Capturing the output printed by a program

[Anonymous Student]

How do I capture the output printed by a program and extract just the part that I want?

[Simon]

Unix programs print output on one or both of two output streams called stdout (standard out) and stderr (standard error). The former is meant for actual output and the latter for error messages, although programs are free to use either for any purpose.

Here’s a program that prints to stdout for testing purposes: it’s just the echo command:

$ echo -e "the first line\nthe second line"

which will print this to stdout

the first line
the second line

Now let’s capture that and do something useful with it. We ‘pipe’ the output to the next program using “|” (pronounced “pipe”):

$ echo -e "the first line\nthe second line" | grep second

which gives

the second line

where grep finds the pattern of interest. Or how about cutting vertically to get a certain character range

$ echo -e "the first line\nthe second line" | cut -c 5-9

which gives

first
secon

Now combine them:

$ echo -e "the first line\nthe second line" | grep first | cut -c 5-9

to print only

first

[Anonymous Student]

That works, but now I need to capture that value so I can manipulate it. I don’t really understand how to do that in the shell?

[Simon]

Try this (again, I’m using echo just as an example of a program that produces output on stdout – you are probably trying to capture the output of a more interesting program):

$ echo -e "this is 1\nand now 2"
$ echo -e "this is 1\nand now 2" | grep this
$ echo -e "this is 1\nand now 2" | grep this | cut -d" " -f3

cut cuts vertically and the above arguments say “define the delimiter as the space character, cut into fields using that delimiter, and give me the third field”

To capture the output of a program, or of a pipeline of programs as we have above, we need to run it inside “backticks”. So, let’s capture the output of that pipeline of commands and store it in a shell variable called MYVAR:

$ MYVAR=`echo -e "this is 1\nand now 2" | grep this | cut -d" " -f3`
$ echo The value is: ${MYVAR}

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