Can I capture the output of another process that I started?

I'm currently using > /dev/null & to have Perl script A run Perl script B totally independently, and it works fine. Script B runs without throwing back any output and stays alive when Script A ends, even when my terminal session ends.

Not saying I need it, but is there a way to recapture its output if I wanted to?

Thanks

asked Nov 19 '18 at 18:50

Sergio D. Caplan

199110

Redirect to a file or named pipe instead of /dev/null? With the pipe be cautious, it will fill up and then block by default if not read from.

– tink
Nov 19 '18 at 18:52

@tink Your answer scared me, does > /dev/null ever fill up?

– Sergio D. Caplan
Nov 19 '18 at 18:54

3

No, it doesn't. That's the beauty of it. But what goes to /dev/null stays in /dev/null ;)

– tink
Nov 19 '18 at 18:56

If you want to start a program in the background, look at the nohup command-line utility. It

– ikegami
Nov 19 '18 at 23:57

add a comment |

Not saying I need it, but is there a way to recapture its output if I wanted to?

Thanks

asked Nov 19 '18 at 18:50

Sergio D. Caplan

199110

Redirect to a file or named pipe instead of /dev/null? With the pipe be cautious, it will fill up and then block by default if not read from.

– tink
Nov 19 '18 at 18:52

@tink Your answer scared me, does > /dev/null ever fill up?

– Sergio D. Caplan
Nov 19 '18 at 18:54

3

No, it doesn't. That's the beauty of it. But what goes to /dev/null stays in /dev/null ;)

– tink
Nov 19 '18 at 18:56

If you want to start a program in the background, look at the nohup command-line utility. It

– ikegami
Nov 19 '18 at 23:57

add a comment |

Not saying I need it, but is there a way to recapture its output if I wanted to?

Thanks

asked Nov 19 '18 at 18:50

Sergio D. Caplan

199110

Not saying I need it, but is there a way to recapture its output if I wanted to?

Thanks

linux perl system

asked Nov 19 '18 at 18:50

Sergio D. Caplan

199110

asked Nov 19 '18 at 18:50

Sergio D. Caplan

199110

asked Nov 19 '18 at 18:50

Sergio D. Caplan

199110

asked Nov 19 '18 at 18:50

Sergio D. Caplan

199110

asked Nov 19 '18 at 18:50

Sergio D. Caplan

199110

Redirect to a file or named pipe instead of /dev/null? With the pipe be cautious, it will fill up and then block by default if not read from.

– tink
Nov 19 '18 at 18:52

@tink Your answer scared me, does > /dev/null ever fill up?

– Sergio D. Caplan
Nov 19 '18 at 18:54

3

No, it doesn't. That's the beauty of it. But what goes to /dev/null stays in /dev/null ;)

– tink
Nov 19 '18 at 18:56

If you want to start a program in the background, look at the nohup command-line utility. It

– ikegami
Nov 19 '18 at 23:57

add a comment |

Redirect to a file or named pipe instead of /dev/null? With the pipe be cautious, it will fill up and then block by default if not read from.

– tink
Nov 19 '18 at 18:52

@tink Your answer scared me, does > /dev/null ever fill up?

– Sergio D. Caplan
Nov 19 '18 at 18:54

3

No, it doesn't. That's the beauty of it. But what goes to /dev/null stays in /dev/null ;)

– tink
Nov 19 '18 at 18:56

If you want to start a program in the background, look at the nohup command-line utility. It

– ikegami
Nov 19 '18 at 23:57

Redirect to a file or named pipe instead of /dev/null? With the pipe be cautious, it will fill up and then block by default if not read from.

– tink
Nov 19 '18 at 18:52

@tink Your answer scared me, does > /dev/null ever fill up?

– Sergio D. Caplan
Nov 19 '18 at 18:54

No, it doesn't. That's the beauty of it. But what goes to /dev/null stays in /dev/null ;)

– tink
Nov 19 '18 at 18:56

If you want to start a program in the background, look at the nohup command-line utility. It

– ikegami
Nov 19 '18 at 23:57

add a comment |

1 Answer
1

active

oldest

votes

Your code framework may look like this:

#!/usr/bin/perl

# I'm a.pl

#...

system "b.pl > ~/b.out &";

while (1)

{

    my $time = localtime;

    my ($fsize, $mtime) = (stat "/var/log/syslog")[7,9];

    print "syslog: size=$fsize, mtime=$mtime at $timen";

    sleep 60;

}

while the b.pl may look like:

#!/usr/bin/perl

# I'm b.pl

while (1)

{

    my $time = localtime;

    my $fsize_a = (stat "/var/log/auth.log")[7];

    my $fsize_s = (stat "/var/log/syslog")[7];

    print "fsize: syslog=$fsize_s auth.log=$fsize_a at $timen";

    sleep 60;

}

a.pl and b.pl do their job independently.

b.pl is called by a.pl as a background job, which sends its output to b.out(won't mess up the screen of a.pl)

You can read b.out from some other terminal, or after a.pl is finished(or when a.pl is put to background temporarily)

About terminating the two scripts:

`ctrl-c` for a.pl

`killall b.pl` for b.pl

Note:

b.pl will never terminate even when you terminate your terminal (Assumed that your terminal is run as a desktop application), so you don't need the `nohup` command to help. (Perhaps only useful in console)

If your b.pl may spit out error messages from time to time, then you still need to deal with its stderr. It's left as your homework.

answered Nov 20 '18 at 11:11

Charles Jie

707

I am using /dev/null as I have no intention of looking at the output, I just mean if I wanted to can I redirect it to something else, then put it back? I don't have to do this, am just curious if I can,

– Sergio D. Caplan
Nov 20 '18 at 19:47

@PlasticProgrammer - You don't have to ALWAYS redirect b.pl's stdout to b.out. You can make an option to turn it on, while using > /dev/null as default redirection. However, you have to decide how to redirect before you run a.pl. It's not trivial job to switch in midway. And you can not recover anything already sent to /dev/null.

– Charles Jie
Nov 21 '18 at 2:24

add a comment |

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1 Answer
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active

oldest

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1 Answer
1

active

oldest

votes

Your code framework may look like this:

#!/usr/bin/perl

# I'm a.pl

#...

system "b.pl > ~/b.out &";

while (1)

{

    my $time = localtime;

    my ($fsize, $mtime) = (stat "/var/log/syslog")[7,9];

    print "syslog: size=$fsize, mtime=$mtime at $timen";

    sleep 60;

}

while the b.pl may look like:

#!/usr/bin/perl

# I'm b.pl

while (1)

{

    my $time = localtime;

    my $fsize_a = (stat "/var/log/auth.log")[7];

    my $fsize_s = (stat "/var/log/syslog")[7];

    print "fsize: syslog=$fsize_s auth.log=$fsize_a at $timen";

    sleep 60;

}

a.pl and b.pl do their job independently.

b.pl is called by a.pl as a background job, which sends its output to b.out(won't mess up the screen of a.pl)

You can read b.out from some other terminal, or after a.pl is finished(or when a.pl is put to background temporarily)

About terminating the two scripts:

`ctrl-c` for a.pl

`killall b.pl` for b.pl

Note:

b.pl will never terminate even when you terminate your terminal (Assumed that your terminal is run as a desktop application), so you don't need the `nohup` command to help. (Perhaps only useful in console)

If your b.pl may spit out error messages from time to time, then you still need to deal with its stderr. It's left as your homework.

answered Nov 20 '18 at 11:11

Charles Jie

707

I am using /dev/null as I have no intention of looking at the output, I just mean if I wanted to can I redirect it to something else, then put it back? I don't have to do this, am just curious if I can,

– Sergio D. Caplan
Nov 20 '18 at 19:47

@PlasticProgrammer - You don't have to ALWAYS redirect b.pl's stdout to b.out. You can make an option to turn it on, while using > /dev/null as default redirection. However, you have to decide how to redirect before you run a.pl. It's not trivial job to switch in midway. And you can not recover anything already sent to /dev/null.

– Charles Jie
Nov 21 '18 at 2:24

add a comment |

Your code framework may look like this:

#!/usr/bin/perl

# I'm a.pl

#...

system "b.pl > ~/b.out &";

while (1)

{

    my $time = localtime;

    my ($fsize, $mtime) = (stat "/var/log/syslog")[7,9];

    print "syslog: size=$fsize, mtime=$mtime at $timen";

    sleep 60;

}

while the b.pl may look like:

#!/usr/bin/perl

# I'm b.pl

while (1)

{

    my $time = localtime;

    my $fsize_a = (stat "/var/log/auth.log")[7];

    my $fsize_s = (stat "/var/log/syslog")[7];

    print "fsize: syslog=$fsize_s auth.log=$fsize_a at $timen";

    sleep 60;

}

a.pl and b.pl do their job independently.

b.pl is called by a.pl as a background job, which sends its output to b.out(won't mess up the screen of a.pl)

You can read b.out from some other terminal, or after a.pl is finished(or when a.pl is put to background temporarily)

About terminating the two scripts:

`ctrl-c` for a.pl

`killall b.pl` for b.pl

Note:

b.pl will never terminate even when you terminate your terminal (Assumed that your terminal is run as a desktop application), so you don't need the `nohup` command to help. (Perhaps only useful in console)

If your b.pl may spit out error messages from time to time, then you still need to deal with its stderr. It's left as your homework.

answered Nov 20 '18 at 11:11

Charles Jie

707

I am using /dev/null as I have no intention of looking at the output, I just mean if I wanted to can I redirect it to something else, then put it back? I don't have to do this, am just curious if I can,

– Sergio D. Caplan
Nov 20 '18 at 19:47

@PlasticProgrammer - You don't have to ALWAYS redirect b.pl's stdout to b.out. You can make an option to turn it on, while using > /dev/null as default redirection. However, you have to decide how to redirect before you run a.pl. It's not trivial job to switch in midway. And you can not recover anything already sent to /dev/null.

– Charles Jie
Nov 21 '18 at 2:24

add a comment |

Your code framework may look like this:

#!/usr/bin/perl

# I'm a.pl

#...

system "b.pl > ~/b.out &";

while (1)

{

    my $time = localtime;

    my ($fsize, $mtime) = (stat "/var/log/syslog")[7,9];

    print "syslog: size=$fsize, mtime=$mtime at $timen";

    sleep 60;

}

while the b.pl may look like:

#!/usr/bin/perl

# I'm b.pl

while (1)

{

    my $time = localtime;

    my $fsize_a = (stat "/var/log/auth.log")[7];

    my $fsize_s = (stat "/var/log/syslog")[7];

    print "fsize: syslog=$fsize_s auth.log=$fsize_a at $timen";

    sleep 60;

}

a.pl and b.pl do their job independently.

b.pl is called by a.pl as a background job, which sends its output to b.out(won't mess up the screen of a.pl)

You can read b.out from some other terminal, or after a.pl is finished(or when a.pl is put to background temporarily)

About terminating the two scripts:

`ctrl-c` for a.pl

`killall b.pl` for b.pl

Note:

b.pl will never terminate even when you terminate your terminal (Assumed that your terminal is run as a desktop application), so you don't need the `nohup` command to help. (Perhaps only useful in console)

If your b.pl may spit out error messages from time to time, then you still need to deal with its stderr. It's left as your homework.

answered Nov 20 '18 at 11:11

Charles Jie

707

Your code framework may look like this:

#!/usr/bin/perl

# I'm a.pl

#...

system "b.pl > ~/b.out &";

while (1)

{

    my $time = localtime;

    my ($fsize, $mtime) = (stat "/var/log/syslog")[7,9];

    print "syslog: size=$fsize, mtime=$mtime at $timen";

    sleep 60;

}

while the b.pl may look like:

#!/usr/bin/perl

# I'm b.pl

while (1)

{

    my $time = localtime;

    my $fsize_a = (stat "/var/log/auth.log")[7];

    my $fsize_s = (stat "/var/log/syslog")[7];

    print "fsize: syslog=$fsize_s auth.log=$fsize_a at $timen";

    sleep 60;

}

a.pl and b.pl do their job independently.

b.pl is called by a.pl as a background job, which sends its output to b.out(won't mess up the screen of a.pl)

You can read b.out from some other terminal, or after a.pl is finished(or when a.pl is put to background temporarily)

About terminating the two scripts:

`ctrl-c` for a.pl

`killall b.pl` for b.pl

Note:

b.pl will never terminate even when you terminate your terminal (Assumed that your terminal is run as a desktop application), so you don't need the `nohup` command to help. (Perhaps only useful in console)

If your b.pl may spit out error messages from time to time, then you still need to deal with its stderr. It's left as your homework.

answered Nov 20 '18 at 11:11

Charles Jie

707

answered Nov 20 '18 at 11:11

Charles Jie

707

answered Nov 20 '18 at 11:11

Charles Jie

707

answered Nov 20 '18 at 11:11

Charles Jie

707

I am using /dev/null as I have no intention of looking at the output, I just mean if I wanted to can I redirect it to something else, then put it back? I don't have to do this, am just curious if I can,

– Sergio D. Caplan
Nov 20 '18 at 19:47

@PlasticProgrammer - You don't have to ALWAYS redirect b.pl's stdout to b.out. You can make an option to turn it on, while using > /dev/null as default redirection. However, you have to decide how to redirect before you run a.pl. It's not trivial job to switch in midway. And you can not recover anything already sent to /dev/null.

– Charles Jie
Nov 21 '18 at 2:24

add a comment |

I am using /dev/null as I have no intention of looking at the output, I just mean if I wanted to can I redirect it to something else, then put it back? I don't have to do this, am just curious if I can,

– Sergio D. Caplan
Nov 20 '18 at 19:47

@PlasticProgrammer - You don't have to ALWAYS redirect b.pl's stdout to b.out. You can make an option to turn it on, while using > /dev/null as default redirection. However, you have to decide how to redirect before you run a.pl. It's not trivial job to switch in midway. And you can not recover anything already sent to /dev/null.

– Charles Jie
Nov 21 '18 at 2:24

I am using /dev/null as I have no intention of looking at the output, I just mean if I wanted to can I redirect it to something else, then put it back? I don't have to do this, am just curious if I can,

– Sergio D. Caplan
Nov 20 '18 at 19:47

@PlasticProgrammer - You don't have to ALWAYS redirect b.pl's stdout to b.out. You can make an option to turn it on, while using > /dev/null as default redirection. However, you have to decide how to redirect before you run a.pl. It's not trivial job to switch in midway. And you can not recover anything already sent to /dev/null.

– Charles Jie
Nov 21 '18 at 2:24

add a comment |

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