I have a C program that reads command line arguments from argv. Is it possible to make a pipe to redirect the contents of a file as command line arguments to my program? Suppose I have a file arguments.dat with this content:
0 0.2 302 0
And I want my program to be called with:
./myprogram 0 0.2 302 0
I tried the following:
cat arguments.dat | ./myprogram
without success.
With most shells, you can insert the contents of a file into a command line with $(<filename):
./myprogram $(<arguments.dat)
If your shell doesn't support that, then one of the older ways will work:
./myprogram $(cat arguments.dat)
./myprogram `cat arguments.dat` # need this one with csh/tcsh
(You do know the difference between command line arguments and file input, right? Why would you expect to pipe command line arguments into a program?)
xargs is your answer:
cat arguments.dat | xargs ./myprogram
Or easier:
xargs -a arguments.dat ./myprogram
Check the manual for the many ways to customize xargs. For example, you can read line-by-line rather than by word, and you can use the arguments in more complex replacements.
...which is to say: The below answers apply to cases where it wouldn't be acceptable for ./myprogram --first-argument "first value" to be silently changed into ./myprogram --first-argument; ./myprogram "first value".
That is, if your input looks like:
--first-argument
first value
--second-argument
second value
and you mean this to run:
./myprogram --first-argument "first value" --second-argument "second value"
...then you should use (with bash 4.0 or later):
readarray -t args <arguments.dat
./myprogram "${args[@]}"
...or (for bash 3.x as well):
args=( )
while IFS= read -r arg; do
args+=( "$arg" )
done <arguments.dat
./myprogram "${args[@]}"
That is, if your file contains something like (note that newlines and unquoted spaces behave identically here):
--first-argument "first value"
--second-argument "second value"
...and you mean this to run:
./myprogram --first-argument "first value" --second-argument "second value"
...then you should use:
args=( )
while IFS= read -r -d '' arg; do
args+=( "$arg" )
done < <(xargs printf '%s\0' <arguments.dat)
Use NUL-delimited values. That is, create the file as so:
printf '%s\0' "argument one" "argument two" >arguments.dat
...and parse it as follows:
args=( )
while IFS= read -r -d '' arg; do
args+=( "$arg" )
done <arguments.dat
./myprogram "${args[@]}"
This will work with all possible argument values, even ones with literal newlines, literal quotes, literal backslashes, or other nonprintable characters. (Literal NULs are not possible in UNIX command lines, since command lines are composed of NUL-terminated strings; thus, NUL is the only character which is completely safe to use to unambiguously separate arguments in a string).
This subsection is relevant if the desired result (when there are more arguments in your file than can be passed to an invocation of your program) is multiple distinct invocations of the program, each one receiving a subset of arguments. This is a family of cases where xargs is the right tool for the job.
If on a GNU platform, you may want to run xargs -a arguments.dat instead of redirecting stdin; however, this isn't supported with BSD xargs (as on MacOS), and so is not demonstrated here.
With GNU xargs (most Linux platforms):
xargs -d $'\n' ./myprogram <arguments.dat
With BSD xargs (MacOS, FreeBSD/OpenBSD/etc):
xargs -0 ./myprogram < <(tr '\n' '\0' <arguments.dat)
xargs ./myprogram <arguments.dat
xargs -0 ./myprogram <arguments.dat
