Edit multiple lines of text file using bash script

Question

I want to run a CFD simulation which is split into 2 steps. After the end of 1st step, I need to change a text file (related to boundary conditions - CFD terminology) and re-run the case further that is step 2.

Until now, I was doing this manually and lost lot of time to run the simulations overnight and over the weekends.

So, is it possible to edit a text file using a bash script?

Example: My file structure:

TestRun

Folder1

-- TextFile1

TestRun (main folder) consists of a sub-folder (Folder1) and Folder1 has a text file to be edited (TextFile1) by a bash script.

Content of TextFile1 (for example):

internalField nonuniform List<scalar>
7
(
0
1
2
3
4
5
6
);
boundaryField
{
 left
 {
  type fixedValue;
  value uniform 1;
 }
}

Now, the bash file shall change the file as:

internalField nonuniform List<scalar>
7
(
0
1
2
3
4
5
6
);
boundaryField
{
 left
 {
  type groovyBC;
  valueExpression "-pc";
  variables       "pc@left=pc;";
  value           uniform 0;
 }
}

Please note, that the number of lines to be edited is more than 1 and I do not know the line number in specific. I did come across few posts editing one specific line number using sed.

In my case, I must find the word "left" (as in vim: /left) and replace the lines inbetween '{ }' following the searched word.

Are there other instances of the word "left" anywhere, or can we assume it will only appear once, in that context? — Nic3500
– Nic3500, Commented Nov 24, 2017 at 15:22
@Nic3500 it occurs only once in the entire text file. That is a positive in my output text file. Forgot to mention!! — Saideep Pavuluri
– Saideep Pavuluri, Commented Nov 24, 2017 at 15:40

Nic3500 · Accepted Answer · 2017-11-24 15:40:40Z

1

Taking for granted the word "left" will only appear once in the file, you can use this script to perform the edit you want:

#!/bin/bash
#
insideleft='false'
while read line
do
    # using grep -c to make sure it matches even if there are spaces
    if [ $(echo $line | grep -c left) -eq 1 ]
    then
        insideleft='true'
        cat <<HEREDOC
  left
  {
    type groovyBC;
    valueExpression "-pc";
    variables       "pc@left=pc;";
    value           uniform 0;
  }
HEREDOC
    else
        if [ $insideleft == 'false' ]
        then
            echo $line
        else
            if [ $(echo $line | grep -c '}') -eq 1 ]
            then
                insideleft='false'
            fi
        fi
    fi
done <data

Basically, once the line "left" is found, output the new text, and loop over the input file lines until the }, closing the left section is found. I tried it with your input and output sample, it works fine.

Note: in the last line, "data" is the filename you want to modify.

answered Nov 24, 2017 at 15:40

Nic3500

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2 Comments

Saideep Pavuluri Over a year ago

How can I overwrite the file (ofcourse I will keep a copy of the original) as the bash script you shared outputs the text to the screen.

Nic3500 Over a year ago

ah yes. Lets say you call the script "edit.bash". Run it like this: edit.bash >output_file.txt.

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