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.

Nicobarese

Answer 1

This should work:

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

Charnel

I like this simplicity, but for completeness should have the -n flag on echo as mentionned in another answer. – Nicobarese

For anyone else wondering about the - in the cat args, from the man pages: With no FILE, or when FILE is -, read standard input. – Bibliotaph

The -n flag doesn't work on all variants of echo, but that is mostly a historical note. – Dianthus

I've always been using { echo "PREPENDED STRING"; cat /tmp/file; } | but this is much more elegant, thank you! – Dianthus

Where the -n flag on echo is needed to suppress the trailing newline. – Scriptural

Note that the -n must come before the string: echo -n "mystring" | cat - astrophy – Reckon

Answer 2

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.

Educt

Answer 3

Or just use only sed

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

Penknife

for "one-liner-ness" use 2 -e's: sed -e 's/test/test2/g' -e 's/^/PREPEND STRING/' ... – Pilpul

This of course will prepend the string to every line of the input file. – Pilpul

Which may be what's wanted. If not: sed '1s/^/PREPENDED STRING/; s/test/test2/g' /tmp/file > /tmp/result – Perennate

Answer 4

Or also:

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

Brok

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

Answer 5

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

echo -e 'To: [email protected]\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.

Otherness

Answer 6

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

Pilpul