Print bash arguments in reverse order

Question

I have to write a script, which will take all arguments and print them in reverse.

I've made a solution, but find it very bad. Do you have a smarter idea?

#!/bin/sh
> tekst.txt

for i in $* 
do
    echo $i | cat - tekst.txt > temp && mv temp tekst.txt
done

cat tekst.txt

You could add each to the front of a shell variable.

Davislor
– Davislor

2015-09-18 08:34:38 +00:00
Commented Sep 18, 2015 at 8:34 — Davislor
– Davislor, Commented Sep 18, 2015 at 8:34

123 · Accepted Answer · 2015-09-18 08:29:34Z

9

Could do this

for (( i=$#;i>0;i-- ));do
        echo "${!i}"
done

This uses the below
c style for loop
Parameter indirect expansion (${!i}towards the bottom of the page)

And $# which is the number of arguments to the script

answered Sep 18, 2015 at 8:29

123

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2 Comments

F. Hauri - Give Up GitHub Over a year ago

In ARITHMETIC EVALUATION i>0 is useless: u could: for((i=$#;i;i--)){ echo "${!i}";}

123 Over a year ago

@F.Hauri It's not useless in arithmetic evaluation. In this particular case, sure you could use i on it's own, but only because 0 evaluates to false. Imagine you want every second param and there is an odd number. You will skip zero and cause a segfault

Mohammad Razeghi · Accepted Answer · 2015-09-18 09:00:10Z

5

you can use this one liner:

echo $@ | tr ' ' '\n' | tac | tr '\n' ' '

answered Sep 18, 2015 at 9:00

Mohammad Razeghi

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2 Comments

123 Over a year ago

Geat unless you have spaces in any of your arguments

gniourf_gniourf Over a year ago

will also break if you have glob characters in your arguments; will also break consecutive spaces; a better (and shorter and more efficient) method: printf '%s\n' "$@" | tac. Though, this will also break if you have newlines in your arguments.

gniourf_gniourf · Accepted Answer · 2015-09-18 09:22:51Z

2

bash:

#!/bin/bash
for i in "$@"; do
    echo "$i"
done | tac

call this script like:

./reverse 1 2 3 4

it will print:

edited Sep 18, 2015 at 9:22

gniourf_gniourf

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answered Sep 18, 2015 at 8:33

Vrata Blazek

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2 Comments

gniourf_gniourf Over a year ago

Will break if arguments contain newlines. A shorter way is: printf '%s\n' "$@" | tac.

F. Hauri - Give Up GitHub Over a year ago

@gniourf_gniourf I finally have it: bash reverse function, without fork and without loop. The only eval use a limited predefined string.

user8017719 · Accepted Answer · 2020-01-06 21:01:56Z

1

Portably and POSIXly, without arrays and working with spaces and newlines:

Reverse the positional parameters:

flag=''; c=1; for a in "$@"; do set -- "$a" ${flag-"$@"}; unset flag; done

Print them:

printf '<%s>' "$@"; echo

answered Jan 6, 2020 at 21:01

user8017719

1 Comment

Paul Jan 10 at 4:08

local zCopy=; for a in "$@"; do set -- "$a" ${zCopy:+"$@"}; zCopy=t; done # Using :+ instead of - eliminates the possibility of being affected by or affecting a globally declared variable when used inside a function.

Paul · Accepted Answer · 2025-01-10 04:39:19Z

0

A loop is generally slow in bash, so I chose to use readarray to create a reversed array...

declare -a aReversed
readarray -d '' -t aReversed < <(printf '%s\0' "$@" | tac -s $'\0')

# aReversed now contains an array of arguments in reverse order

printf 'ARG %s\n' "${aReversed[@]}"

# or

for zArg in "${aReversed[@]}"; do
  printf 'ARG %s\n' "$zArg"
done

This handles newlines in arguments because...

readarray -d '' # tells readarray to split on null terminator (the space is needed)
readarray -t # tells readarray to exclude the terminator from the array values.
printf '\0' # tells printf to output a null byte.
tac -s '$\0' # tells tac that the file is null byte terminated.

edited Jan 10 at 4:39

answered Jan 10 at 4:19

Paul

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