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awk '/^nameserver/ && !modif { printf("nameserver 127.0.0.1\n"); modif=1 } {print}' testfile.txt It is displaying output but I want to write the output to same file. In my example testfile.txt.
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To print a blank line, use print "" , where "" is the empty string. To print a fixed piece of text, use a string constant, such as "Don't Panic" , as one item. If you forget to use the double-quote characters, your text is taken as an awk expression, and you will probably get an error.
Use command substitution to assign the output of a command to a variable. The syntax is: var1=$(command) . Show activity on this post. The awk command needs to have some input!
If you notice awk 'print $1' prints first word of each line. If you use $3, it will print 3rd word of each line.
Not possible per se. You need a second temporary file because you can't read and overwrite the same file. Something like:
awk '(PROGRAM)' testfile.txt > testfile.tmp && mv testfile.tmp testfile.txt The mktemp program is useful for generating unique temporary file names.
There are some hacks for avoiding a temporary file, but they rely mostly on caching and read buffers and quickly get unstable for larger files.
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Since GNU Awk 4.1.0, there is the "inplace" extension, so you can do:
$ gawk -i inplace '{ gsub(/foo/, "bar") }; { print }' file1 file2 file3 To keep a backup copy of original files, try this:
$ gawk -i inplace -v INPLACE_SUFFIX=.bak '{ gsub(/foo/, "bar") } > { print }' file1 file2 file3 This can be used to simulate the GNU sed -i feature.
See: Enabling In-Place File Editing
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