13

I'm quite new to Bash scripting. I have a "testscript", which I used as the basis for a more advanced / useful script:

#!/bin/bash
files=$1
for a in $files
do
    echo "$a"
done

When I call this without any quotes it just picks up one file in a directory:

testscript *.txt

But when I call it with quotes it works correctly and picks out all the text files:

testscript '*.txt'

What is going on here?

29

When you call a program

testscript *.txt

then your shell does the expansion and works out all the values. So it might, effectively call your program as

testscript file1.txt file2.txt file3.txt file4.txt

Now your program only looks at $1 and so only works on file1.txt.

By quoting on the command line you are passing the literal string *.txt to the script, and that is what is stored in $1. Your for loop then expands it.

Normally you would use "$@" and not $1 in scripts like this.

This is a "gotcha" for people coming from CMD scripting, where the command shell doesn't do globbing (as it's known) and always passes the literal string.

  • 6
    Just to clarify (for people other than the author of the above answer), using "$@" (as opposed to $@ or $1 $2 $3) will cause each filename to be quoted "file1.txt" "file2.txt" etc. For file1.txt this is meaningless, but if you have my file.txt, the quoting is critical to prevent the shell parsing to turn it into two filenames, one named my and one named file.txt. Always quote user input and glob expansion lest you be very unhappy some day. – Seth Robertson Aug 26 '16 at 16:44
  • 2
    And this isn't just a theoretical - Mac OS X once shipped with an update script that didn't properly quote arguments and ended up deleting peoples' hard drives in some circumstances. – fluffy Aug 26 '16 at 21:48
  • 2
    @fluffy, do you have a link about that? – Wildcard Aug 27 '16 at 8:05
  • @Wildcard Unfortunately I'm unable to find any articles about it, but it was big news in the tech world when it happened. I want to say it was in 2003/2004 or thereabouts, back when Apple was still getting the hang of being a UNIX distributor. – fluffy Aug 27 '16 at 20:29
  • 1
    @wildcard Ah, found it! xlr8yourmac.com/OSX/itunes2_erased_drives.html - it was actually an iTunes upgrade script that was the culprit. – fluffy Aug 27 '16 at 20:34
7

Without quotes the shell expands *.txt before invoking the script, so $1 is only the first file that gets expanded. All of the txt files are arguments to your script at that point (assuming there aren't too many).

With quotes that string is passed without being expanded into the script, which then lets the for do the expansion, as you're hoping for.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.