How to alter a variable with a shell script using sed

Question

I am attempting to alter the values in a .dat file for a model I am running. The issue is that the code cannot simply be changed with the sed -i 's/old_text/new_text/'

The .dat file I am attempting to change looks like

0.000E-00          !Argon
7.956E-03          !Methane
0.000E-00          !Ethane
1.945E-03          !Carbon Dioxide
9.901E-01          !Nitrogen
1.000E-40          !Oxygen
0.000E-00          !Hydrogen
0.000E-00          !Nitrogen Dioxide
22                 !Tropopause layer

I am attempting to alter the values for methane, nitrogen, and carbon dioxide, however I cannot simply tell it to replace '7.965E-03 !Methane' because after the first time I run the shell script, that value will change.

I have attempted to replace !Methane with $'variable name' !Methane, but that does not solve the issue of removing the number, nor will it work after the first time I compile the script.

The current script is

#file=atmos-master-copy1/CLIMA/IO/mixing_ratios.dat                                                                                        
#sed -i 's/Methane.*/'$muCH4_1'    !Methane/' $file

But I dont know how to make it start by altering the contents at the beginning of the file.

I am trying to replace that line with $muCH4_1 !Methane, where $muCH4_1 is a variable the shell script takes from a different .dat file — jlan160
– jlan160, Commented Jul 10, 2023 at 5:29
Do you want $muCH4_1 to appear in the output or do you want a value that the variable $muCH4_1 holds to be in the output? Please show the expected output — Ted Lyngmo
– Ted Lyngmo, Commented Jul 10, 2023 at 5:38
The value that is held in '$muCH4_1' would appear in the output, not the name of the variable. The output would be the same format as in the current snippet of the .dat file, it would just be a different value — jlan160
– jlan160, Commented Jul 10, 2023 at 5:42

Renaud Pacalet · Accepted Answer · 2023-07-11 07:00:47Z

2

If you want to replace the value for !Methane with the value held by the variable $muCH4_1, then:

sed -Ei 's/^\S+\s+(!Methane)$/'"$muCH4_1"' \1/' "$file"

^ - Start of line anchor
\S+ - One or more non-whitespace characters
\s+ - One or more whitespace characters
(!Methane) - Capture !Methane into capture group 1
$ - End of line anchor

Substitute with

'"$muCH4_1"' - The value held by the variable muCH4_1
- A literal space
\1 - !Methane from capture group 1

Note: For the second column to line up just like in the original file, muCH4_1 must be padded with spaces to the right. If that's what you want, you can use printf to do the padding:

sed -Ei 's/^\S+\s+(!Methane)$/'"$(printf "%-18s" "$muCH4_1")"' \1/' "$file"

edited Jul 11, 2023 at 7:00

Renaud Pacalet

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answered Jul 10, 2023 at 5:43

Ted Lyngmo

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2 Comments

Ted Lyngmo Over a year ago

@jlan160 Oups, I accidentally made it -iE instead of -Ei, try now :-)

jlan160 Over a year ago

Yes, this worked, thank you! One issue that also may have contributed was that there is a single space before each number in the .dat file, which I needed to delete in order for this to work, but it solved the problem, thanks.

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