using linux how can I pass the contents of a file

Question

let's say I want to run ./program with a string argument

instead of typing ./program string each time, how can I do ./program <file> where <file> is a file that contains string?

thankyou

Answer 1

Command

./program "$(< file)"

Explanation

1. $(cmd) runs a command and converts its output into a string. $(cmd) can also be written with backticks as `cmd`. I prefer the newer $(cmd) as it nests better than the old school backticks.

2. The command we want is cat file. And then from the bash man page:

   The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).

3. The quotes around it make sure that whitespace is preserved, just in case you have spaces or newlines inside of your file. Properly quoting things in shell scripts is a skill that is sorely lacking. Quoting variables and command substitutions is a good habit to good into so you don't get bit later when your file names or variable values have spaces or control characters or other weirdnesses.

Answer 2

Another option is xargs:

cat file | xargs ./program

I personally find this version easier to read, especially if you're doing more than just catting a file.

From the man page:

xargs reads items from the standard input, delimited by blanks or newlines, and executes the command one or more times with any initial-arguments followed by items read from standard input.

In plain speak, xargs will execute ./program on each line/word that is being piped to it.

See man xargs for more options.

Answer 3

This should do the trick:

./program `cat file`

If you want the entire file content in a single argument, add double quotation (tested in bash. I think It may vary shell to shell) :

./program "`cat file`" 

Answer 4

This is extension to @robbrit answer.

Consider a file.txt with content:

1 2 3
4 5 6

And the a program.c with content:

#include<stdio.h>
void main(int argc, char** argv) 
{ 
    for (int i = 0; i < argc; i++) 
        printf("Arg %d: %s\n", i, argv[i]); 
}

Here is what happen if you pass command line arguments with quotes and without:

$ gcc  program.c -o program

$ ./program `cat file`
Arg 0: ./program
Arg 1: 1
Arg 2: 2
Arg 3: 3
Arg 4: 4
Arg 5: 5
Arg 6: 6

$ ./program "`cat file`"
Arg 0: ./program
Arg 1: 1 2 3
4 5 6

Answer 5

All the above answers work but I find the cleanest and easiest to type option is

xargs < file ./program

Answer 6

You can use one of:

./program $(cat file)
./program "$(cat file)"

The latter is useful if there may be multiple words in the file and you want them to be treated as a single argument. In any case, I prefer the use if $() to backticks simply due to it's ability to nest (which is probably not a requirement in this case).

Also keep in mind that this answer (and others) are more related to the shell in use rather than Linux itself. Since the predominant ^{and the best :-)} shell seems to be bash, I've coded specifically for that.

This is actually a fairly common way of killing processes under UNIX lookalikes. A daemon (like cron) will write its process ID to a file (like /var/cron.pid) and you can signal it with something like:

kill -HUP $(cat /var/cron.pid)