Running program and providing input from shell script

Question

I have a shell script as follows

ln -s -f /ctf/message/dave.inbox /home/alice/imoprt.inbox
/ctf/message/sendmsg
echo hello

The purpose of the script is to first make a link, then run a program, and provide hello as input to the program. It all works except providing hello as the input. I can do this easily by writing a script just like

echo hello

but then I am unable to do some other things, such as linking before. Is there a way to do this sequence of commands?

What if I want to do a series of input? Like multiple inputs to the program one after the other — Ben
– Ben, Commented Sep 21, 2020 at 22:34
feed it all the input you want. Perhaps you'll want to use a heredoc to give multi-line input, but you can also do printf 'line1\nline2\n' | sendmsg. It depends on what you want to do. — William Pursell
– William Pursell, Commented Sep 21, 2020 at 22:36
Or, you may want to create a fifo and run sendmsg asynchronously and then let the script continue, feeding data to the program through the fifo. — William Pursell
– William Pursell, Commented Sep 21, 2020 at 22:37
@Ben - I think more details of just what you want to do are needed. Based on your comment, it sounds like you need some form of loop. — ash
– ash, Commented Sep 21, 2020 at 22:42

William Pursell · Accepted Answer · 2020-09-22 11:57:10Z

2

There are a few common ways to provide input to a command:

echo input | cmd

{ echo one; echo two; echo three; } | cmd

cmd <<EOF
line 1 of input 
line 2 of input
EOF

trap 'rm -f fifo' 0
mkfifo fifo
cmd < fifo &
{
echo line 1 of input
var=$( echo line 2 of input generated dynamically )
echo "$var"
echo another line of input
} > fifo
wait

edited Sep 22, 2020 at 11:57

answered Sep 21, 2020 at 22:41

William Pursell

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2 Comments

user1934428 Over a year ago

Great for the use of mkfifo, but the heredoc version could be simpler written without a pipe, using just cmd <<EOF.

William Pursell Over a year ago

@user1934428 Thank you. It is amazing! Even after decades of railing against UUOC how easy it is to fall into it.

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