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    <pre><code>header="/name/of/file/containing/header" for file in "$@" do cat "$header" "$file" &gt; /tmp/xx.$$ mv /tmp/xx.$$ "$file" done </code></pre> <p>You might prefer to locate the temporary file on the same file system as the file you are editing, but anything that requires inserting data at the front of the file is going to end up working very close to this. If you are going to be doing this all day, every day, you might assemble something a little slicker, but the chances are the savings will be minuscule (fractions of a second per file).</p> <p>If you really, really must use <code>sed</code>, then I suppose you could use:</p> <pre><code>header="/name/of/file/containing/header" for file in "$@" do sed -e "0r $header" "$file" &gt; /tmp/xx.$$ mv /tmp/xx.$$ "$file" done </code></pre> <p>The command reads the content of header 'after' line 0 (before line 1), and then everything else is passed through unchanged. This isn't as swift as <code>cat</code> though.</p> <p>An analogous construct using <code>awk</code> is:</p> <pre><code>header="/name/of/file/containing/header" for file in "$@" do awk '{print}' "$header" "$file" &gt; /tmp/xx.$$ mv /tmp/xx.$$ "$file" done </code></pre> <p>This simply prints each input line on the output; again, not as swift as <code>cat</code>.</p> <p>One more advantage of <code>cat</code> over <code>sed</code> or <code>awk</code>; <code>cat</code> will work even if the big files are mainly binary data (it is oblivious to the content of the files). Both <code>sed</code> and <code>awk</code> are designed to handle data split into lines; while modern versions will probably handle even binary data fairly well, it is not what they are designed for. </p>
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    1. USJonathan Leffler
    1. USJonathan Leffler
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    1. CO+1, excellent answer
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      1. USglenn jackman
    2. COThat is so cool answer. I have learned much from your directions. Thanks a lot.
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      1. USjianfeng.mao
    3. CO@Jonathan, I also need to add header lines to an existing large file. In my case, the header lines correspond to numbers that are stored in bash variables. How, could I use `awk/sed` to do it?. I did sth. like: `awk -v param1="$param1" -v param2="$param2" 'BEGIN{print "description1"; print param1 " " param2; print "description2"}{print}' test.data > ${tmpFile}`. In files larger than 2GB, I get the following error: `awk: cannot open test.data (Value too large for defined data type)`. Do you have any suggestions on how to deal with large files?
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