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1

I have a command that runs forever and periodically output stuff until it gets killed by something else (similar to tail -f), and I want to make it so that whenever there's new output another command gets executed.

Caveats:

• I can't use Bash


• That command obviously isn't tail, it just behaves in a similar manner


• That command's output doesn't always come in lines, and I do not intend to execute that other command for each line of the output


• Polling is not an acceptable solution

shell-script shell

share|improve this question

asked 1 hour ago

Hwi417Hwi417

312

add a comment | 

1

I have a command that runs forever and periodically output stuff until it gets killed by something else (similar to tail -f), and I want to make it so that whenever there's new output another command gets executed.

Caveats:

• I can't use Bash


• That command obviously isn't tail, it just behaves in a similar manner


• That command's output doesn't always come in lines, and I do not intend to execute that other command for each line of the output


• Polling is not an acceptable solution

shell-script shell

share|improve this question

asked 1 hour ago

Hwi417Hwi417

312

add a comment | 

I have a command that runs forever and periodically output stuff until it gets killed by something else (similar to tail -f), and I want to make it so that whenever there's new output another command gets executed.

Caveats:

• I can't use Bash


• That command obviously isn't tail, it just behaves in a similar manner


• That command's output doesn't always come in lines, and I do not intend to execute that other command for each line of the output


• Polling is not an acceptable solution

shell-script shell

share|improve this question

asked 1 hour ago

Hwi417Hwi417

312

share|improve this question

asked 1 hour ago

Hwi417Hwi417

312

share|improve this question

asked 1 hour ago

Hwi417Hwi417

312

asked 1 hour ago

Hwi417Hwi417

312

asked 1 hour ago

Hwi417Hwi417

312

1 Answer
1

active

oldest

votes

2

Here is an example of using dd to trigger a shell command whenever something is read from stdin, whether terminated by newline or not:

 printf %s not-nl-terminated
 sleep 1
 printf '%sn' nl-terminated
 sleep 1
 printf 'other1junk'
 | 
while : ; do
 input=$(dd bs=1G count=1 2>/dev/null)
 [ "$input" ] || break
 printf 'input of size %dn' "$#input" 
done

will give

input of size 17
input of size 13
input of size 10

This snippet from the standard spec may help understand dd's behavior when bs= is used explicily:

If the bs= expr operand is
specified and no conversions other than sync, noerror, or notrunc
are requested, the data returned from each input block shall be
written as a separate output block; if the read returns less than a
full block and the sync conversion is not specified, the resulting
output block shall be the same size as the input block. If the bs=
expr operand is not specified, or a conversion other than sync,
noerror, or notrunc is requested, the input shall be processed and
collected into full-sized output blocks until the end of the input
is reached.

Note:

If the input data may contain NUL bytes, you may want something like

input=$(dd bs=1G count=1 2>/dev/null | tr '' _)

or parse the status lines that dd writes to stderr.

share|improve this answer

edited 3 mins ago

answered 35 mins ago

mosvymosvy

11.1k11340

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  did you mean printf '%sn' nl-terminated in stead of printf %s nl-terminated?
  
  – iruvar
  3 mins ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  yes, thanks for the correction.
  
  – mosvy
  49 secs ago
  

  
  
  
  

add a comment | 

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2

Here is an example of using dd to trigger a shell command whenever something is read from stdin, whether terminated by newline or not:

 printf %s not-nl-terminated
 sleep 1
 printf '%sn' nl-terminated
 sleep 1
 printf 'other1junk'
 | 
while : ; do
 input=$(dd bs=1G count=1 2>/dev/null)
 [ "$input" ] || break
 printf 'input of size %dn' "$#input" 
done

will give

input of size 17
input of size 13
input of size 10

This snippet from the standard spec may help understand dd's behavior when bs= is used explicily:

If the bs= expr operand is
specified and no conversions other than sync, noerror, or notrunc
are requested, the data returned from each input block shall be
written as a separate output block; if the read returns less than a
full block and the sync conversion is not specified, the resulting
output block shall be the same size as the input block. If the bs=
expr operand is not specified, or a conversion other than sync,
noerror, or notrunc is requested, the input shall be processed and
collected into full-sized output blocks until the end of the input
is reached.

Note:

If the input data may contain NUL bytes, you may want something like

input=$(dd bs=1G count=1 2>/dev/null | tr '' _)

or parse the status lines that dd writes to stderr.

share|improve this answer

edited 3 mins ago

answered 35 mins ago

mosvymosvy

11.1k11340

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  did you mean printf '%sn' nl-terminated in stead of printf %s nl-terminated?
  
  – iruvar
  3 mins ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  yes, thanks for the correction.
  
  – mosvy
  49 secs ago
  

  
  
  
  

add a comment | 

2

Here is an example of using dd to trigger a shell command whenever something is read from stdin, whether terminated by newline or not:

 printf %s not-nl-terminated
 sleep 1
 printf '%sn' nl-terminated
 sleep 1
 printf 'other1junk'
 | 
while : ; do
 input=$(dd bs=1G count=1 2>/dev/null)
 [ "$input" ] || break
 printf 'input of size %dn' "$#input" 
done

will give

input of size 17
input of size 13
input of size 10

This snippet from the standard spec may help understand dd's behavior when bs= is used explicily:

If the bs= expr operand is
specified and no conversions other than sync, noerror, or notrunc
are requested, the data returned from each input block shall be
written as a separate output block; if the read returns less than a
full block and the sync conversion is not specified, the resulting
output block shall be the same size as the input block. If the bs=
expr operand is not specified, or a conversion other than sync,
noerror, or notrunc is requested, the input shall be processed and
collected into full-sized output blocks until the end of the input
is reached.

Note:

If the input data may contain NUL bytes, you may want something like

input=$(dd bs=1G count=1 2>/dev/null | tr '' _)

or parse the status lines that dd writes to stderr.

share|improve this answer

edited 3 mins ago

answered 35 mins ago

mosvymosvy

11.1k11340

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  did you mean printf '%sn' nl-terminated in stead of printf %s nl-terminated?
  
  – iruvar
  3 mins ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  yes, thanks for the correction.
  
  – mosvy
  49 secs ago
  

  
  
  
  

add a comment | 

Here is an example of using dd to trigger a shell command whenever something is read from stdin, whether terminated by newline or not:

 printf %s not-nl-terminated
 sleep 1
 printf '%sn' nl-terminated
 sleep 1
 printf 'other1junk'
 | 
while : ; do
 input=$(dd bs=1G count=1 2>/dev/null)
 [ "$input" ] || break
 printf 'input of size %dn' "$#input" 
done

will give

input of size 17
input of size 13
input of size 10

This snippet from the standard spec may help understand dd's behavior when bs= is used explicily:

If the bs= expr operand is
specified and no conversions other than sync, noerror, or notrunc
are requested, the data returned from each input block shall be
written as a separate output block; if the read returns less than a
full block and the sync conversion is not specified, the resulting
output block shall be the same size as the input block. If the bs=
expr operand is not specified, or a conversion other than sync,
noerror, or notrunc is requested, the input shall be processed and
collected into full-sized output blocks until the end of the input
is reached.

Note:

If the input data may contain NUL bytes, you may want something like

input=$(dd bs=1G count=1 2>/dev/null | tr '' _)

or parse the status lines that dd writes to stderr.

share|improve this answer

edited 3 mins ago

answered 35 mins ago

mosvymosvy

11.1k11340

 printf %s not-nl-terminated
 sleep 1
 printf '%sn' nl-terminated
 sleep 1
 printf 'other1junk'
 | 
while : ; do
 input=$(dd bs=1G count=1 2>/dev/null)
 [ "$input" ] || break
 printf 'input of size %dn' "$#input" 
done

input of size 17
input of size 13
input of size 10

input=$(dd bs=1G count=1 2>/dev/null | tr '' _)

share|improve this answer

edited 3 mins ago

answered 35 mins ago

mosvymosvy

11.1k11340

answered 35 mins ago

mosvymosvy

11.1k11340

answered 35 mins ago

mosvymosvy

11.1k11340