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1

I am trying to overwrite a file with command output, but only if there is any output. That is, I usually want

mycommand > myfile

but if this would overwrite myfile with empty data, I wish to retain the old version of myfile. I thought that something using ifne should be possible, a la

mycommand | ifne (cat > myfile) 

but that does not work ...

An indirect approach

mycommand | tee mytempfile | ifne mv mytempfile myfile

works, but I consider the use of that temp file unelegant.

Q: Why does my first idea not work? Can it be made work? Or is there another nice and perhaps completely different solution for my original problem?

bash

share|improve this question

asked 8 hours ago

Hagen von EitzenHagen von Eitzen

10855 bronze badges

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  "Redirecting to a separate file, checking the size of that file and possibly renaming it" seems pretty straight-forward to me. Why not use a temporary file?
  
  – Jeff Schaller♦
  8 hours ago
  

  
  
  
  

add a comment | 

1

I am trying to overwrite a file with command output, but only if there is any output. That is, I usually want

mycommand > myfile

but if this would overwrite myfile with empty data, I wish to retain the old version of myfile. I thought that something using ifne should be possible, a la

mycommand | ifne (cat > myfile) 

but that does not work ...

An indirect approach

mycommand | tee mytempfile | ifne mv mytempfile myfile

works, but I consider the use of that temp file unelegant.

Q: Why does my first idea not work? Can it be made work? Or is there another nice and perhaps completely different solution for my original problem?

bash

share|improve this question

asked 8 hours ago

Hagen von EitzenHagen von Eitzen

10855 bronze badges

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  "Redirecting to a separate file, checking the size of that file and possibly renaming it" seems pretty straight-forward to me. Why not use a temporary file?
  
  – Jeff Schaller♦
  8 hours ago
  

  
  
  
  

add a comment | 

I am trying to overwrite a file with command output, but only if there is any output. That is, I usually want

mycommand > myfile

but if this would overwrite myfile with empty data, I wish to retain the old version of myfile. I thought that something using ifne should be possible, a la

mycommand | ifne (cat > myfile) 

but that does not work ...

An indirect approach

mycommand | tee mytempfile | ifne mv mytempfile myfile

works, but I consider the use of that temp file unelegant.

Q: Why does my first idea not work? Can it be made work? Or is there another nice and perhaps completely different solution for my original problem?

bash

share|improve this question

asked 8 hours ago

Hagen von EitzenHagen von Eitzen

10855 bronze badges

mycommand > myfile

mycommand | ifne (cat > myfile) 

mycommand | tee mytempfile | ifne mv mytempfile myfile

share|improve this question

asked 8 hours ago

Hagen von EitzenHagen von Eitzen

10855 bronze badges

share|improve this question

asked 8 hours ago

Hagen von EitzenHagen von Eitzen

10855 bronze badges

asked 8 hours ago

Hagen von EitzenHagen von Eitzen

10855 bronze badges

asked 8 hours ago

Hagen von EitzenHagen von Eitzen

10855 bronze badges

2 Answers
2

active

oldest

votes

6

Your first approach works, you just need to give a command to ifne (see man ifne):

NAME
 ifne - Run command if the standard input is not empty

SYNOPSIS
 ifne [-n] command

DESCRIPTION
 ifne runs the following command if and only if the standard input is
 not empty.

So you need to give it a command to run. You're almost there, tee will work:

command | ifne tee myfile > /dev/null

If your command doesn't produce an enormous amount of data, if it's small enough to fit in a variable, you can also do:

var=$(mycommand)
[[ -n $var ]] && printf '%sn' "$var" > myfile

share|improve this answer

edited 7 hours ago

ilkkachu

67.2k1010 gold badges112112 silver badges193193 bronze badges

answered 8 hours ago

terdon♦terdon

141k3434 gold badges289289 silver badges467467 bronze badges

add a comment | 

4

The pedestrian solution:

tmpfile=$(mktemp)

mycommand >"$tmpfile"
if [ -s "$tmpfile" ]; then
 cat "$tmpfile" >myfile
fi

rm -f "$tmpfile"

That is, save the output to a temporary file, then test whether it's empty or not. If it's not empty, copy its contents over your file. In the end, delete the temporary file.

I'm using cat "$tmpfile" >myfile rather than cp "$tmpfile" myfile (or mv) to get the same effect as you would have gotten from mycommand >myfile, i.e. to truncate the existing file and preserve ownership and permissions.

If $TMPDIR (used by mktemp) is on a memory-mounted filesystem, then this would not write to disk other than possibly when writing to myfile. It would additionally be more portable than using ifne.

share|improve this answer

edited 5 hours ago

answered 8 hours ago

Kusalananda♦Kusalananda

160k1818 gold badges316316 silver badges503503 bronze badges

add a comment | 

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6

Your first approach works, you just need to give a command to ifne (see man ifne):

NAME
 ifne - Run command if the standard input is not empty

SYNOPSIS
 ifne [-n] command

DESCRIPTION
 ifne runs the following command if and only if the standard input is
 not empty.

So you need to give it a command to run. You're almost there, tee will work:

command | ifne tee myfile > /dev/null

If your command doesn't produce an enormous amount of data, if it's small enough to fit in a variable, you can also do:

var=$(mycommand)
[[ -n $var ]] && printf '%sn' "$var" > myfile

share|improve this answer

edited 7 hours ago

ilkkachu

67.2k1010 gold badges112112 silver badges193193 bronze badges

answered 8 hours ago

terdon♦terdon

141k3434 gold badges289289 silver badges467467 bronze badges

add a comment | 

6

Your first approach works, you just need to give a command to ifne (see man ifne):

NAME
 ifne - Run command if the standard input is not empty

SYNOPSIS
 ifne [-n] command

DESCRIPTION
 ifne runs the following command if and only if the standard input is
 not empty.

So you need to give it a command to run. You're almost there, tee will work:

command | ifne tee myfile > /dev/null

If your command doesn't produce an enormous amount of data, if it's small enough to fit in a variable, you can also do:

var=$(mycommand)
[[ -n $var ]] && printf '%sn' "$var" > myfile

share|improve this answer

edited 7 hours ago

ilkkachu

67.2k1010 gold badges112112 silver badges193193 bronze badges

answered 8 hours ago

terdon♦terdon

141k3434 gold badges289289 silver badges467467 bronze badges

add a comment | 

Your first approach works, you just need to give a command to ifne (see man ifne):

NAME
 ifne - Run command if the standard input is not empty

SYNOPSIS
 ifne [-n] command

DESCRIPTION
 ifne runs the following command if and only if the standard input is
 not empty.

So you need to give it a command to run. You're almost there, tee will work:

command | ifne tee myfile > /dev/null

If your command doesn't produce an enormous amount of data, if it's small enough to fit in a variable, you can also do:

var=$(mycommand)
[[ -n $var ]] && printf '%sn' "$var" > myfile

share|improve this answer

edited 7 hours ago

ilkkachu

67.2k1010 gold badges112112 silver badges193193 bronze badges

answered 8 hours ago

terdon♦terdon

141k3434 gold badges289289 silver badges467467 bronze badges

NAME
 ifne - Run command if the standard input is not empty

SYNOPSIS
 ifne [-n] command

DESCRIPTION
 ifne runs the following command if and only if the standard input is
 not empty.

command | ifne tee myfile > /dev/null

var=$(mycommand)
[[ -n $var ]] && printf '%sn' "$var" > myfile

share|improve this answer

edited 7 hours ago

ilkkachu

67.2k1010 gold badges112112 silver badges193193 bronze badges

answered 8 hours ago

terdon♦terdon

141k3434 gold badges289289 silver badges467467 bronze badges

edited 7 hours ago

ilkkachu

67.2k1010 gold badges112112 silver badges193193 bronze badges

edited 7 hours ago

ilkkachu

67.2k1010 gold badges112112 silver badges193193 bronze badges

answered 8 hours ago

terdon♦terdon

141k3434 gold badges289289 silver badges467467 bronze badges

answered 8 hours ago

terdon♦terdon

141k3434 gold badges289289 silver badges467467 bronze badges

4

The pedestrian solution:

tmpfile=$(mktemp)

mycommand >"$tmpfile"
if [ -s "$tmpfile" ]; then
 cat "$tmpfile" >myfile
fi

rm -f "$tmpfile"

That is, save the output to a temporary file, then test whether it's empty or not. If it's not empty, copy its contents over your file. In the end, delete the temporary file.

I'm using cat "$tmpfile" >myfile rather than cp "$tmpfile" myfile (or mv) to get the same effect as you would have gotten from mycommand >myfile, i.e. to truncate the existing file and preserve ownership and permissions.

If $TMPDIR (used by mktemp) is on a memory-mounted filesystem, then this would not write to disk other than possibly when writing to myfile. It would additionally be more portable than using ifne.

share|improve this answer

edited 5 hours ago

answered 8 hours ago

Kusalananda♦Kusalananda

160k1818 gold badges316316 silver badges503503 bronze badges

add a comment | 

4

The pedestrian solution:

tmpfile=$(mktemp)

mycommand >"$tmpfile"
if [ -s "$tmpfile" ]; then
 cat "$tmpfile" >myfile
fi

rm -f "$tmpfile"

That is, save the output to a temporary file, then test whether it's empty or not. If it's not empty, copy its contents over your file. In the end, delete the temporary file.

I'm using cat "$tmpfile" >myfile rather than cp "$tmpfile" myfile (or mv) to get the same effect as you would have gotten from mycommand >myfile, i.e. to truncate the existing file and preserve ownership and permissions.

If $TMPDIR (used by mktemp) is on a memory-mounted filesystem, then this would not write to disk other than possibly when writing to myfile. It would additionally be more portable than using ifne.

share|improve this answer

edited 5 hours ago

answered 8 hours ago

Kusalananda♦Kusalananda

160k1818 gold badges316316 silver badges503503 bronze badges

add a comment | 

The pedestrian solution:

tmpfile=$(mktemp)

mycommand >"$tmpfile"
if [ -s "$tmpfile" ]; then
 cat "$tmpfile" >myfile
fi

rm -f "$tmpfile"

That is, save the output to a temporary file, then test whether it's empty or not. If it's not empty, copy its contents over your file. In the end, delete the temporary file.

I'm using cat "$tmpfile" >myfile rather than cp "$tmpfile" myfile (or mv) to get the same effect as you would have gotten from mycommand >myfile, i.e. to truncate the existing file and preserve ownership and permissions.

If $TMPDIR (used by mktemp) is on a memory-mounted filesystem, then this would not write to disk other than possibly when writing to myfile. It would additionally be more portable than using ifne.

share|improve this answer

edited 5 hours ago

answered 8 hours ago

Kusalananda♦Kusalananda

160k1818 gold badges316316 silver badges503503 bronze badges

tmpfile=$(mktemp)

mycommand >"$tmpfile"
if [ -s "$tmpfile" ]; then
 cat "$tmpfile" >myfile
fi

rm -f "$tmpfile"

share|improve this answer

edited 5 hours ago

answered 8 hours ago

Kusalananda♦Kusalananda

160k1818 gold badges316316 silver badges503503 bronze badges

answered 8 hours ago

Kusalananda♦Kusalananda

160k1818 gold badges316316 silver badges503503 bronze badges

answered 8 hours ago

Kusalananda♦Kusalananda

160k1818 gold badges316316 silver badges503503 bronze badges