Prepending Data to a File
Problem
You want to prepend data to an existing file, for example to add a header after sorting.
Solution
Use cat in a subshell.
temp_file="temp.$RANDOM$RANDOM$$" (echo 'static header line1'; cat data_file) > $temp_file \ && cat $temp_file > data_file rm $temp_file unset temp_file
You could also use sed, the streaming editor. To prepend static text, note that back-slash escape sequences are expanded in GNU sed but not in some other versions. Also, under some shells the trailing backslashes may need to be doubled:
# Any sed, e.g., Solaris 10 /usr/bin/sed $ sed -e '1i\ >static header line1
> ' data_file static header line1 1 foo 2 bar 3 baz $ sed -e '1i\ >static header line1\
>static header line2
> ' data_file static header line1 static header line2 1 foo 2 bar 3 baz # GNU sed $ sed -e '1istatic header line1\nstatic header line2
' data_file static header line1 static header line2 1 foo 2 bar 3 baz
To prepend an existing file:
$ sed -e '$r data_file' header_file Header Line1 Header Line2 1 foo 2 bar 3 baz
Discussion
This one seems to be a love/hate kind of thing. People either love the cat solution or love the sed solution, but not both. The cat version is probably faster and simpler, the sed solution is arguably more flexible.
You can also store a sed script in a file,
instead of leaving it on the command line. And of course you would
usually redirect the output into a new file, like sed -e '$r
data' header>
new_file
, but note that will change the file’s inode and ...
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