Can Linux Cat Command Be Used for Writing Text to File

Can linux cat command be used for writing text to file?

That's what echo does:

echo "Some text here." > myfile.txt

How to cat EOF a file containing code?

You only need a minimal change; single-quote the here-document delimiter after <<.

cat <<'EOF' >> brightup.sh

or equivalently backslash-escape it:

cat <<\EOF >>brightup.sh

Without quoting, the here document will undergo variable substitution, backticks will be evaluated, etc, like you discovered.

If you need to expand some, but not all, values, you need to individually escape the ones you want to prevent.

cat <<EOF >>brightup.sh
#!/bin/sh
# Created on $(date # : <<-- this will be evaluated before cat;)
echo "\$HOME will not be evaluated because it is backslash-escaped"
EOF

will produce

#!/bin/sh
# Created on Fri Feb 16 11:00:18 UTC 2018
echo "$HOME will not be evaluated because it is backslash-escaped"

As suggested by @fedorqui, here is the relevant section from man bash:

Here Documents

This type of redirection instructs the shell to read input from the
current source until a line containing only delimiter (with no
trailing blanks) is seen. All of the lines read up to that point are
then used as the standard input for a command.

The format of here-documents is:

      <<[-]word
              here-document
      delimiter

No parameter expansion, command substitution, arithmetic expansion,
or pathname expansion is performed on word. If any characters in word
are quoted, the delimiter is the result of quote removal on word, and
the lines in the here-document are not expanded. If word is
unquoted, all lines of the here-document are subjected to parameter
expansion, command substitution, and arithmetic expansion. In the
latter case, the character sequence \<newline> is ignored, and \
must be used to quote the characters \, $, and `.

With cat command create a file which is a merge of 2 files(linux)

You can use "cat" on multiple files, in which case it'll print them out in order.

cat test.txt Copy

How to parse file as an argument using cat command

This is my first answer on stackoverflow, so bear with me...

"Running cat filename reads the contents of the specified file and writes them to standard output. | between two commands means connect standard output of the left command to standard input of the right command." cat and pipe

When you propose to read from "file from the cat"...
read([file from the cat], buf, sizeof(buf));

...you're meant to specify a file descriptor for the read command read(2)

It turns out, you can pass in the file descriptor of "0" to read from standard input.

So you would simply do the following:

read(0, buf, sizeof(buf));

How does cat EOF work in bash?

This is called heredoc format to provide a string into stdin. See https://en.wikipedia.org/wiki/Here_document#Unix_shells for more details.

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From man bash:

Here Documents

This type of redirection instructs the shell to read input from
the current source until a line
containing only word (with no trailing
blanks) is seen.

All of the lines read up to that point are then used as the
standard input for a command.

The format of here-documents is:

          <<[-]word
                  here-document
          delimiter

No parameter expansion, command substitution, arithmetic expansion, or
pathname expansion is performed on
word. If any characters in word are
quoted, the
delimiter is the result of quote removal on word, and the lines
in the here-document are not expanded.
If word is unquoted, all lines of the
here-document are subjected to parameter expansion, command
substitution, and arithmetic
expansion. In the latter case, the
character sequence \<newline> is
ignored, and \ must be used to quote the characters \, $, and `.

If the redirection operator is <<-, then all leading tab characters
are stripped from input lines and the
line containing delimiter. This
allows here-documents within shell scripts to be indented in a natural fashion.

how to append string before print text using cat?

You can find and replace the text using the below command.

sed -i 's/.txt/.jpg/g' test_set.txt

This command will replace ".txt" to ".jpg". Hope this will solve your problem.

Output a text file including the command that was used to generate it

You could use another symbol as separator in sed. You could write your set statement like below.

cat /proc/meminfo > meminfo.txt; sed -i '1s#^#/proc/meminfo >  meminfo.txt \n\n#' meminfo.txt

You could do it like below. First set your command and output to variables. And execute them as variables.

mycmd="ls -l"
out="myout.txt"
eval $mycmd > $out; sed -i -e "1s#^#$mycmd > $out \n\n#" $out

Hope this helps.

variables are not interpreted when I use the cat command to put the contents of a text file into a variable

You could use envsubst for this:

content=$(envsubst < "file.txt")

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