Cat command and echo

Question

I'd like to concatenate the output from echo with content of a file. I've tried the following comand:

echo "abc" | cat 1.txt > 2.txt

but the 2.txt file only contains the content from 1.txt. Why doesn't it work?

Answer 1

It does not work because the cat program in your pipe sequence was not instructed to read the echo program's output from standard input.

You can use - as a pseudo file name to indicate standard input to cat. From man cat on an msys2 installation:

EXAMPLES
       cat f - g
              Output f's contents, then standard input, then g's contents.

So try

echo "abc" | cat - 1.txt > 2.txt

instead.

Answer 2

As noted by others, your original command fails because cat 1.txt disregards its standard input. Either indicate that it should be its first argument (cat - 1.txt), or use block redirection to redirect echo abc and cat 1.txt together. To wit:

{ echo abc; cat 1.txt; } > 2.txt 

Relevant excerpt from the manual (man bash):

Compound Commands
A compound command is one of the following.  In most cases a list in a command's description may be separated from the rest of the command by one or more newlines, and may be followed by a newline in place of a semicolon.

(list)

list is executed in a subshell environment (see COMMAND EXECUTION ENVIRONMENT below). Variable assignments and builtin commands that affect the shell's environment do not remain in effect after the command completes.  The return status is the exit status of list.

{ list; }

list is simply executed in the current shell environment.  list must be terminated with a newline or semicolon.  This is known as a group command.  The return status is the exit status of list.  Note that unlike the metacharacters ( and ), { and } are reserved words and must occur where a reserved word is permitted to be recognized.  Since they do not cause a word break, they must be separated from list by whitespace or another shell metacharacter.

The first option (subshell environment) has a bunch of side effects, most if not all of which are irrelevant to your scenario; however, if redirecting a bunch of commands' output is all you need, then Option #2 here (group command) is preferred.

Answer 3

( echo "abc"; cat 1.txt ) > 2.txt

You piped the echo output to cat, but cat had no use for input, and ignored it. Now the commands run one after the other, and their output is grouped (the parentheses) and directed into 2.txt.

Answer 4

In any POSIX shell you could use command substitution to use cat file as input for echo:

echo $(cat file1.txt) "This Too"

In Bash you could use process substitution and use echo as another "file" for cat, as in:

cat file1.txt <(echo This Too)

and then pipe or redirect the output as you see fit.

Like every other answer says, cat ignores stdin if it's got a file to look at. (I do like Daniel & mvw's answers too, +1 for them)

Answer 5

Your command cat 1.txt doesn't do anything with the output of echo "abc".

Instead: (echo "abc"; cat 1.txt) > 2.txt will write the output of both commands into 2.txt.

Answer 6

It is just a guess, but it looks like you have a repeatable process, which is a good candidate for an alias or a function. Aliases are usually shortcuts for invoking Bash commands, such as abbreviation, on a single input stream as its argument(s).

alias hg="history | grep"

However, in this case, a function would be more readable, since you are combining multiple, discrete (2) input streams as well as multiple bash commands. You have two arguments, the first being a string and the other a filepath. In the end, you want the result to be written to the standard output stream.

From a CLI prompt, type this:

# ecat()
{
echo ${1}
cat ${2}
}

Your function is named ecat, which is memorable.

Now you can invoke as

ecat "abc" 1.txt

To append, simply supply a different output destination to standard output:

ecat "abc" 1.txt >> 2.txt

The append redirection operator '>>' will add the output to the end of the specified file.

If you like it, then append to your ~/.bashrc file for reuse.

declare -f ecat >> ~/.bashrc

That also means you can decorate, etc., within your function definition.

It is also a good idea to protect files from being overwritten by adding this to your ~/.bashrc file file:

set noclobber

Answer 7

The following code will also work:

echo abc >> 1.txt && cat 1.txt > 2.txt

The output of echo abc will be appended to 1.txt with >> After that the && will tell it to run the next set of commands which will cat 1.txt and then output it to 2.txt. Basically, it appends the output of the first command into 1.txt and then sends the output of 1.txt into 2.txt.

If you want the abc to appear first, you can use the following code:

echo abc >> 2.txt && cat 1.txt >> 2.txt