CODEWITHSUNDEEP
bash
Preston Connors
Preston Connors

Reputation: 407

How Do You Read Input As Redirection From Program In Bash?

I'm trying to read input from a file in bash, but want to discard the first line of a file because of the header rows in the CSV. The tail -n+2 "${csv}" works by itself but when using the < I am getting error:

syntax error near unexpected token `-n+2'

The code I'm trying to make work is:

while IFS=, read c1 c2 c3 c4 c5 c6 c7 c8 c9 c10 c11 c12 c13 c14 c15 c16 c17 c18 c19 c20 c21 c22 c23 c24
do
    description[$i]="${c3}"
    current_ip[$i]="${c10}"
    new_ip[$i]="${c12}"
    new_netmask[$i]="${c13}"
    new_gateway[$i]="${c14}"
    i=$i+1
done < tail -n+2 "${csv}"

Can someone please help provide a working form of < tail -n+2 "${csv}" and explain the solution?

Upvotes: 0

Views: 55

Answers (2)

chepner
chepner

Reputation: 532268

Instead of using tail, just read directly from the file, but use a compound command ({...} and two reads to discard the first two lines.

{ read; read;
  while IFS=, read ...; do
      ...
  done
} < "$csv"

Upvotes: 1

Charles Duffy
Charles Duffy

Reputation: 295815

The easy answer here is to use process substitution:

while ... done < <(tail -n+2 "$csv")

This works because <(tail) is replaced, by the shell, with a filename which connects to the output of tail, either via a named pipe or, on platforms that support it, a /dev/fd alias.

Thus, when run, this will become something like (pseudocode):

tail -n+2 "$csv" >/dev/fd/63 &
while ... done < /dev/fd/63

...or (and this latter is what you might do if trying to write this code for POSIX sh, or otherwise a shell without comparable functionality):

tempdir=$(mktemp -t -d csvpipe.d.XXXXXX)
mkfifo "$tempdir/csvpipe.tmp"
tail -n+2 "$csv" >"$tempdir/csvpipe.tmp" &
while ... done <"$tempdir/csvpipe.tmp"
rm -rf "$tempdir"

...though your shell won't need the overhead of a temporary directory to use in creating its FIFO, this is somewhat safer than using mktemp -u, and far safer than a fixed/hardcoded temporary file name.

Upvotes: 4

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