, tmpBuffer->bytes);
// the written bytes are equal to the buffer size, the write is successful - remove the buffer and continue
if (bytes == tmpBuffer->bytes) {
DelFromQueue(&queue);
queue.pWrites++;
} else if (bytes > 0) {
// on a positive bytes value there was a partial write. we shrink the current buffer
// and handle this as a write failure
ShrinkInQueue(&queue, bytes);
bytes = -1;
}
}
// There are several cases here:
// 1.) The Queue is empty -> bytes is still set from the write to STDOUT. in this case, we try to write the read data directly to the pipe
// 2.) The Queue was not empty but is now -> bytes is set from the last write (which was successful) and is bigger 0. also try to write the data
// 3.) The Queue was not empty and still is not -> there was a write error before (even partial), and bytes is -1. Thus this line is skipped.
if (bytes != -1) bytes = write(writefd, buffer.data, buffer.bytes);
// again, there are several cases what can happen here
// 1.) the write before was successful -> in this case bytes is equal to buffer.bytes and nothing happens
// 2.) the write just before is partial or failed all together - bytes is either -1 or smaller than buffer.bytes -> add the remaining data to the queue
// 3.) the write before did not happen as the buffer flush already had an error. In this case bytes is -1 -> add the remaining data to the queue
if (bytes != buffer.bytes)
PushToQueue(&queue, &buffer, bytes);
else
queue.pWrites++;
}
// once we are done with STDIN, try to flush the buffer to the named pipe
if (queue.active > 0) {
//set output buffer to block - here we wait until we can write everything to the named pipe
// --> this does not seem to work - just in case there is a busy loop that waits for buffer flush aswell.
int saved_flags = fcntl(writefd, F_GETFL);
int new_flags = saved_flags & ~O_NONBLOCK;
int res = fcntl(writefd, F_SETFL, new_flags);
sBuffer *tmpBuffer = NULL;
//TODO: this does not handle partial writes yet
while ((tmpBuffer = PeakAtQueue(&queue)) != NULL) {
int bytes = write(writefd, tmpBuffer->data, tmpBuffer->bytes);
if (bytes != -1) DelFromQueue(&queue);
}
}
close(writefd);
}
/** init a given Queue **/
void InitQueue (sQueue *queue, int bufferSize) {
queue->data = calloc(bufferSize, sizeof(sBuffer));
queue->bufferSize = bufferSize;
queue->start = 0;
queue->end = 0;
queue->active = 0;
queue->maxUse = 0;
queue->drops = 0;
queue->sWrites = 0;
queue->pWrites = 0;
}
/** push a buffer into the Queue**/
void PushToQueue(sQueue *queue, sBuffer *p, int offset)
{
if (offset < 0) offset = 0; // offset cannot be smaller than 0 - if that is the case, we were given an error code. Set it to 0 instead
if (offset == p->bytes) return; // in this case there are 0 bytes to add to the queue. Nothing to write
// this should never happen - offset cannot be bigger than the buffer itself. Panic action
if (offset > p->bytes) {perror("got more bytes to buffer than we read\n"); exit(EXIT_FAILURE);}
// debug output on a partial write. TODO: remove this line
// if (offset > 0 ) fprintf(stderr, "partial write to buffer\n");
// copy the data from the buffer into the queue and remember its size
memcpy(queue->data[queue->end].data, p->data + offset , p->bytes-offset);
queue->data[queue->end].bytes = p->bytes - offset;
// move the buffer forward
queue->end = (queue->end + 1) % queue->bufferSize;
// there is still space in the buffer
if (queue->active < queue->bufferSize)
{
queue->active++;
if (queue->active > queue->maxUse) queue->maxUse = queue->active;
} else {
// Overwriting the oldest. Move start to next-oldest
queue->start = (queue->start + 1) % queue->bufferSize;
queue->drops++;
}
}
/** return the oldest entry in the Queue and remove it or return NULL in case the Queue is empty **/
sBuffer *RetrieveFromQueue(sQueue *queue)
{
if (!queue->active) { return NULL; }
queue->start = (queue->start + 1) % queue->bufferSize;
queue->active--;
return &(queue->data[queue->start]);
}
/** return the oldest entry in the Queue or NULL if the Queue is empty. Does not remove the entry **/
sBuffer *PeakAtQueue(sQueue *queue)
{
if (!queue->active) { return NULL; }
return &(queue->data[queue->start]);
}
/*** Shrinks the oldest entry i the Queue by bytes. Removes the entry if buffer of the oldest entry runs empty*/
void ShrinkInQueue(sQueue *queue, int bytes) {
// cannot remove negative amount of bytes - this is an error case. Ignore it
if (bytes <= 0) return;
// remove the entry if the offset is equal to the buffer size
if (queue->data[queue->start].bytes == bytes) {
DelFromQueue(queue);
return;
};
// this is a partial delete
if (queue->data[queue->start].bytes > bytes) {
//shift the memory by the offset
memmove(queue->data[queue->start].data, queue->data[queue->start].data + bytes, queue->data[queue->start].bytes - bytes);
queue->data[queue->start].bytes = queue->data[queue->start].bytes - bytes;
return;
}
// panic is the are to remove more than we have the buffer
if (queue->data[queue->start].bytes < bytes) {
perror("we wrote more than we had - this should never happen\n");
exit(EXIT_FAILURE);
return;
}
}
/** delete the oldest entry from the queue. Do nothing if the Queue is empty **/
void DelFromQueue(sQueue *queue)
{
if (queue->active > 0) {
queue->start = (queue->start + 1) % queue->bufferSize;
queue->active--;
}
}
/** Stats output on SIGUSR1 **/
static void sigUSR1(int signo) {
fprintf(stderr, "Buffer use: %i (%i/%i), STDOUT: %i PIPE: %i:%i\n", queue.active, queue.maxUse, queue.bufferSize, queue.sWrites, queue.pWrites, queue.drops);
}
/** handle signal for terminating **/
static void sigINT(int signo) {
quit++;
if (quit > 1) exit(EXIT_FAILURE);
}
This version takes one more (optional) argument which specifies the number of the blocks that are to buffered for the pipe. My sample call now looks like this:
some_process | bftee >(onlineAnalysis.pl > results) 16384 | gzip > raw_data.gz
resulting in 16384 blocks to be buffered before discards happen. this uses about 32 Mbyte more memory, but... who cares ?
Of course, in the real environment I am using a named pipe so that I can attach and detach as needed. There is looks like this:
mkfifo named_pipe
some_process | bftee named_pipe 16384 | gzip > raw_data.gz &
cat named_pipe | onlineAnalysis.pl > results
Also, the process reacts on signals as follows:
SIGUSR1 -> print counters to STDERR
SIGTERM, SIGINT -> first exits the main loop and flushed the buffer to the pipe, the second terminated the program immediatly.
Maybe this helps someone in the future...
Enjoy
However, this would create a ever growing log file even if not used until the drive runs out of space.
Why not periodically rotate the logs? There's even a program to do it for you logrotate.
There's also a system for generating log messages and doing different things with them according to type. It's called syslog.
You could even combine the two. Have your program generate syslog messages, configure syslog to place them in a file and use logrotate to ensure they don't fill the disk.
If it turned out that you were writing for a small embedded system and the program's output is heavy there are a variety of techniques you might consider.
Remote syslog: send the syslog messages to a syslog server on the network.
Use the severity levels availble in syslog to do different things with the messages. E.g. discard "INFO" but log and forward "ERR" or greater. E.g. to console
Use a signal handler in your program to reread configuration on HUP and vary log generation "on demand" this way.
Have your program listen on a unix socket and write messages down it when open. You could even implement and interactive console into your program this way.
Using a configuration file, provide granular control of logging output.
It seems like bash <> redirection operator (3.6.10 Opening File Descriptors for Reading and WritingSee) makes writing to file/fifo opened with it non-blocking.
This should work:
$ mkfifo /tmp/mylog
$ exec 4<>/tmp/mylog
$ myprogram 2>&1 | tee >&4
$ cat /tmp/mylog # on demend
Solution given by gniourf_gniourf on #bash IRC channel.
BusyBox often used on embedded devices can create a ram buffered log by
syslogd -C
which can be filled by
logger
and read by
logread
Works quite well, but only provides one global log.
The logging could be directed to a UDP socket. Since UDP is connection-less, it won't block the sending program. Of course logs will be lost if the receiver or network can't keep up.
myprogram 2>&1 | socat - udp-datagram:localhost:3333
Then when you want to observe the logging:
socat udp-recv:3333 -
There are some other cool benefits like being able to attach multiple listeners at the same time or broadcast to multiple devices.
If you can install screen on the embedded device then you can run 'myprogram' in it and detach it, and reattach it anytime you want to see the log. Something like:
$ screen -t sometitle myprogram
Hit Ctrl+A, then d to detach it.
Whenever you want to see the output, reattach it:
$ screen -DR sometitle
Hit Ctrl-A, then d to detach it again.
This way you won't have to worry about the program output using disk space at all.
The problem with the given fifo approach is that the whole thing will hang when the pipe buffer is getting filled up and no reading process is taking place.
For the fifo approach to work I think you would have to implement a named pipe client-server model similar to the one mentioned in BASH: Best architecture for reading from two input streams (see slightly modified code below, sample code 2).
For a workaround you could also use a while ... read construct instead of teeing stdout to a named pipe by implementing a counting mechanism inside the while ... read loop that will overwrite the log file periodically by a specified number of lines. This would prevent an ever growing log file (sample code 1).
# sample code 1
# terminal window 1
rm -f /tmp/mylog
touch /tmp/mylog
while sleep 2; do date '+%Y-%m-%d_%H.%M.%S'; done 2>&1 | while IFS="" read -r line; do
lno=$((lno+1))
#echo $lno
array[${lno}]="${line}"
if [[ $lno -eq 10 ]]; then
lno=$((lno+1))
array[${lno}]="-------------"
printf '%s\n' "${array[#]}" > /tmp/mylog
unset lno array
fi
printf '%s\n' "${line}"
done
# terminal window 2
tail -f /tmp/mylog
#------------------------
# sample code 2
# code taken from:
# https://stackoverflow.com/questions/6702474/bash-best-architecture-for-reading-from-two-input-streams
# terminal window 1
# server
(
rm -f /tmp/to /tmp/from
mkfifo /tmp/to /tmp/from
while true; do
while IFS="" read -r -d $'\n' line; do
printf '%s\n' "${line}"
done </tmp/to >/tmp/from &
bgpid=$!
exec 3>/tmp/to
exec 4</tmp/from
trap "kill -TERM $bgpid; exit" 0 1 2 3 13 15
wait "$bgpid"
echo "restarting..."
done
) &
serverpid=$!
#kill -TERM $serverpid
# client
(
exec 3>/tmp/to;
exec 4</tmp/from;
while IFS="" read -r -d $'\n' <&4 line; do
if [[ "${line:0:1}" == $'\177' ]]; then
printf 'line from stdin: %s\n' "${line:1}" > /dev/null
else
printf 'line from fifo: %s\n' "$line" > /dev/null
fi
done &
trap "kill -TERM $"'!; exit' 1 2 3 13 15
while IFS="" read -r -d $'\n' line; do
# can we make it atomic?
# sleep 0.5
# dd if=/tmp/to iflag=nonblock of=/dev/null # flush fifo
printf '\177%s\n' "${line}"
done >&3
) &
# kill -TERM $!
# terminal window 2
# tests
echo hello > /tmp/to
yes 1 | nl > /tmp/to
yes 1 | nl | tee /tmp/to
while sleep 2; do date '+%Y-%m-%d_%H.%M.%S'; done 2>&1 | tee -a /tmp/to
# terminal window 3
cat /tmp/to | head -n 10
If your process writes to any log file and then wipes the file and starts again every now and again, so it doesn't get too big, or uses logrotate.
tail --follow=name --retry my.log
Is all you need. You will get as much scroll-back as your terminal.
Nothing non standard is needed. I've not tried it with small log files but all our logs rotate like this and I have never noticed loosing lines.
To follow in Fabraxias foot steps I'm going to share my small modification of racic's code. In one of my use cases I needed to suppress the writes to STDOUT, so I've added another parameter: swallow_stdout. If that is not 0, then output to STDOUT will be turned off.
Since I'm no C coder I've added comments while reading the code, maybe they are useful for others.
/* ftee - clone stdin to stdout and to a named pipe
(c) racic#stackoverflow
WTFPL Licence */
// gcc /tmp/ftee.c -o /usr/local/bin/ftee
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <signal.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
int readfd, writefd; // read & write file descriptors
struct stat status; // read file descriptor status
char *fifonam; // name of the pipe
int swallow_stdout; // 0 = write to STDOUT
char buffer[BUFSIZ]; // read/write buffer
ssize_t bytes; // bytes read/written
signal(SIGPIPE, SIG_IGN);
if(3!=argc)
{
printf("Usage:\n someprog 2>&1 | %s [FIFO] [swallow_stdout] \n"
"FIFO - path to a named pipe (created beforehand with mkfifo), required argument\n"
"swallow_stdout - 0 = output to PIPE and STDOUT, 1 = output to PIPE only, required argument\n", argv[0]);
exit(EXIT_FAILURE);
}
fifonam = argv[1];
swallow_stdout = atoi(argv[2]);
readfd = open(fifonam, O_RDONLY | O_NONBLOCK); // open read file descriptor in non-blocking mode
if(-1==readfd) // read descriptor error!
{
perror("ftee: readfd: open()");
exit(EXIT_FAILURE);
}
if(-1==fstat(readfd, &status)) // read descriptor status error! (?)
{
perror("ftee: fstat");
close(readfd);
exit(EXIT_FAILURE);
}
if(!S_ISFIFO(status.st_mode)) // read descriptor is not a FIFO error!
{
printf("ftee: %s in not a fifo!\n", fifonam);
close(readfd);
exit(EXIT_FAILURE);
}
writefd = open(fifonam, O_WRONLY | O_NONBLOCK); // open write file descriptor non-blocking
if(-1==writefd) // write file descriptor error!
{
perror("ftee: writefd: open()");
close(readfd);
exit(EXIT_FAILURE);
}
close(readfd); // reading complete, close read file descriptor
while(1) // infinite loop
{
bytes = read(STDIN_FILENO, buffer, sizeof(buffer)); // read STDIN into buffer
if (bytes < 0 && errno == EINTR)
continue; // skip over errors
if (bytes <= 0)
break; // no more data coming in or uncaught error, let's quit since we can't write anything
if (swallow_stdout == 0)
bytes = write(STDOUT_FILENO, buffer, bytes); // write buffer to STDOUT
if(-1==bytes) // write error!
perror("ftee: writing to stdout");
bytes = write(writefd, buffer, bytes); // write a copy of the buffer to the write file descriptor
if(-1==bytes);// ignore errors
}
close(writefd); // close write file descriptor
return(0); // return exit code 0
}
On Linux, is it possible to somehow disable signaling for programs externally... that is, without modifying their source code?
Context:
I'm calling a C (and also a Java) program from within a bash script on Linux. I don't want any interruptions for my bash script, and for the other programs that the script launches (as foreground processes).
While I can use a...
trap '' INT
... in my bash script to disable the Ctrl C signal, this works only when the program control happens to be in the bash code. That is, if I press Ctrl C while the C program is running, the C program gets interrupted and it exits! This C program is doing some critical operation because of which I don't want it be interrupted. I don't have access to the source code of this C program, so signal handling inside the C program is out of question.
#!/bin/bash
trap 'echo You pressed Ctrl C' INT
# A C program to emulate a real-world, long-running program,
# which I don't want to be interrupted, and for which I
# don't have the source code!
#
# File: y.c
# To build: gcc -o y y.c
#
# #include <stdio.h>
# int main(int argc, char *argv[]) {
# printf("Performing a critical operation...\n");
# for(;;); // Do nothing forever.
# printf("Performing a critical operation... done.\n");
# }
./y
Regards,
/HS
The process signal mask is inherited across exec, so you can simply write a small wrapper program that blocks SIGINT and executes the target:
#include <signal.h>
#include <unistd.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
sigset_t sigs;
sigemptyset(&sigs);
sigaddset(&sigs, SIGINT);
sigprocmask(SIG_BLOCK, &sigs, 0);
if (argc > 1) {
execvp(argv[1], argv + 1);
perror("execv");
} else {
fprintf(stderr, "Usage: %s <command> [args...]\n", argv[0]);
}
return 1;
}
If you compile this program to noint, you would just execute ./noint ./y.
As ephemient notes in comments, the signal disposition is also inherited, so you can have the wrapper ignore the signal instead of blocking it:
#include <signal.h>
#include <unistd.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
struct sigaction sa = { 0 };
sa.sa_handler = SIG_IGN;
sigaction(SIGINT, &sa, 0);
if (argc > 1) {
execvp(argv[1], argv + 1);
perror("execv");
} else {
fprintf(stderr, "Usage: %s <command> [args...]\n", argv[0]);
}
return 1;
}
(and of course for a belt-and-braces approach, you could do both).
The "trap" command is local to this process, never applies to children.
To really trap the signal, you have to hack it using a LD_PRELOAD hook. This is non-trival task (you have to compile a loadable with _init(), sigaction() inside), so I won't include the full code here. You can find an example for SIGSEGV on Phack Volume 0x0b, Issue 0x3a, Phile #0x03.
Alternativlly, try the nohup and tail trick.
nohup your_command &
tail -F nohup.out
I would suggest that your C (and Java) application needs rewriting so that it can handle an exception, what happens if it really does need to be interrupted, power fails, etc...
I that fails, J-16 is right on the money. Does the user need to interract with the process, or just see the output (do they even need to see the output?)
The solutions explained above are not working for me, even by chaining the both commands proposed by Caf.
However, I finally succeeded in getting the expected behavior this way :
#!/bin/zsh
setopt MONITOR
TRAPINT() { print AAA }
print 1
( ./child & ; wait)
print 2
If I press Ctrl-C while child is running, it will wait that it exits, then will print AAA and 2. child will not receive any signals.
The subshell is used to prevent the PID from being shown.
And sorry... this is for zsh though the question is for bash, but I do not know bash enough to provide an equivalent script.
This is example code of enabling signals like Ctrl+C for programs which block it.
fixControlC.c
#include <stdio.h>
#include <signal.h>
int sigaddset(sigset_t *set, int signo) {
printf("int sigaddset(sigset_t *set=%p, int signo=%d)\n", set, signo);
return 0;
}
Compile it:
gcc -fPIC -shared -o fixControlC.so fixControlC.c
Run it:
LD_LIBRARY_PATH=. LD_PRELOAD=fixControlC.so mysqld