CODEWITHSUNDEEP
bashsedpipeconditional-statements
Joe Riel
Joe Riel

Reputation: 46

Conditionally pipe output through sed

Is there a way to conditionally pipe the output of a command through sed, in a bash script? Depending upon a script option, I either want to pipe the output of a long pipe through sed, or omit the pipe through sed. Currently I'm doing

if [ $pipeit ]; then
   sed_args='/omit this line/d'
else
   sed_args='/$^/d'  # pass-thru (what's a better sed pass thru?)
fi

some_cmd | sed "$sed_args"

Upvotes: 2

Views: 721

Answers (2)

Jay jargot
Jay jargot

Reputation: 2868

By default sed prints all lines:

if [ $pipeit ]; then
  sed_args='/omit this line/d'
else
  sed_args=""  # pass-thru
fi

some_cmd | sed "${sed_args}"

There is this other tested solution:

some_cmd | if [ $pipeit ]; then
  sed "/omit this line/d"
else 
  sed ""
fi

cat could be used instead of the sed ""

Finally, a string can be built and executed using eval.

some_cmd='printf "foo\n\nbar\n"'
if [ $pipeit ]; then
  conditional_pipe='| sed "/foo/d"'
else
  conditional_pipe=""
fi

eval "${some_cmd}" "${conditional_pipe}"

If some_cmd is complex it migth be tricky to build a string that would behave as expected with eval.

----

First solution for history

Using an impossible match with sed would make it print all lines to stdout:

$ printf "foo\n\nbar\n" | sed "/./{/^$/d}"
foo

bar

/./ selects a line with at least one char. /^$/ selects an empty line.

Upvotes: 1

hek2mgl
hek2mgl

Reputation: 158060

I would keep it as simple as:

if [ $pipeit ]; then
   some_cmd | sed '/omit this line/d'
else
   some_cmd
fi

Why should you call sed if you don't need it? Just for your information, a possible sed command that does not change the input would be sed -n p

Btw, if some_cmd is kind of a large beast and you want to avoid duplicating it, wrap it into a function.

Upvotes: 3

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