Using xargs to assign stdin to a variable

Question

All that I really want to do is make sure everything in a pipeline succeeded and assign the last stdin to a variable. Consider the following dumbed down scenario:

x=`exit 1|cat`

When I run declare -a, I see this:

declare -a PIPESTATUS='([0]="0")'

I need some way to notice the exit 1, so I converted it to this:

exit 1|cat|xargs -I {} x={}

And declare -a gave me:

declare -a PIPESTATUS='([0]="1" [1]="0" [2]="0")'

That is what I wanted, so I tried to see what would happen if the exit 1 didn't happen:

echo 1|cat|xargs -I {} x={}

But it fails with:

xargs: x={}: No such file or directory

Is there any way to have xargs assign {} to x? What about other methods of having PIPESTATUS work and assigning the stdin to a variable?

Note: these examples are dumbed down. I'm not really doing an exit 1, echo 1 or a cat, but used these commands to simplify so we can focus on my particular issue.

Dennis Williamson · Accepted Answer · 2010-08-10 21:03:57Z

3

When you use backticks (or the preferred $()) you're running those commands in a subshell. The PIPESTATUS you're getting is for the assignment rather than the piped commands in the subshell.

When you use xargs, it knows nothing about the shell so it can't make variable assignments.

Try set -o pipefail then you can get the status from $?.

answered Aug 10, 2010 at 21:03

Dennis Williamson

364k95 gold badges386 silver badges446 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

User1 Over a year ago

pipefail is a good option to consider. However, I wish I could get teh PIPESTATUS for everything in the pipeline.

Douglas Leeder · Accepted Answer · 2010-08-11 08:00:02Z

3

xargs is run in a child process, as are all the commands you call. So they can't effect the environment of your shell.

You might be able to do something with named pipes (mkfifo), or possible bash's read function?

EDIT:

Maybe just redirect the output to a file, then you can use PIPESTATUS:

command1 | command2 | command3 >/tmp/tmpfile
## Examine PIPESTATUS
X=$(cat /tmp/tmpfile)

edited Aug 11, 2010 at 8:00

answered Aug 10, 2010 at 16:27

Douglas Leeder

53.5k9 gold badges100 silver badges138 bronze badges

1 Comment

User1 Over a year ago

I was thinking read could do something like that. echo 1|read x didn't work. Do you have an example?

timo · Accepted Answer · 2010-08-10 21:10:32Z

0

How about ...

read x <<<"$(echo 1)"
read x < <(echo 1)

echo "$x"

answered Aug 10, 2010 at 21:10

timo

11 bronze badge

1 Comment

User1 Over a year ago

Interesting thought. Using (exit 1|cat) still doesn't properly populate PIPESTATUS.

timo · Accepted Answer · 2010-08-11 10:46:00Z

0

Why not just populate a new array?

IFS=$'\n' read -r -d '' -a result < <(echo a | cat | cat; echo "PIPESTATUS='${PIPESTATUS[*]}'" )
IFS=$'\n' read -r -d '' -a result < <(echo a | exit 1 | cat; echo "PIPESTATUS='${PIPESTATUS[*]}'" )

echo "${#result[@]}"
echo "${result[@]}"
echo "${result[0]}"
echo "${result[1]}"

answered Aug 11, 2010 at 10:46

timo

1

Comments

will · Accepted Answer · 2018-11-20 12:51:17Z

There are already a few helpful solutions. It turns out that I actually had an example that matches the question as framed above; close-enough anyway.

Consider this:

 XX=$(ls -l *.cpp | wc -l  | xargs -I{} echo {})
 echo $XX
 3

Meaning that I had 3 x .cpp files to in my working directory. Now $XX is 3 and I can make use of that result in my script. It is contrived, because I don't actually need the xargs in this example. It works though.

In the example from the question ...

x=`exit 1|cat`

I don't think that will give you what was specified. exit will quit the sub-shell before the cat gets a mention. Also on that note,

I might start with something like

declare -a PIPESTATUS='([0]="0")'
x=$?

x now has the status from the last command.

TRicks43 · Accepted Answer · 2019-12-17 18:38:22Z

0

Assign each line of input to an array, e.g. all python files in a directory

declare -a pyFiles=($(ls -l *.py | awk '{print $9}'))

where $9 is the nineth field in ls -l corresponding to the filename

answered Dec 17, 2019 at 18:38

TRicks43

1531 silver badge7 bronze badges

Collectives™ on Stack Overflow

Using xargs to assign stdin to a variable

6 Answers 6

1 Comment

1 Comment

1 Comment

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

1 Comment

1 Comment

1 Comment

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related