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With a single input file that only contains comments (starting with #) and VARIABLE=value lines, is it possible to replace a value for a single variable if found and, otherwise, append the pair to the end of file if not found?

My current method works by deleting it in a first pass, then appending it to the end of the file in a second pass, but this method messes up the line ordering (and is also two different commands):

sed -r "/^FOOBAR=.*$/d"      -i samefile &&
sed -r "$ a\FOOBAR=newvalue" -i samefile

Is there anyway to do this, ie. keeping line order, in a single sed line? If some other utility (awk, ...) does this, I'ld take it over sed.

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5 Answers 5

up vote 3 down vote accepted

It's actually quite simple with sed: if a line matches just copy it to the hold space then substitute the value.
On the la$t line exchange hold space and pattern space then check if the latter is empty. If it's not empty, it means the substitution was already made so nothing to do. If it's empty, that means no match was found so replace the pattern space with the desired variable=value then append to the current line in the hold buffer. Finally, exchange again:

sed '/^FOOBAR=/{h;s/=.*/=newvalue/};${x;/^$/{s//FOOBAR=newvalue/;H};x}' infile

The above is gnu sed syntax. Portable:

sed '/^FOOBAR=/{
}' infile
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much more elegant, thank you – gogoud Oct 27 at 11:51

This can probably be shortened. It's not a single sed command and it also uses grep, but this seems to be basically what you're wanting. It's a single line, and it edits the file in-place (no temp files).

grep -q "^FOOBAR=" file && sed "s/^FOOBAR=.*/FOOBAR=newvalue/" -i file || 
    sed "$ a\FOOBAR=newvalue" -i file
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It's a bit easier in awk, although the "in place editing" is not automatic:

awk -v varname="FOOBAR" -v newval="newvalue" '
    BEGIN {FS = OFS = "="}
    $1 == varname {$2 = newval; found = 1}
    END {if (! found) {print varname, newval}}
' file > tempfile &&
mv tempfile file
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Based on the other answers, if what you want to do is replace a variable's value if that variable is present in the file and append it to the end of the file if it is not (which is not what your posted sed commands do), you could try this:

perl -ne '$c=1 if s/^FOOBAR=.*$/FOOBAR=newvalue/;  
             END{print "FOBAR=newvalue" unless $c==1}' file > tmpfile && 
mv tmpfile file
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to do this with perl and in-place this works fine for me:

grep ^FOOBAR= my.file && perl -i -ple "s/^FOOBAR=.+/FOOBAR=newvalue/g" my.file || echo FOOBAR=newvalue >> my.file
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This answer is bad in so many ways! grep and echo and perl instead of doing everything in perl? Also, writing to a file (>> my.file) as you're reading from it (grep my.file)? – vladr Nov 6 at 18:53

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