mjhjmtu

I have a file that I want to update relatively frequently.

What needs to be done is that a specific part of the file would be changed to include some more lines. I am doing it with sed and a option:

So this works:

sed -i "/import:/a$IMPORTED" file 

This works as expected the first time. On the next re-runs this commands appends over and over the same lines.

How would I approach this so that I somehow remove everything I imported before I run it again?

E.g. use some following lines to anchor in the beginning that I know were not added automatically or something like that?

Your command:

sed "/import:/a$IMPORTED" infile

Is inserting the literal $IMPORTED text on linux and GNU sed. The problem is that the shell is interpreting the backslash before the $. You can either quote correctly:

sed '/import:/a'"$IMPORTED" infile

Or, split the value in its own script part:

sed -e '/import:/a' -e "$IMPORTED" infile

It is always better to single quote sed scripts.

To replace an old value with a new one that may contain an undefined number of lines, you need to

• 
  

  First: capture all lines in the pattern space:

  
  
  

  sed 'H;1h;$!d;x' infile

  
  


• 
  

  Set values to change:

  
  
  

  old='original'
  new='newvalue'

  
  


• 
  

  Remove old value (which must exist):

  
  
  

  sed -E -e 'H;1h;$!d;x' -e 's/(import:)(n)'"$old"'/1/g' infile

  
  


• 
  

  Add the new input:

  
  
  

  sed -E -e 'H;1h;$!d;x' -e 's/(import:)(n)'"$old"'/1/g' -e 's/(import:)/1n'"$new"'/g' infile

  
  

There is an corner issue in that if the variable $old contains a null value (empty) a newline will gets removed. To solve that we need:

sed -E -e 'H;1h;$!d;x' -e 's/(import:)'"$old:+"(\n)$old""'/1/g' -e 's/(import:)/1n'"$new"'/g' infile