Combining echo and cat on Unix

Question

Really simple question, how do I combine echo and cat in the shell, I'm trying to write the contents of a file into another file with a prepended string?

If /tmp/file looks like this:

this is a test

I want to run this:

echo "PREPENDED STRING"
cat /tmp/file | sed 's/test/test2/g' > /tmp/result

so that /tmp/result looks like this:

PREPENDED STRINGthis is a test2

Thanks.

Douglas · Accepted Answer · 2010-06-09T12:09:35.640

50

This should work:

echo "PREPENDED STRING" | cat - /tmp/file | sed 's/test/test2/g' > /tmp/result

edited Jun 09 '10 at 12:09

answered Jun 09 '10 at 11:47

Douglas

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I like this simplicity, but for completeness should have the -n flag on echo as mentionned in another answer. – Dan Jun 09 '10 at 12:10
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For anyone else wondering about the `-` in the `cat` args, from the man pages: `With no FILE, or when FILE is -, read standard input.` – SooDesuNe Jan 13 '12 at 14:33
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The `-n` flag doesn't work on all variants of `echo`, but that is mostly a historical note. – reinierpost Apr 26 '13 at 08:05
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I've always been using `{ echo "PREPENDED STRING"; cat /tmp/file; } |` but this is much more elegant, thank you! – reinierpost Apr 26 '13 at 08:06
Where the -n flag on echo is needed to suppress the trailing newline. – hertzsprung Feb 13 '19 at 13:10
Note that the ``-n`` must come before the string: ``echo -n "mystring" | cat - astrophy`` – Juha Untinen Apr 03 '19 at 12:02

score 11 · Answer 2 · edited Jun 09 '10 at 13:00

11

Try:

(printf "%s" "PREPENDED STRING"; sed 's/test/test2/g' /tmp/file) >/tmp/result

The parentheses run the command(s) inside a subshell, so that the output looks like a single stream for the >/tmp/result redirect.

edited Jun 09 '10 at 13:00

pixelbeat

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answered Jun 09 '10 at 11:47

Greg Hewgill

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score 2 · Answer 3 · answered Jun 09 '10 at 11:55

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Or just use only sed

  sed -e 's/test/test2/g
s/^/PREPEND STRING/' /tmp/file > /tmp/result

answered Jun 09 '10 at 11:55

Iacopo

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for "one-liner-ness" use 2 `-e`'s: `sed -e 's/test/test2/g' -e 's/^/PREPEND STRING/' ...` – glenn jackman Jun 09 '10 at 13:45
This of course will prepend the string to every line of the input file. – glenn jackman Jun 09 '10 at 13:55
Which may be what's wanted. If not: `sed '1s/^/PREPENDED STRING/; s/test/test2/g' /tmp/file > /tmp/result` – Dennis Williamson Jun 09 '10 at 14:18

score 2 · Answer 4 · answered Jun 09 '10 at 13:12

2

Or also:

{ echo "PREPENDED STRING" ; cat /tmp/file | sed 's/test/test2/g' } > /tmp/result

answered Jun 09 '10 at 13:12

mathk

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Useless use of `cat`, e.g. this would also work: `{ echo "PREPENDED STRING" ; sed 's/test/test2/g'; } < /tmp/file > /tmp/result` – reinierpost Apr 26 '13 at 08:08

score 1 · Answer 5 · answered Apr 26 '13 at 08:01

If this is ever for sending an e-mail, remember to use CRLF line-endings, like so:

echo -e 'To: cookimonster@kibo.org\r' | cat - body-of-message \
| sed 's/test/test2/g' | sendmail -t

Notice the -e-flag and the \r inside the string.

Setting To: this way in a loop gives you the world's simplest bulk-mailer.

glenn jackman · Answer 6 · 2010-06-09T13:58:05.303

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Another option: assuming the prepended string should only appear once and not for every line:

gawk 'BEGIN {printf("%s","PREPEND STRING")} {gsub(/test/, "&2")} 1' in > out

edited Jun 09 '10 at 13:58

answered Jun 09 '10 at 13:50

glenn jackman

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Combining echo and cat on Unix

6 Answers6