3

I want to pipe the output of one command to another script. In this other script I want to check whether the input was empty or just consists of spaces. If this is the case, I'll ignore it. Otherwise, I want all the input to be forwarded to another few commands.

For more clarity, I'll run something like cat input.txt | ./script.sh where input.txt looks like 

line 1
line 2
line 3

and script.sh currently looks like:

read input_text

if [ -z "$input_text" ]
    # ignore emtpy input
    then exit 0
else
    # do something here with the input
fi

The problem is that in this scenario, the first line of the input "line 1" is read into the input_text variable and therefore not forwarded with the rest of the input (which then solely consists of lines 2 and 3) into the code following after else.

So, how can I first check whether the input consists of more than just empty spaces and then forward the whole input to another command?

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3 Answers 3

Reset to default
2

That would mean you want to forward the input as soon as you see a non-blank character.

awk -v cmd='otherCommand' '
  forward {print | cmd; next}
  {initial_output = initial_output $0 "\n"}
  NF {printf "%s", initial_output | cmd; forward = 1}'

Example:

$ printf '%b\n' ' ' '' '\t' | awk -v cmd='echo START; sed "s/.*/<&>/"' '
  forward {print | cmd; next}
  {initial_output = initial_output $0 "\n"}
  NF {printf "%s", initial_output | cmd; forward = 1}'
$ printf '%b\n' ' ' '' '\t' something | awk -v cmd='echo START; sed "s/.*/<&>/"' '
  forward {print | cmd; next}
  {initial_output = initial_output $0 "\n"}
  NF {printf "%s", initial_output | cmd; forward = 1}'
START
< >
<>
<       >
<something>
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1
  • This looks like it does what I want it to do. Would you provide a short explanation of what's happening there? Commented Nov 27, 2015 at 18:14
0 
#!/bin/sh
{   tee /dev/fd/3 <&4&:|
    grep -q '[^[:space:]]' &&
    cat - /dev/fd/4 <&3
}   3<<"" 4<&0 | another_few_commands

# cause i dont know how else to end this with a blank line
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-1

how about using a loop? (this will end on any empty or space-only lines)

#!/bin/sh
while read input_text; do
  [ -z "$input_text" ] && break
  # do something with the non-blank input
done
exit 0
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2
  • It is necessary that the input is forwarded "en block" to the next command, not just line by line. Is there a way accomplish that? Commented Nov 27, 2015 at 12:28
  • This strips off whitespace at the beginning and at the end of each line, and also performs some backslash expansion. Commented Nov 27, 2015 at 22:32

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