Bash – Print shell arguments in reverse order

bashparametershell-script

I am a bit stuck. My task is to print the arguments to my script in reverse order except the third and fourth.

What I have is this code:

#!/bin/bash

i=$#
for arg in "$@"
do
    case $i
    in
        3) ;;
        4) ;;
        *) eval echo "$i. Parameter: \$$i";;
    esac
    i=`expr $i - 1`
done

As I hate eval (greetings to PHP), I am looking for a solution without it but I am not able to find one.

How can I define the position of the argument dynamically?

PS: No its not a homework, I am learning shell for an exam so I try to solve old exams.

Best Answer

eval is the only portable way to access a positional parameter by its dynamically-chosen position. Your script would be clearer if you explicitly looped on the index rather than the values (which you aren't using). Note that you don't need expr unless you want your script to run in antique Bourne shells; $((…)) arithmetic is in POSIX. Limit the use of eval to the smallest possible fragment; for example, don't use eval echo, assign the value to a temporary variable.

i=$#
while [ "$i" -gt 0 ]; do
  if [ "$i" -ne 3 ] && [ "$i" -ne 2 ]; then
    eval "value=\${$i}"
    echo "Parameter $i is $value"
  fi
  i=$((i-1))
done

In bash, you can use ${!i} to mean the value of the parameter whose name is $i. This works when $i is either a named parameter or a number (denoting a positional parameter). While you're at it, you can make use of other bash convenience features.

for ((i=$#; i>0; i--)); do
  if ((i != 3 && i != 4)); then
    echo "Parameter $i is ${!i}"
  fi
done

OUTPUT

$eight = declared
$eight = declared_and_preferred
$eight = 8
$eight = some_default

In the above example the explicitly declared environment variable is preferred to the command-line argument, but the command-line argument is used when the environment variable is either empty or unset. When both the 8th positional and the environment variable $eight are empty or unset the default value some_default is assigned to $eight. In either case the : can be removed from the :- or := statements if empty should be an acceptable value.

The variable $eight could as well have been set like:

printf '$eight = %s\n' "${eight:=${8:-some_default}}"

... and the previous line omitted entirely but I wanted to demonstrate that declaring a variable in that way does result in a persistent value, and so I did it in two commands. Either way $eight is set to the final value of that compound parameter expansion.

getopts - for robust scripts - often is the best way to handle command options. Positional parameters, on the other hand, are almost always the most simple means of robustly handling operands in a script.

For example:

touch file$(seq -ws\ file 100)
./foo *

OUTPUT

$eight = file008

There we only see the eighth operand, but there are 101 of them globbed from my test directory.

Bash – Print All Words on Lines of a File in Reverse Order

All examples presented below work for general case where there's an arbitrary number of words on the line. The essential idea is the same everywhere - we have to read the file line by line and print the words in reverse. AWK facilitates this the best because it already has all the necessary tools for text processing done programmatically, and is most portable - it can be used with any awk derivative, and most systems have it. Python also has quite a few good utilities for string processing that allow us to do the job. It's a tool for more modern systems, I'd say. Bash, IMHO, is the least desirable approach, due to portability, potential hazards, and the amount of "trickery" that needs to be done.

AWK

$ awk '{for(i=NF;i>=1;i--) printf "%s ", $i;print ""}' input.txt                                                  
Earth Hello 
Mars Hello

The way this works is fairly simple: we're looping backwards through each word on the line, printing words separated with space - that's done by printf "%s ",$i function (for printing formatted strings) and for-loop. NF variable corresponds to number of fields. The default field separator is assumed to be space. We start by setting a throw-away variable i to the number of words, and on each iteration, decrement the variable. Thus, if there's 3 words on line, we print field $3, then $2, and $1. After the last pass, variable i becomes 0, the condition i>=1 becomes false, and the loop terminates. To prevent lines being spliced together, we insert a newline using print "". AWK code blocks {} are processed for each line in this case (if there's a matching condition in front of code block, it depends on the match for the code block to be executed or not).

Python

For those who like alternative solutions, here's python:

$ python -c "import sys;print '\n'.join([ ' '.join(line.split()[::-1]) for line in sys.stdin ])"  < input.txt     
Earth Hello
Mars Hello

The idea here is slightly different. < operator tells your current shell to redirect input.txt into python's stdin stream, and we read that line by line. Here we use list comprehension to create a list of lines - that's what the [ ' '.join(line.split()[::-1]) for line in sys.stdin ] part does. The part ' '.join(line.split()[::-1]) takes a line, splits it into list of words, reverses the list via [::-1] , and then ' '.join() creates a space-separated string out of it. We have as a result a list of larger strings. Finally, '\n'.join() makes an even larger string, with each item joined via newline.

In short, this method is basically a "break and rebuild" approach.

BASH

#!/bin/bash

while IFS= read -r line
do
     bash -c 'i=$#; while [ $i -gt 0 ];do printf "%s " ${!i}; i=$(($i-1)); done' sh $line   
     echo 
done < input.txt

And a test run:

$ ./reverse_words.sh                                                                                              
Earth Hello 
Mars Hello

Bash itself doesn't have strong text processing capabilities. What happens here is that we read the file line by line via

while IFS= read -r line
do
   # some code
done < text.txt

This is a frequent technique and is widely used in shell scripting to read output of a command or a text file line-by-line. Each line is stored into $line variable.

On the inside we have

bash -c 'i=$#; while [ $i -gt 0 ];do printf "%s " ${!i}; i=$(($i-1)); done' sh $line

Here we use bash with -c flag to run a set of commands enclosed into single-quotes. When -c is used, bash will start assigning command-line arguments into variables starting with $0. Because that $0 is traditionally used to signify a program's name, I use sh dummy variable first.

The unquoted $line will be broken down into individual items due to the behavior known as word-splitting. Word splitting is often undesirable in shell scripting, and you will often hear people say "always quote your variables, like "$foo"." In this case, however, word-splitting is desirable for processing simple text. If your text contains something like $var, it might break this approach. For this, and several other reasons, I'd say python and awk approach are better.

As for the inner code, it's also simple: the unquoted $line is split into words and is passed to the inner code for processing. We take the number of arguments $#, store it into the throw away variable i, and again - print out each item using something known as variable indirection - that's the ${!i} part (note that this is bashism - it's not available in other shells). And again, we use printf "%s " to print out each word, space-separated. Once that's done, echo will append a newline.

Essentially this approach is a mix of both awk and python. We read the file line by line, but divide and conquer each line, using several of bash's features to do the job.

A simpler variation can be done with the GNU tac command, and again playing with word splitting. tac is used to reverse lines of input stream or file, but in this case we specify -s " " to use space as separator. Thus, var will contain a newline-separated list of words in reverse order, but due to $var not being quoted, newline will be substituted with space. Trickery, and again not the most reliable, but works.

#!/bin/bash

while IFS= read -r line
do
     var=$(tac -s " " <<< "$line" )
     echo  $var
done < input.txt

Test runs:

And here's the 3 methods with arbitrary lines of input

$ cat input.txt                                                                                                   
Hello Earth end of line
Hello Mars  another end of line
abra cadabra magic
$ ./reverse_words.sh                                                                                              
line of end Earth Hello 
line of end another Mars Hello 
magic cadabra abra 
$ python -c "import sys;print '\n'.join([ ' '.join(line.split()[::-1]) for line in sys.stdin ])"  < input.txt  
line of end Earth Hello
line of end another Mars Hello
magic cadabra abra
$ awk '{for(i=NF;i>=1;i--) printf "%s ", $i;print ""}' input.txt
line of end Earth Hello 
line of end another Mars Hello 
magic cadabra abra

Extra: perl and ruby

Same idea as with python - we split each line into array of words, reverse the array, and print it out.

$ perl -lane '@r=reverse(@F); print "@r"' input.txt                           
line of end Earth Hello
line of end another Mars Hello
magic cadabra abra


$ ruby -ne 'puts $_.chomp.split().reverse.join(" ")' < input.txt                                                  
line of end Earth Hello
line of end another Mars Hello
magic cadabra abra

Best Answer

Related Solutions

Shell – When running a shell script, is it possible to pass specific positional parameters without having to enter them all in order

OUTPUT

OUTPUT

Bash – Print All Words on Lines of a File in Reverse Order

AWK

Python

BASH

Test runs:

Extra: perl and ruby

Related Question