# communicating with programs

The most naive way to interact with another program is simply to run it, let it communicate directly with the user, and wait for it to finish. This is done with the run command.
 i1 : run "uname -a" Linux debian 5.10.0-9-amd64 #1 SMP Debian 5.10.70-1 (2021-09-30) x86_64 GNU/Linux o1 = 0
To run a program and provide it with input, one way is use the operator <<, with a file name whose first character is an exclamation point; the rest of the file name will be taken as the command to run, as in the following example.
 i2 : "!grep a" << " ba \n bc \n ad \n ef \n" << close ba ad o2 = !grep a o2 : File
More often, one wants to write Macaulay2 code to obtain and manipulate the output from the other program. If the program requires no input data, then we can use get with a file name whose first character is an exclamation point. In the following example, we also peek at the string to see whether it includes a newline character.
 i3 : peek get "!uname -a" o3 = "Linux debian 5.10.0-9-amd64 #1 SMP Debian 5.10.70-1 (2021-09-30) x86_64 " GNU/Linux
Bidirectional communication with a program is also possible. We use openInOut to create a file that serves as a bidirectional connection to a program. That file is called an input output file. In this example we open a connection to the unix utility egrep and use it to locate the symbol names in Macaulay2 that begin with in.
 i4 : f = openInOut "!egrep '^in'" o4 = !egrep '^in' o4 : File i5 : scan(keys Core.Dictionary, key -> f << key << endl) i6 : f << closeOut o6 = !egrep '^in' o6 : File i7 : get f o7 = interpreterDepth infoHelp integrate inducedMap installPackage installMethod insert instance info interval intersect inversePermutation installedPackages input index indexComponents installAssignmentMethod infinity intersection inducesWellDefinedMap inverse indices incomparable installHilbertFunction indeterminate in instances independentSets
With this form of bidirectional communication there is always a danger of blocking, because the buffers associated with the communication channels (pipes) typically hold only 4096 bytes. In this example we succeeded because the entire output from egrep was smaller than 4096 bytes. In general, one should be careful to arrange things so that the two programs take turns using the communication channel, so that when one is writing data, the other is reading it.

A useful function in this connection is isReady, which will tell you whether an input file has any input available for reading, or whether it has arrived at the end. We illustrate it in the following example by simulating a computation that takes 5 seconds to complete, printing one dot per second while waiting.

 i8 : f = openIn "!sleep 5; echo -n the answer is 4" o8 = !sleep 5; echo -n the answer is 4 o8 : File i9 : isReady f o9 = false i10 : while not isReady f do (sleep 1; << "." << flush) ...... i11 : read f o11 = the answer is 4 i12 : isReady f o12 = true i13 : atEndOfFile f o13 = true i14 : close f o14 = !sleep 5; echo -n the answer is 4 o14 : File
We also allow for bidirectional communication through sockets over the internet. See openInOut and openListener, or the next section.

Another useful function is wait, which can be used to wait for input to be available from any of a list of input files.