How to read file contents in an array using shell script?

Question

I have a shell script which reads a file in to an array. It works perfectly with RHEL, but when i run the same script on Ubuntu i am getting an error. Here is the script.

file=/root/monitor.txt
while IFS=',' read -ra line ; do
        echo -e "export MASTER_IP=${line[2]}" >> ~/.bashrc
        source ~/.bashrc
done < $file

The above script works perfectly in RHEL. The error i am getting when i run it on Ubuntu is

read: Illegal option -a

The -a option is just for bash. Check this page Unix - Using Shell Arrays for reference. — fedorqui
– fedorqui, Commented Sep 2, 2014 at 8:25
For pure POSIX sh, arrays don't exist -- they're a ksh feature adopted by bash, zsh, etc. Fortunately, you don't need them. — Charles Duffy
– Charles Duffy, Commented Sep 3, 2014 at 12:06

benj · Accepted Answer · 2014-09-03 09:15:03Z

5

read -a is a bash feature, but on Ubuntu /bin/sh points to dash, which doesn't support the -a option. There are a few ways you could solve this problem:

On Ubuntu, just run the script under bash, by changing the first line to #!/bin/bash (as suggested by Charles Duffy)
Instead of read -a, just give a list of variables, such as read f1 f2 f3 f4 f5 and then refer to the fields as $f1, $f2 etc. In this case, if you have more than 5 fields, you need to add more variables, otherwise $f5 will contain the rest of the line.
Use awk instead of read. Awk is a good fit for this kind of field-processing task: you could replace the while loop with something like awk -F, '{print "export MASTER_IP=" $3}' $file >> ~/.bashrc, and then source ~/.bashrc at the end.

edited Sep 3, 2014 at 9:15

answered Sep 2, 2014 at 23:13

benj

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2 Comments

Charles Duffy Over a year ago

I'd at least directly mention changing the shebang as an option.

benj Over a year ago

I didn't think of that, that's a much nicer solution. I've edited it in. Thanks!

Charles Duffy · Accepted Answer · 2014-09-02 23:30:29Z

while IFS=, read -r _ _ ip _; do
  printf 'export MASTER_IP=%s\n' "$ip"
done <"$file" >>"$HOME/.bashrc"

This is better in a few respects:

Doesn't require any bashisms; will work with /bin/sh on Ubuntu.
Opens the output file only once for the whole loop, instead of inefficiently reopening it every time a new line is to be written.

However, you can do better -- improving your script's security -- if you change its shebang to be #!/bin/bash (or /usr/bin/env bash, or otherwise as appropriate for your platform):

#!/bin/bash
while IFS=, read -r _ _ ip _; do
  printf 'export MASTER_IP=%q\n' "$ip"
done <"$file" >>"$HOME/.bashrc"

Because this uses %q instead of %s, it also escapes the addresses, so they can't contain malicious content like $(rm -rf $HOME) -- something you probably don't want injected into your ~/.bashrc.

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