Using process substitution, we can get every lines of output of a command .
# Echoes every seconds using process substitution
while read line; do
echo $line
done < <(for i in $(seq 1 10); do echo $i && sleep 1; done)
By the same way above, I want to get the stdout output of 'wpa_supplicant' command, while discarding stderr.
But nothing can be seen on screen!
while read line; do
echo $line
done < <(wpa_supplicant -Dwext -iwlan1 -c${MY_CONFIG_FILE} 2> /dev/null)
I confirmed that typing the same command in prompt shows its output normaly.
$ wpa_supplicant -Dwext -iwlan1 -c${MY_CONFIG_FILE} 2> /dev/null
What is the mistake? Any help would be appreciated.
Finally I found the answer here!
The problem was easy... the buffering. Using stdbuf (and piping), the original code will be modified as below.
stdbuf -oL wpa_supplicant -iwlan1 -Dwext -c${MY_CONFIG_FILE} | while read line; do
echo "! $line"
done
'stdbuf -oL' make the stream line buffered, so I can get every each line from the running process.
Related
I have a script which watches dmesg and kills a process after a specific log message
#!/bin/bash
while sleep 1;
do
# dmesg -w | grep --max-count=1 -q 'protocol'
dmesg -w | sed '/protocol/Q'
mkdir -p /home/user/dmesg/
eval "dmesg -T > /home/user/dmesg/dmesg-`date +%d_%m_%Y-%H:%M`.log";
eval "dmesg -c";
pkill -x -9 programm
done
The Problem is that sed as well as grep only trigger after two messages.
So the script will not continue after only one message.
Is there anything I am missing?
You have a script that periodically executes dmesg. Instead, write a script that watches the output of dmesg.
dmesg | while IFS= read -r line; do
case "$line" in
*protocol*)
echo "do something when line has protocol"
;;
esac
done
Consider reading https://mywiki.wooledge.org/BashFAQ/001 .
I'd like to know when an application hasn't print a line in stdout for N seconds.
Here is a reproducible example:
#!/bin/bash
dmesg -w | {
while IFS= read -t 3 -r line
do
echo "$line"
done
echo "NO NEW LINE"
}
echo "END"
I can see the NO NEW LINE but the pipe doesn't stop and the bash doesn't continue. END is never displayed.
How to exit from the braces' code?
Source: https://unix.stackexchange.com/questions/117501/in-bash-script-how-to-capture-stdout-line-by-line
How to exit from the brackets' code?
Not all commands exit when they can't write to output or receive SIGPIPE, and they will not exit until they actually notice they can't write to output. Instead, run the command in the background. If the intention is not to wait on the process, in bash you could just use process substitution:
{
while IFS= read -t 3 -r line; do
printf "%s\n" "$line"
done
echo "end"
} < <(dmesg -w)
You could also use coprocess. Or just run the command in the background with a pipe and kill it when done with it.
The script monitors incoming HTTP messages and forwards them to a monitoring application called zabbix, It works fine, however after about 1-2 days it stops working. Heres what I know so far:
Using pgrep i see the script is still running
the logfile file gets updated properly (first command of script)
The FIFO pipe seems to be working
The problem must be somewhere in WHILE loop or tail command.
Im new at scripting so maybe someone can spot the problem right away?
#!/bin/bash
tcpflow -p -c -i enp2s0 port 80 | grep --line-buffered -oE 'boo.php.* HTTP/1.[01]' >> /usr/local/bin/logfile &
pipe=/tmp/fifopipe
trap "rm -f $pipe" EXIT
if [[ ! -p $pipe ]]; then
mkfifo $pipe
fi
tail -n0 -F /usr/local/bin/logfile > /tmp/fifopipe &
while true
do
if read line <$pipe; then
unset sn
for ((c=1; c<=3; c++)) # c is no of max parameters x 2 + 1
do
URL="$(echo $line | awk -F'[ =&?]' '{print $'$c'}')"
if [[ "$URL" == 'sn' ]]; then
((c++))
sn="$(echo $line | awk -F'[ =&?]' '{print $'$c'}')"
fi
done
if [[ "$sn" ]]; then
hosttype="US2G_"
host=$hosttype$sn
zabbix_sender -z nuc -s $host -k serial -o $sn -vv
fi
fi
done
You're inputting from the fifo incorrectly. By writing:
while true; do read line < $pipe ....; done
you are closing and reopening the fifo on each iteration of the loop. The first time you close it, the producer to the pipe (the tail -f) gets a SIGPIPE and dies. Change the structure to:
while true; do read line; ...; done < $pipe
Note that every process inside the loop now has the potential to inadvertently read from the pipe, so you'll probably want to explicitly close stdin for each.
I'm trying to read full stdin into a variable :
script.sh
#/bin/bash
input=""
while read line
do
echo "$line"
input="$input""\n""$line"
done < /dev/stdin
echo "$input" > /tmp/test
When I run ls | ./script.sh or mostly any other commands, it works fine.
However It doesn't work when I run cat | ./script.sh , enter my message, and then hit Ctrl-C to exit cat.
Any ideas ?
I would stick to the one-liner
input=$(cat)
Of course, Ctrl-D should be used to signal end-of-file.
I am using RedHat EL 4. I am using Bash 3.00.15.
I am writing SystemVerilog and I want to emulate stdin and stdout. I can only use files as the normal stdin and stdout is not supported in the environment. I would like to use named pipes to emulate stdin and stdout.
I understand how to create a to_sv and from_sv file using mkpipe, and how to open them and use them in SystemVerilog.
By using "cat > to_sv" I can output strings to the SystemVerilog simulation. But that also outputs what I'm typing in the shell.
I would like, if possible, a single shell where it acts almost like a UART terminal. Whatever I type goes directly out to "to_sv", and whatever is written to "from_sv" gets printed out.
If I am going about this completely wrong, then by all means suggest the correct way! Thank you so much,
Nachum Kanovsky
Edit: You can output to a named pipe and read from an other one in the same terminal. You can also disable keys to be echoed to the terminal using stty -echo.
mkfifo /tmp/from
mkfifo /tmp/to
stty -echo
cat /tmp/from & cat > /tmp/to
Whit this command everything you write goes to /tmp/to and is not echoed and everything written to /tmp/from will be echoed.
Update: I have found a way to send every chars inputed to the /tmp/to one at a time. Instead of cat > /tmp/to use this command:
while IFS= read -n1 c;
do
if [ -z "$c" ]; then
printf "\n" >> /tmp/to;
fi;
printf "%s" "$c" >> /tmp/to;
done
You probably want to use exec as in:
exec > to_sv
exec < from_sv
See sections 19.1. and 19.2. in the Advanced Bash-Scripting Guide - I/O Redirection
Instead of cat /tmp/from & you may use tail -f /tmp/from & (at least here on Mac OS X 10.6.7 this prevented a deadlock if I echo more than once to /tmp/from).
Based on Lynch's code:
# terminal window 1
(
rm -f /tmp/from /tmp/to
mkfifo /tmp/from
mkfifo /tmp/to
stty -echo
#cat -u /tmp/from &
tail -f /tmp/from &
bgpid=$!
trap "kill -TERM ${bgpid}; stty echo; exit" 1 2 3 13 15
while IFS= read -n1 c;
do
if [ -z "$c" ]; then
printf "\n" >> /tmp/to
fi;
printf "%s" "$c" >> /tmp/to
done
)
# terminal window 2
(
tail -f /tmp/to &
bgpid=$!
trap "kill -TERM ${bgpid}; stty echo; exit" 1 2 3 13 15
wait
)
# terminal window 3
echo "hello from /tmp/from" > /tmp/from