Shell script interpretation code and parameters

Question

I have the following shell script and they ask me to know how works and what the $1 and $2 parameters are.

#!/bin/bash

for i in `sort $1`; do   
    if grep $2 $i > /dev/null; then 
        echo A  
        cp $i /tmp  
        exit  
    fi  
done  
echo B;

My question is this: $1 can not be a directory because you can not sort on directories. Therefore, it must be a file. But the "for i in file" function implies that we will work for each line and the 'grep' tool does not work with lines.

I have considered that it should be a file that contains files, but for now I can only think of the tar files and it does not work with them.

Thanks!

Don'tReadLinesWithFor -- this code is intrinsically broken. See BashFAQ #1 for best practices around reading files line-by-line. — Charles Duffy
– Charles Duffy, Commented Mar 28, 2018 at 16:26
(That said, line-oriented files as a whole can't safely be used to store lists of arbitrary filenames, because files can have names that contain newlines. The only safe storage is for arbitrary filenames, like arbitrary C strings, environment variables, &c. is NUL-delimited). — Charles Duffy
– Charles Duffy, Commented Mar 28, 2018 at 16:27
Beyond that, it's not clear what you're actually asking. Is it for an explanation of how this code works? It doesn't work correctly -- it's broken. (In more ways than I covered already, too -- if you have * as a line in your file, it'll be replaced with a list of everything in your current working directory). — Charles Duffy
– Charles Duffy, Commented Mar 28, 2018 at 16:28
...you say "my question is this", but what follows is a statement, not a question. — Charles Duffy
– Charles Duffy, Commented Mar 28, 2018 at 16:29
Also, see How to deal with questions of the type "I don't understand how this code works?" on Meta StackOverflow -- the consensus is that such questions should be closed as either "unclear what you're asking", or "too broad". — Charles Duffy
– Charles Duffy, Commented Mar 28, 2018 at 16:30

Tom Fenech · Accepted Answer · 2018-03-28 17:09:47Z

1

sort $1 most probably means that $1 contains the path to a file. The contents of the file are most probably paths to other files. In its unquoted form, it will be subject to word-splitting and glob expansion. This is either an oversight or intended behaviour (e.g. it would expand 'dat?.lst' to a list of files, as mentioned in the comments).

The for loop makes the assumption that none of these paths contain any awkward characters (spaces, glob characters like *), and goes through every word in the sorted output.

$2 (which again should really be "$2") is a pattern to search in the file $i. If the contents of the file match the pattern, then the file is copied to /tmp and the script exits.

Given the caveat of not working with filenames containing newlines, the simplest fix would be:

#!/bin/bash

sort "$1" | while read -r file; do
    if grep -q "$2" "$file" 2>/dev/null; then
        echo A  
        cp "$file" /tmp  
        exit  
    fi
done

This assumes that the script is passed the path to a file as the first argument, like:

/path/to/first/file
/path/to/second/file

And a regular expression as the second argument, with $s escaped to prevent the shell from attempting to expand them.

edited Mar 28, 2018 at 17:09

answered Mar 28, 2018 at 16:25

Tom Fenech

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4 Comments

user unknown Over a year ago

Whether $1 or "$1" changes the semantics of the program. Maybe the writer want's to pass "dat?.lst" as parameter, which needs to be expanded?

gniourf_gniourf Over a year ago

@userunknown: that would then be a really awkward interface. If this is the case, the script should handle multiple arguments, and let the shell perform the globbing; it's then better to redesign the interface, have the pattern as the first argument, and the remaining arguments would then be the files. Usage would then be script thing-to-match file1 file2 file3, and allow script thing-to-match dat?.lst.

gniourf_gniourf Over a year ago

@userunknown: since the scripter didn't even bother quoting the expansion $2 for the grep statement, I'd argue that it's more an error from the scripter than an actual design feature.

user unknown Over a year ago

@gniourf_gniourf: AFAIK, the question is an interview question, where you should answer what the script does - not how to improve it.

user unknown · Accepted Answer · 2018-03-28 17:00:58Z

!/bin/bash

for i in `sort $1`  
do   
if grep $2 $i > /dev/null  
    then 
         echo A  
         cp $i /tmp  
         exit  
    fi  
done  
echo B;

Probably, the 1st param is filename or a list of filenames without whitespace, which itself contains a list of filenames. The $2 is a keyword which is searched for in these files, which were sorted by name first.

The first match leads to a copy of that file to /tmp - probably for further processing, and an echo message A and termination of the program. If no matching file is found, a different message is printed.

Note, that masking the $1 like this:

for i in $(sort "$1")
do

will prevent the expansion of wildcards,

script "dat-?.lst" foobar

and possibly break the script, so I would be careful with suggestions to improve it.

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