[SOLVED] How to extract (read and delete) a line from file with a single command?

Issue

I would like to extract the first line from a file, read into a variable and delete right afterwards, with a single command. I know sed can read the first line as follows:

sed '1q' file.txt

or delete it as follows:

sed '1q;d' file.txt

but can I somehow do both with a single command? The reason for this is that multiple processes will be reading the first line of the file, and I want to minimize the chances of them getting the same line.

Solution

It's impossible.

Except you read the manpage, and have Gnu-sed:

echo -e {1..3}"\n" > input
cat input
1
 2
 3

sed -n '1p;2,$ Woutput' input 
1
cat output 
 2
 3

Explanation:

 sed -n '1p;2,$ Woutput' input 

• -n no output by default
• 1p; print line 1
• 2,$ from line 2 until $ last line
  • W (non posix) Write buffer to file

From the man page gnu sed:

   w filename
          Write the current pattern space to filename.

   W filename
          Write the first line of the current pattern space to filename.  This is a GNU extension.

However, reading and experimenting takes longer, than opening the file in a full blown office suite and deleting the line by hand, or invoking a text-to-speech framework and training it, to do the job.

It doesn't work if invoked in posix style:

sed -n --posix '1p;2,$ Woutput' input

And you still have the hard hanwork of renaming output to input again.

I didn't try to write to input in place, because that could damage my carefully crafted input file - try it on own risk:

sed -n '1p;2,$ Winput' input 

However, you might set up a filesystem notify job, which always rename freshly created output files to input again. But I fear you can't do it from within the sed command. Except ... (to be continued)

Answered By - user unknown

Answer Checked By - Willingham (WPSolving Volunteer)