Bash: Capture output of command run in background - bash

I'm trying to write a bash script that will get the output of a command that runs in the background. Unfortunately I can't get it to work, the variable I assign the output to is empty - if I replace the assignment with an echo command everything works as expected though.
#!/bin/bash
function test {
echo "$1"
}
echo $(test "echo") &
wait
a=$(test "assignment") &
wait
echo $a
echo done
This code produces the output:
echo
done
Changing the assignment to
a=`echo $(test "assignment") &`
works, but it seems like there should be a better way of doing this.

Bash has indeed a feature called Process Substitution to accomplish this.
$ echo <(yes)
/dev/fd/63
Here, the expression <(yes) is replaced with a pathname of a (pseudo device) file that is connected to the standard output of an asynchronous job yes (which prints the string y in an endless loop).
Now let's try to read from it:
$ cat /dev/fd/63
cat: /dev/fd/63: No such file or directory
The problem here is that the yes process terminated in the meantime because it received a SIGPIPE (it had no readers on stdout).
The solution is the following construct
$ exec 3< <(yes) # Save stdout of the 'yes' job as (input) fd 3.
This opens the file as input fd 3 before the background job is started.
You can now read from the background job whenever you prefer. For a stupid example
$ for i in 1 2 3; do read <&3 line; echo "$line"; done
y
y
y
Note that this has slightly different semantics than having the background job write to a drive backed file: the background job will be blocked when the buffer is full (you empty the buffer by reading from the fd). By contrast, writing to a drive-backed file is only blocking when the hard drive doesn't respond.
Process substitution is not a POSIX sh feature.
Here's a quick hack to give an asynchronous job drive backing (almost) without assigning a filename to it:
$ yes > backingfile & # Start job in background writing to a new file. Do also look at `mktemp(3)` and the `sh` option `set -o noclobber`
$ exec 3< backingfile # open the file for reading in the current shell, as fd 3
$ rm backingfile # remove the file. It will disappear from the filesystem, but there is still a reader and a writer attached to it which both can use it.
$ for i in 1 2 3; do read <&3 line; echo "$line"; done
y
y
y
Linux also recently got added the O_TEMPFILE option, which makes such hacks possible without the file ever being visible at all. I don't know if bash already supports it.
UPDATE:
#rthur, if you want to capture the whole output from fd 3, then use
output=$(cat <&3)
But note that you can't capture binary data in general: It's only a defined operation if the output is text in the POSIX sense. The implementations I know simply filter out all NUL bytes. Furthermore POSIX specifies that all trailing newlines must be removed.
(Please note also that capturing the output will result in OOM if the writer never stops (yes never stops). But naturally that problem holds even for read if the line separator is never written additionally)

One very robust way to deal with coprocesses in Bash is to use... the coproc builtin.
Suppose you have a script or function called banana you wish to run in background, capture all its output while doing some stuff and wait until it's done. I'll do the simulation with this:
banana() {
for i in {1..4}; do
echo "gorilla eats banana $i"
sleep 1
done
echo "gorilla says thank you for the delicious bananas"
}
stuff() {
echo "I'm doing this stuff"
sleep 1
echo "I'm doing that stuff"
sleep 1
echo "I'm done doing my stuff."
}
You will then run banana with the coproc as so:
coproc bananafd { banana; }
this is like running banana & but with the following extras: it creates two file descriptors that are in the array bananafd (at index 0 for output and index 1 for input). You'll capture the output of banana with the read builtin:
IFS= read -r -d '' -u "${bananafd[0]}" banana_output
Try it:
#!/bin/bash
banana() {
for i in {1..4}; do
echo "gorilla eats banana $i"
sleep 1
done
echo "gorilla says thank you for the delicious bananas"
}
stuff() {
echo "I'm doing this stuff"
sleep 1
echo "I'm doing that stuff"
sleep 1
echo "I'm done doing my stuff."
}
coproc bananafd { banana; }
stuff
IFS= read -r -d '' -u "${bananafd[0]}" banana_output
echo "$banana_output"
Caveat: you must be done with stuff before banana ends! if the gorilla is quicker than you:
#!/bin/bash
banana() {
for i in {1..4}; do
echo "gorilla eats banana $i"
done
echo "gorilla says thank you for the delicious bananas"
}
stuff() {
echo "I'm doing this stuff"
sleep 1
echo "I'm doing that stuff"
sleep 1
echo "I'm done doing my stuff."
}
coproc bananafd { banana; }
stuff
IFS= read -r -d '' -u "${bananafd[0]}" banana_output
echo "$banana_output"
In this case, you'll obtain an error like this one:
./banana: line 22: read: : invalid file descriptor specification
You can check whether it's too late (i.e., whether you've taken too long doing your stuff) because after the coproc is done, bash removes the values in the array bananafd, and that's why we obtained the previous error.
#!/bin/bash
banana() {
for i in {1..4}; do
echo "gorilla eats banana $i"
done
echo "gorilla says thank you for the delicious bananas"
}
stuff() {
echo "I'm doing this stuff"
sleep 1
echo "I'm doing that stuff"
sleep 1
echo "I'm done doing my stuff."
}
coproc bananafd { banana; }
stuff
if [[ -n ${bananafd[#]} ]]; then
IFS= read -r -d '' -u "${bananafd[0]}" banana_output
echo "$banana_output"
else
echo "oh no, I took too long doing my stuff..."
fi
Finally, if you really don't want to miss any of gorilla's moves, even if you take too long for your stuff, you could copy banana's file descriptor to another fd, 3 for example, do your stuff and then read from 3:
#!/bin/bash
banana() {
for i in {1..4}; do
echo "gorilla eats banana $i"
sleep 1
done
echo "gorilla says thank you for the delicious bananas"
}
stuff() {
echo "I'm doing this stuff"
sleep 1
echo "I'm doing that stuff"
sleep 1
echo "I'm done doing my stuff."
}
coproc bananafd { banana; }
# Copy file descriptor banana[0] to 3
exec 3>&${bananafd[0]}
stuff
IFS= read -d '' -u 3 output
echo "$output"
This will work very well! the last read will also play the role of wait, so that output will contain the complete output of banana.
That was great: no temp files to deal with (bash handles everything silently) and 100% pure bash!
Hope this helps!

One way to capture background command's output is to redirect it's output in a file and capture output from file after background process has ended:
test "assignment" > /tmp/_out &
wait
a=$(</tmp/_out)

I also use file redirections. Like:
exec 3< <({ sleep 2; echo 12; }) # Launch as a job stdout -> fd3
cat <&3 # Lock read fd3
More real case
If I want the output of 4 parallel workers: toto, titi, tata and tutu.
I redirect each one to an different file descriptor (in fd variable).
Then reading these file descriptor will block until EOF <= pipe broken <= command completed
#!/usr/bin/env bash
# Declare data to be forked
a_value=(toto titi tata tutu)
msg=""
# Spawn child sub-processes
for i in {0..3}; do
((fd=50+i))
echo -e "1/ Launching command: $cmd with file descriptor: $fd!"
eval "exec $fd< <({ sleep $((i)); echo ${a_value[$i]}; })"
a_pid+=($!) # Store pid
done
# Join child: wait them all and collect std-output
for i in {0..3}; do
((fd=50+i));
echo -e "2/ Getting result of: $cmd with file descriptor: $fd!"
msg+="$(cat <&$fd)\n"
((i_fd--))
done
# Print result
echo -e "===========================\nResult:"
echo -e "$msg"
Should output:
1/ Launching command: with file descriptor: 50!
1/ Launching command: with file descriptor: 51!
1/ Launching command: with file descriptor: 52!
1/ Launching command: with file descriptor: 53!
2/ Getting result of: with file descriptor: 50!
2/ Getting result of: with file descriptor: 51!
2/ Getting result of: with file descriptor: 52!
2/ Getting result of: with file descriptor: 53!
===========================
Result:
toto
titi
tata
tutu
Note1: coproc is supporting only one coprocess and not multiple
Note2: wait command is buggy for old bash version (4.2) and cannot retrieve the status of the jobs I launched. It works well in bash 5 but file redirection works for all versions.

Just group the commands, when you run them in background and wait for both.
{ echo a & echo b & wait; } | nl
Output will be:
1 a
2 b
But notice that the output can be out of order, if the second task runs faster than the first.
{ { sleep 1; echo a; } & echo b & wait; } | nl
Reverse output:
1 b
2 a
If it is necessary to separate the output of both background jobs, it is necessary to buffer the output somewhere, typically in a file. Example:
#! /bin/bash
t0=$(date +%s) # Get start time
trap 'rm -f "$ta" "$tb"' EXIT # Remove temp files on exit.
ta=$(mktemp) # Create temp file for job a.
tb=$(mktemp) # Create temp file for job b.
{ exec >$ta; echo a1; sleep 2; echo a2; } & # Run job a.
{ exec >$tb; echo b1; sleep 3; echo b2; } & # Run job b.
wait # Wait for the jobs to finish.
cat "$ta" # Print output of job a.
cat "$tb" # Print output of job b.
t1=$(date +%s) # Get end time
echo "t1 - t0: $((t1-t0))" # Display execution time.
The overall runtime of the script is three seconds, although the combined sleeping time of both background jobs is five seconds. And the output of the background jobs is in order.
a1
a2
b1
b2
t1 - t0: 3
You can also use a memory buffer to store the output of your jobs. But this works only, if your buffer is big enough to store the whole output of your jobs.
#! /bin/bash
t0=$(date +%s)
trap 'rm -f /tmp/{a,b}' EXIT
mkfifo /tmp/{a,b}
buffer() { dd of="$1" status=none iflag=fullblock bs=1K; }
pids=()
{ echo a1; sleep 2; echo a2; } > >(buffer /tmp/a) &
pids+=($!)
{ echo b1; sleep 3; echo b2; } > >(buffer /tmp/b) &
pids+=($!)
# Wait only for the jobs but not for the buffering `dd`.
wait "${pids[#]}"
# This will wait for `dd`.
cat /tmp/{a,b}
t1=$(date +%s)
echo "t1 - t0: $((t1-t0))"
The above will also work with cat instead of dd. But then you can not control the buffer size.

If you have GNU Parallel you can probably use parset:
myfunc() {
sleep 3
echo "The input was"
echo "$#"
}
export -f myfunc
parset a,b,c myfunc ::: myarg-a "myarg b" myarg-c
echo "$a"
echo "$b"
echo "$c"
See: https://www.gnu.org/software/parallel/parset.html

Related

Bash string interpolation without subshell

I have a function like this
print_stuff_and_set_vars() {
IMPORTANT_VAL_1=""
IMPORTANT_VAL_2=""
echo -n "some stuff"
echo -n "some more stuff"
echo -n "result"
}
and I call it like this:
my_main_func() {
print_stuff_and_set_vars
print_stuff_and_set_vars
print_stuff_and_set_vars
echo "IMPORTANT_VAL_1 was $IMPORTANT_VAL_1"
}
Instead, I want to save all the echoed results to a string
my_main_func() {
# doesn't work -- result is empty
result="${print_stuff_and_set_vars}${print_stuff_and_set_vars}${print_stuff_and_set_vars}"
echo "the result length was ${#result}"
echo "$result"
echo "IMPORTANT_VAL_1 was $IMPORTANT_VAL_1"
}
This does work if I use $() instead of ${} to start a subshell, but then the global variables are not set.
Is there any way to save the result of a function to a string without starting a subshell? I know the obvious answer in this example would be to save "result" to a global variable instead of echoing it but in my actual script that would require a lot of changes and I want to avoid it if possible.
Actually, I only need to know the length so if there is a way to keep track of how much has been printed since the start of the function that would work fine too. I'm actually using zsh if that makes a difference, too.
Assuming the output from the function print_stuff_and_set_vars does not contain newline characters, how about:
mkfifo p # create a named pipe "p"
exec 3<>p # open fd 3 for both reading and writing
rm p # now "p" can be closed
print_stuff_and_set_vars() {
IMPORTANT_VAL_1="foo"
IMPORTANT_VAL_2="bar"
echo -n "some stuff "
echo -n "some more stuff "
echo -n "result "
}
my_main_func() {
print_stuff_and_set_vars 1>&3 # redirect to fd 3
print_stuff_and_set_vars 1>&3 # same as above
print_stuff_and_set_vars 1>&3 # same as above
echo 1>&3 # send newline as an end of input
IFS= read -r -u 3 result # read a line from fd 3
echo "the result length was ${#result}"
echo "$result"
echo "IMPORTANT_VAL_1 was $IMPORTANT_VAL_1"
}
my_main_func
exec 3>&- # close fd 3
Output:
the result length was 102
some stuff some more stuff result some stuff some more stuff result some stuff some more stuff result
IMPORTANT_VAL_1 was foo
This is very easy to do with temp file. Example:
print_stuff_and_set_vars() {
IMPORTANT_VAL_1="x"
IMPORTANT_VAL_2="y"
echo -n "some stuff"
echo -n "some more stuff"
echo -n "result"
}
print_stuff_and_set_vars > /tmp/$$
myVar=$(< /tmp/$$)
echo $myVar
echo $IMPORTANT_VAL_1
echo $IMPORTANT_VAL_2
It is possible to implement this using named pipe or file descriptor as well.

Capturing the output of a detached command makes the execution sequential [duplicate]

I'm trying to write a bash script that will get the output of a command that runs in the background. Unfortunately I can't get it to work, the variable I assign the output to is empty - if I replace the assignment with an echo command everything works as expected though.
#!/bin/bash
function test {
echo "$1"
}
echo $(test "echo") &
wait
a=$(test "assignment") &
wait
echo $a
echo done
This code produces the output:
echo
done
Changing the assignment to
a=`echo $(test "assignment") &`
works, but it seems like there should be a better way of doing this.
Bash has indeed a feature called Process Substitution to accomplish this.
$ echo <(yes)
/dev/fd/63
Here, the expression <(yes) is replaced with a pathname of a (pseudo device) file that is connected to the standard output of an asynchronous job yes (which prints the string y in an endless loop).
Now let's try to read from it:
$ cat /dev/fd/63
cat: /dev/fd/63: No such file or directory
The problem here is that the yes process terminated in the meantime because it received a SIGPIPE (it had no readers on stdout).
The solution is the following construct
$ exec 3< <(yes) # Save stdout of the 'yes' job as (input) fd 3.
This opens the file as input fd 3 before the background job is started.
You can now read from the background job whenever you prefer. For a stupid example
$ for i in 1 2 3; do read <&3 line; echo "$line"; done
y
y
y
Note that this has slightly different semantics than having the background job write to a drive backed file: the background job will be blocked when the buffer is full (you empty the buffer by reading from the fd). By contrast, writing to a drive-backed file is only blocking when the hard drive doesn't respond.
Process substitution is not a POSIX sh feature.
Here's a quick hack to give an asynchronous job drive backing (almost) without assigning a filename to it:
$ yes > backingfile & # Start job in background writing to a new file. Do also look at `mktemp(3)` and the `sh` option `set -o noclobber`
$ exec 3< backingfile # open the file for reading in the current shell, as fd 3
$ rm backingfile # remove the file. It will disappear from the filesystem, but there is still a reader and a writer attached to it which both can use it.
$ for i in 1 2 3; do read <&3 line; echo "$line"; done
y
y
y
Linux also recently got added the O_TEMPFILE option, which makes such hacks possible without the file ever being visible at all. I don't know if bash already supports it.
UPDATE:
#rthur, if you want to capture the whole output from fd 3, then use
output=$(cat <&3)
But note that you can't capture binary data in general: It's only a defined operation if the output is text in the POSIX sense. The implementations I know simply filter out all NUL bytes. Furthermore POSIX specifies that all trailing newlines must be removed.
(Please note also that capturing the output will result in OOM if the writer never stops (yes never stops). But naturally that problem holds even for read if the line separator is never written additionally)
One very robust way to deal with coprocesses in Bash is to use... the coproc builtin.
Suppose you have a script or function called banana you wish to run in background, capture all its output while doing some stuff and wait until it's done. I'll do the simulation with this:
banana() {
for i in {1..4}; do
echo "gorilla eats banana $i"
sleep 1
done
echo "gorilla says thank you for the delicious bananas"
}
stuff() {
echo "I'm doing this stuff"
sleep 1
echo "I'm doing that stuff"
sleep 1
echo "I'm done doing my stuff."
}
You will then run banana with the coproc as so:
coproc bananafd { banana; }
this is like running banana & but with the following extras: it creates two file descriptors that are in the array bananafd (at index 0 for output and index 1 for input). You'll capture the output of banana with the read builtin:
IFS= read -r -d '' -u "${bananafd[0]}" banana_output
Try it:
#!/bin/bash
banana() {
for i in {1..4}; do
echo "gorilla eats banana $i"
sleep 1
done
echo "gorilla says thank you for the delicious bananas"
}
stuff() {
echo "I'm doing this stuff"
sleep 1
echo "I'm doing that stuff"
sleep 1
echo "I'm done doing my stuff."
}
coproc bananafd { banana; }
stuff
IFS= read -r -d '' -u "${bananafd[0]}" banana_output
echo "$banana_output"
Caveat: you must be done with stuff before banana ends! if the gorilla is quicker than you:
#!/bin/bash
banana() {
for i in {1..4}; do
echo "gorilla eats banana $i"
done
echo "gorilla says thank you for the delicious bananas"
}
stuff() {
echo "I'm doing this stuff"
sleep 1
echo "I'm doing that stuff"
sleep 1
echo "I'm done doing my stuff."
}
coproc bananafd { banana; }
stuff
IFS= read -r -d '' -u "${bananafd[0]}" banana_output
echo "$banana_output"
In this case, you'll obtain an error like this one:
./banana: line 22: read: : invalid file descriptor specification
You can check whether it's too late (i.e., whether you've taken too long doing your stuff) because after the coproc is done, bash removes the values in the array bananafd, and that's why we obtained the previous error.
#!/bin/bash
banana() {
for i in {1..4}; do
echo "gorilla eats banana $i"
done
echo "gorilla says thank you for the delicious bananas"
}
stuff() {
echo "I'm doing this stuff"
sleep 1
echo "I'm doing that stuff"
sleep 1
echo "I'm done doing my stuff."
}
coproc bananafd { banana; }
stuff
if [[ -n ${bananafd[#]} ]]; then
IFS= read -r -d '' -u "${bananafd[0]}" banana_output
echo "$banana_output"
else
echo "oh no, I took too long doing my stuff..."
fi
Finally, if you really don't want to miss any of gorilla's moves, even if you take too long for your stuff, you could copy banana's file descriptor to another fd, 3 for example, do your stuff and then read from 3:
#!/bin/bash
banana() {
for i in {1..4}; do
echo "gorilla eats banana $i"
sleep 1
done
echo "gorilla says thank you for the delicious bananas"
}
stuff() {
echo "I'm doing this stuff"
sleep 1
echo "I'm doing that stuff"
sleep 1
echo "I'm done doing my stuff."
}
coproc bananafd { banana; }
# Copy file descriptor banana[0] to 3
exec 3>&${bananafd[0]}
stuff
IFS= read -d '' -u 3 output
echo "$output"
This will work very well! the last read will also play the role of wait, so that output will contain the complete output of banana.
That was great: no temp files to deal with (bash handles everything silently) and 100% pure bash!
Hope this helps!
One way to capture background command's output is to redirect it's output in a file and capture output from file after background process has ended:
test "assignment" > /tmp/_out &
wait
a=$(</tmp/_out)
I also use file redirections. Like:
exec 3< <({ sleep 2; echo 12; }) # Launch as a job stdout -> fd3
cat <&3 # Lock read fd3
More real case
If I want the output of 4 parallel workers: toto, titi, tata and tutu.
I redirect each one to an different file descriptor (in fd variable).
Then reading these file descriptor will block until EOF <= pipe broken <= command completed
#!/usr/bin/env bash
# Declare data to be forked
a_value=(toto titi tata tutu)
msg=""
# Spawn child sub-processes
for i in {0..3}; do
((fd=50+i))
echo -e "1/ Launching command: $cmd with file descriptor: $fd!"
eval "exec $fd< <({ sleep $((i)); echo ${a_value[$i]}; })"
a_pid+=($!) # Store pid
done
# Join child: wait them all and collect std-output
for i in {0..3}; do
((fd=50+i));
echo -e "2/ Getting result of: $cmd with file descriptor: $fd!"
msg+="$(cat <&$fd)\n"
((i_fd--))
done
# Print result
echo -e "===========================\nResult:"
echo -e "$msg"
Should output:
1/ Launching command: with file descriptor: 50!
1/ Launching command: with file descriptor: 51!
1/ Launching command: with file descriptor: 52!
1/ Launching command: with file descriptor: 53!
2/ Getting result of: with file descriptor: 50!
2/ Getting result of: with file descriptor: 51!
2/ Getting result of: with file descriptor: 52!
2/ Getting result of: with file descriptor: 53!
===========================
Result:
toto
titi
tata
tutu
Note1: coproc is supporting only one coprocess and not multiple
Note2: wait command is buggy for old bash version (4.2) and cannot retrieve the status of the jobs I launched. It works well in bash 5 but file redirection works for all versions.
Just group the commands, when you run them in background and wait for both.
{ echo a & echo b & wait; } | nl
Output will be:
1 a
2 b
But notice that the output can be out of order, if the second task runs faster than the first.
{ { sleep 1; echo a; } & echo b & wait; } | nl
Reverse output:
1 b
2 a
If it is necessary to separate the output of both background jobs, it is necessary to buffer the output somewhere, typically in a file. Example:
#! /bin/bash
t0=$(date +%s) # Get start time
trap 'rm -f "$ta" "$tb"' EXIT # Remove temp files on exit.
ta=$(mktemp) # Create temp file for job a.
tb=$(mktemp) # Create temp file for job b.
{ exec >$ta; echo a1; sleep 2; echo a2; } & # Run job a.
{ exec >$tb; echo b1; sleep 3; echo b2; } & # Run job b.
wait # Wait for the jobs to finish.
cat "$ta" # Print output of job a.
cat "$tb" # Print output of job b.
t1=$(date +%s) # Get end time
echo "t1 - t0: $((t1-t0))" # Display execution time.
The overall runtime of the script is three seconds, although the combined sleeping time of both background jobs is five seconds. And the output of the background jobs is in order.
a1
a2
b1
b2
t1 - t0: 3
You can also use a memory buffer to store the output of your jobs. But this works only, if your buffer is big enough to store the whole output of your jobs.
#! /bin/bash
t0=$(date +%s)
trap 'rm -f /tmp/{a,b}' EXIT
mkfifo /tmp/{a,b}
buffer() { dd of="$1" status=none iflag=fullblock bs=1K; }
pids=()
{ echo a1; sleep 2; echo a2; } > >(buffer /tmp/a) &
pids+=($!)
{ echo b1; sleep 3; echo b2; } > >(buffer /tmp/b) &
pids+=($!)
# Wait only for the jobs but not for the buffering `dd`.
wait "${pids[#]}"
# This will wait for `dd`.
cat /tmp/{a,b}
t1=$(date +%s)
echo "t1 - t0: $((t1-t0))"
The above will also work with cat instead of dd. But then you can not control the buffer size.
If you have GNU Parallel you can probably use parset:
myfunc() {
sleep 3
echo "The input was"
echo "$#"
}
export -f myfunc
parset a,b,c myfunc ::: myarg-a "myarg b" myarg-c
echo "$a"
echo "$b"
echo "$c"
See: https://www.gnu.org/software/parallel/parset.html

bash 'read' timeout do not wait for input when using pipeline

## bash script
foo(){
sleep 0.0001
read -s -t 0.0002 var ## get the pipeline input if have
if [ -n "$var" ];then
echo "has pipe input"
fi
if [ $# -gt 0 ];then
echo "has std input"
fi
read -s -t 30 var1 #1 # expect wait for 30 seconds , but acturaly not
echo "failed"
read -s -t 30 var2 #2
echo "failed again"
}
echo "ok1"| foo "ok"
## output:
has pipe input
has std input
failed
failed again
if the foo has pipeline input , the read command at #1 & #2 will return immediately without waiting for input TIMEOUT
in my real script there are three needs :
1 made function could accept pipeline input and parameter at the same time (cause i want my function can take configuration from pipeline input , i think that will be nice for me. for example: )
foo(){
local config
sleep 0.0001
read -t 0.0002 config
eval "$config"
}
then i can pass configuration like this
foo para1 para2 <<EOF
G_TIMEOUT=300
G_ROW_SPACE=2
G_LINE_NUM=10
EOF
2 in my function i need to read user input from keyboard( i need interactive with user by using read )
3 wait for user input should has a timeout ( i want to implament a screensaver ,if there is no action after TIMEOUT seconds passed from user, will call screensaver script , and after any keydown , screensaver will return , and again to wait for use input
if there is a way to redirect pipeline input to fd3 after i get the pipeline input ,and then close fd3 to make pipe broken , then reopen fd0 to standard input (keyboard) and wait for user input ?
It doesn't wait for input because it has reached the end of the pipe.
echo "ok1" | ... writes a single line to the pipe, then closes it. The first read in the function reads ok1 into var. All other read calls return immediately because there is no more data to read and no chance of more data appearing later because the write end of the pipe has already been closed.
If you want the pipe to stay open, you have to do something like
{ echo ok1; sleep 40; echo ok2; } | foo
because function foo has pipeline input , so in child process , the input fd been redirected to pipeline automaticly , just redirect the standard input to keyboard(/proc/pid/0) after get the pipeline input ,will solve the problem
thanks for those guys give me that clue , it is not read command problem ,it is fd problem acturaly
foo(){
local config
sleep 0.0001
read -t 0.0002 config
if [ -n "$config" ];then
config=$(cat -)
fi
echo "$config"
exec 3</dev/tty
read -t 10 -u 3 input
echo "success!"
}
a better approche:
foo(){
local config
sleep 0.0001
read -t 0.0002 config
if [ -n "$config" ];then
config=$(cat -)
fi
exec 0<&- ## close current pipeline input
exec 0</dev/tty ##reopen input fd with standard input
read -t 10 input ##read will wait for input for keyboard :) good !
echo "success!"
}
furthermore if i can detect current input is pipe or standard input , i colud not use read config to judge if there are pipeline input , but how to fullfill that ? [ -t 0 ] is a good idea
a better approche:
foo(){
local config
if [ ! -t 0 ];then
config=$(cat -)
exec 0<&- ## close current pipeline input
exec 0</dev/tty ##reopen input fd with standard input
fi
read -t 10 input ##read will wait for input for keyboard :) great !
echo "success!"
}
As an extra to the answer of melpomene you can see this when executing the following line:
$ echo foo | { read -t 10 var1; echo $?; read -t 10 var2; echo $?; }
0
1
$ read -t 1 var; echo $?
142
This line outputs the return codes of read and the manual states
The return code is zero, unless end-of-file is encountered, read times out (in which case the return code is greater than 128), or an invalid
file descriptor is supplied as the argument to -u
source: man bash
From this we see that the second read in the first command fails because EOF is reached.

Customized progress message for tasks in bash script

I'm currently writing a bash script to do tasks automatically. In my script I want it to display progress message when it is doing a task.
For example:
user#ubuntu:~$ Configure something
->
Configure something .
->
Configure something ..
->
Configure something ...
->
Configure something ... done
All the progress message should appear in the same line.
Below is my workaround so far:
echo -n "Configure something "
exec "configure something 2>&1 /dev/null"
//pseudo code for progress message
echo -n "." and sleep 1 if the previous exec of configure something not done
echo " done" if exec of the command finished successfully
echo " failed" otherwise
Will exec wait for the command to finish and then continue with the script lines later?
If so, then how can I echo message at the same time the exec of configure something is taking place?
How do I know when exec finishes the previous command and return true? use $? ?
Just to put the editorial hat on, what if something goes wrong? How are you, or a user of your script going to know what went wrong? This is probably not the answer you're looking for but having your script just execute each build step individually may turn out to be better overall, especially for troubleshooting. Why not define a function to validate your build steps:
function validateCmd()
{
CODE=$1
COMMAND=$2
MODULE=$3
if [ ${CODE} -ne 0 ]; then
echo "ERROR Executing Command: \"${COMMAND}\" in Module: ${MODULE}"
echo "Exiting."
exit 1;
fi
}
./configure
validateCmd $? "./configure" "Configuration of something"
Anyways, yes as you probably noticed above, use $? to determine what the result of the last command was. For example:
rm -rf ${TMP_DIR}
if [ $? -ne 0 ]; then
echo "ERROR Removing directory: ${TMP_DIR}"
exit 1;
fi
To answer your first question, you can use:
echo -ne "\b"
To delete a character on the same line. So to count to ten on one line, you can do something like:
for i in $(seq -w 1 10); do
echo -en "\b\b${i}"
sleep .25
done
echo
The trick with that is you'll have to know how much to delete, but I'm sure you can figure that out.
You cannot call exec like that; exec never returns, and the lines after an exec will not execute. The standard way to print progress updates on a single line is to simply use \r instead of \n at the end of each line. For example:
#!/bin/bash
i=0
sleep 5 & # Start some command
pid=$! # Save the pid of the command
while sleep 1; do # Produce progress reports
printf '\rcontinuing in %d seconds...' $(( 5 - ++i ))
test $i -eq 5 && break
done
if wait $pid; then echo done; else echo failed; fi
Here's another example:
#!/bin/bash
execute() {
eval "$#" & # Execute the command
pid=$!
# Invoke a shell to print status. If you just invoke
# the while loop directly, killing it will generate a
# notification. By trapping SIGTERM, we suppress the notice.
sh -c 'trap exit SIGTERM
while printf "\r%3d:%s..." $((++i)) "$*"; do sleep 1
done' 0 "$#" &
last_report=$!
if wait $pid; then echo done; else echo failed; fi
kill $last_report
}
execute sleep 3
execute sleep 2 \| false # Execute a command that will fail
execute sleep 1

Test whether stdout has been written to

I have a script that prints in a loop. I want the loop to print differently the first time from all other times (i.e., it should print differently if anything has been printed at all). I am thinking a simple way would be to check whether anything has been printed yet (i.e., stdout has been written to). Is there any way to determine that?
I know I could also write to a variable and test whether it's empty, but I'd like to avoid a variable if I can.
I think that will do what you need. If you echo something between # THE SCRIPT ITSELF and # END, THE FOLLOWING DATA HAS BEEN WRITTEN TO STDOUT will be printed STDOUT HAS NOT BEEN TOUCHED else...
#!/bin/bash
readonly TMP=$(mktemp /tmp/test_XXXXXX)
exec 3<> "$TMP" # open tmp file as fd 3
exec 4>&1 # save current value of stdout as fd 4
exec >&3 # redirect stdout to fd 3 (tmp file)
# THE SCRIPT ITSELF
echo Hello World
# END
exec >&4 # restore save stdout
exec 3>&- # close tmp file
TMP_SIZE=$(stat -f %z "$TMP")
if [ $TMP_SIZE -gt 0 ]; then
echo "THE FOLLOWING DATA HAS BEEN WRITTEN TO STDOUT"
echo
cat "$TMP"
else
echo "STDOUT HAS NOT BEEN TOUCHED"
fi
rm "$TMP"
So, output of the script as is:
THE FOLLOWING DATA HAS BEEN WRITTEN TO STDOUT
Hello World
and if you remove the echo Hello World line:
STDOUT HAS NOT BEEN TOUCHED
And if you really want to test that while running the script itself, you can do that, too :-)
#!/bin/bash
#FIRST ELSE
function echo_fl() {
TMP_SIZE=$(stat -f %z "$TMP")
if [ $TMP_SIZE -gt 0 ]; then
echo $2
else
echo $1
fi
}
TMP=$(mktemp /tmp/test_XXXXXX)
exec 3 "$TMP" # open tmp file as fd 3
exec 4>&1 # save current value of stdout as fd 4
exec >&3 # redirect stdout to fd 3 (tmp file)
# THE SCRIPT ITSELF
for f in fst snd trd; do
echo_fl "$(echo $f | tr a-z A-Z)" "$f"
done
# END
exec >&4 # restore save stdout
exec 3>&- # close tmp file
TMP_SIZE=$(stat -f %z "$TMP")
if [ $TMP_SIZE -gt 0 ]; then
echo "THE FOLLOWING DATA HAS BEEN WRITTEN TO STDOUT"
echo
cat "$TMP"
else
echo "STDOUT HAS NOT BEEN TOUCHED"
fi
rm "$TMP"
output is:
THE FOLLOWING DATA HAS BEEN WRITTEN TO STDOUT
FST
snd
trd
as you can see: Only the first line (FST) has caps on. That's what the echo_fl function does for you: If it's the first line of output, if echoes the first argument, if it's not it echoes the second argument :-)
It's hard to tell what you are trying to do here, but if your script is printing to stdout, you could simply pipe it to perl:
yourcommand | perl -pe 'if ($. == 1) { print "First line is: $_" }'
It all depends on what kind of changes you are attempting to do.
You cannot use the -f option with %z. The line TMP_SIZE=$(stat -f %z "$TMP") produces a long string that fails the test in if [ $TMP_SIZE -gt 0 ].

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