writing a bash array (of strings and special chars) to a file

Question

I have a bash array OUTARRAY that I fill will values from processing an INARRAY. I frequently append to OUTARRAY special chars, namely \t and \n so it may look like:

OUTARRAY[j]="\n"

or

OUTARRAY[j]="${INARRAY[i]}\t"

in the end I dump the OUTARRAY in a file using

printf "%s" "${OUTARRAY[@]}" > ${OUTFILE}

the result I get however is, a single line file with all the special chars printed within:

\n2771\t2899\t7624\t2911\t\n2772\t2904\t7706\t2911\t\n2771\t2909

Instead, I want columned output. Something like

2771    2899    7624    2911
2772    2904    7706    2911

and so on. what do I do wrong? thank you

rici · Accepted Answer · 2014-05-30 15:45:13Z

5

bash does not interpret C-style backslash escapes (\n) in quoted strings.

Instead, use the bash syntax $'\n':

OUTARRAY[j]=$'\n'
OUTARRAY[j]="${INARRAY[i]}"$'\t'

You might find it more readable if you start by defining:

NL=$'\n'
TAB=$'\t'

and then you can insert ${NL} and ${TAB} freely inside your double-quoted strings.

Alternatively, you can use a bash extension:

printf normally expands C-style backslash escapes in formats, but not in arguments. However, if you are using bash, then you can use the bash-specific %b printf format which expands backslash escapes in the corresponding argument. I don't really endorse this solution, but it should work without other modifications:

printf "%b" "${OUTARRAY[@]}" > ${OUTFILE}

By the way, it is not really good style to use ALL CAPS for bash variable names, because it increases the probability that they will clash with bash/system-specific environment variables.

edited May 30, 2014 at 15:45

answered May 30, 2014 at 15:25

rici

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2 Comments

gniourf_gniourf Over a year ago

Oh, it seems we have exactly the same answer... so I can only +1 yours :).

rici Over a year ago

@gniourf_gniourf There are not so many ways to answer the question :)

gniourf_gniourf · Accepted Answer · 2014-05-30 15:27:10Z

5

Instead of inserting the string \t or \n, insert actual tabs and newlines:

outarray[j]=$'\n'
outarray[j]=${inarray[i]}$'\t'

Now this is not really a good strategy. Instead, do not put your formatting in the array, but use printf to format how you want your array to be displayed: if you want 4 fields per line, separated by tabs:

printf '%s\t%s\t%s\t%s\n' "${outarray[@]}"

(btw, I lowercased your variable names, it's much better).

answered May 30, 2014 at 15:27

gniourf_gniourf

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2 Comments

nass Over a year ago

for reasons of parsing strength, I prefer not to assume I'll always have 4 columns :) this would be a good way otherwise.

gniourf_gniourf Over a year ago

@nass: you can build your format string dynamically to take that into account!

Tom Fenech · Accepted Answer · 2014-05-30 15:30:06Z

1

The other answers are the right way to do it but it is possible to print your data as it is. You can use echo -ne and a loop to print your values:

$ arr=( "a\t" "b\n" "c\t" "d\n" )
$ for i in "${arr[@]}"; do echo -ne $i; done
a       b
c       d

The -n switch to echo removes the trailing newline and the -e means that backslash escape characters will be interpreted.

answered May 30, 2014 at 15:30

Tom Fenech

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3 Comments

gniourf_gniourf Over a year ago

Yes, that's another quick possibility too! (with caveats too). +1 anyways. :).

rici Over a year ago

As gniourf_gniourf notes, printf is much more elegant if you have a consistent number of columns.

Tom Fenech Over a year ago

I readily acknowledge that both of your alternatives are superior, I just thought that echo deserved a mention. +1 to both of you.

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