linux shell script read from stdin if no file

Question

I am trying to set my Linux shell script to read from a file (which I have working) but if there isn't any file then I need to read from stdin.

The command for reading a file looks like this:

./stats -row test_file

How would I be able to read what the user enters with something like this:

./stats -row 4 2 3 5 3 4 5 3 6 5 6 3 4

When I enter a command like this I get 'no such file or directory'

I broke my script down to the problem I need help with.

#!/bin/sh

INPUT_FILE=$2         #Argument 2 from command line is the input file
exec 5< $INPUT_FILE   #assign input file to file descriptor #5

while read -u 5 line  #read from file descriptor 5 (input file)
do
    echo "$line"
done

exec 5<&-   #close file descriptor #5

This also won't work for the input I need.

while read line  
do
    echo "$line"
done <$2

If a file exists named '4', and the user enters ./stats -row 4, what behavior do you want? — William Pursell
– William Pursell, Commented Apr 1, 2015 at 22:19
Thanks for pointing that out. I know I can use $@ to get all of the arguments, how do you get the arguments from 2 and on? — Casey Balza
– Casey Balza, Commented Apr 1, 2015 at 22:24
a plus one for "I broke my script down to the problem I need help with" Good luck. — shellter
– shellter, Commented Apr 1, 2015 at 22:34
You can read from /dev/stdin or /dev/fd/0 if there is no input file at all. How do you determine between having a file and having command line arguments? If it is 'number of arguments > 2', then there are various tricks you can use to arrange for the arguments to be fed -- process substitution might be one, for example. — Jonathan Leffler
– Jonathan Leffler, Commented Apr 1, 2015 at 22:44

Community · Accepted Answer · 2017-05-23 12:06:20Z

2

InArtful Solution

A very in-artful if statement will do the trick:

INPUT_FILE=$2         #Argument 2 from command line is the input file

if [ -f "$INPUT_FILE" ]; then

    while read -r line
    do
        echo "$line"
    done <"$INPUT_FILE"

else

    while read -r line
    do
        echo "$line"
    done

fi

Note: this presumes you are still looking for the filename as the 2nd argument.

Artful Solution

I cannot take credit, but the artful solution was already answered here: How to read from file or stdin in bash?

INPUT_FILE=${2:-/dev/stdin}         #Argument 2 from command line is the input file

while read -r line
do
    echo "$line"
done <"$INPUT_FILE"

exit 0

I was picking around with a solution like this but missed the stdin device /dev/stdin as the default for INPUT_FILES. note this solution is limited to OS's with a proc-filesystem.

edited May 23, 2017 at 12:06

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answered Apr 1, 2015 at 22:51

David C. Rankin

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3 Comments

John Kugelman Over a year ago

The script changes behavior depending on the existence of its argument? Ick, I don't care for that too much. It should be designed so the caller can clearly delineate whether they're passing a file name or not. Having to do this is a sign of a poor API.

David C. Rankin Over a year ago

I don't like it either.. That was the purpose of the very in-artful comment at the beginning.

David C. Rankin Over a year ago

@JohnKugelman, INPUT_FILE=${2:-/dev/stdin} was the key to an artful solution.

aks · Accepted Answer · 2015-04-01 23:40:20Z

In bash scripts, I usually put code that reads from a file (or a pipe) in a function, where the redirection can be separated from the logic.

Also, when reading from a file or from STDIN, it's a good idea for the logic to not care which is which. So, it's best to capture STDIN into a temp file and then the rest of the file reading code is the same.

Here's an example script that reads from ARG 1 or from STDIN, and just counts the lines in the file. It also invokes wc -l on the same input and shows the results from both methods.

#!/bin/bash

# default input is this script
input=$0

# If arg given, read from it
if (( $# > 0 )); then
  input=$1
  echo 1>&2 "Reading from $input"
else
  # otherwise, read from STDIN
  # since we're reading twice, need to capture it into
  # a temp file
  input=/tmp/$$.tmp
  cat >$input
  trap "rm -f $input" EXIT ERR HUP INT QUIT
  echo 1>&2 "Reading from STDIN (saved to $input)"
fi

count_lines() {
  local count=0
  while read line ; do
    let count+=1
  done
  echo $count
}

lines1=`count_lines <$input`
lines2=`wc -l <$input`

fmt="%15s: %d\n"
printf "$fmt" 'count_lines' $lines1
printf "$fmt" 'wc -l'       $lines2

exit

Here are two invocations: one with a file on arg 1, and one with no argument, reading from STDIN:

$ ./t2.sh t2.sh
Reading from t2.sh
    count_lines: 35
          wc -l: 35

$ ./t2.sh <t2.sh
Reading from STDIN (saved to /tmp/8757.tmp)
    count_lines: 35
          wc -l: 35

Thanks, creating a temp file and placing the values the user entered worked out after making a few adjustments.

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