bash script to check file name begins with expected string

Question

Running on OS X with a bash script:

sourceFile=`basename $1`
shopt -s nocasematch
if [[ "$sourceFile" =~ "adUsers.txt" ]]; then echo success ; else echo fail ; fi

The above works, but what if the user sources a file called adUsers_new.txt?

I tried:

if [[ "$sourceFile" =~ "adUsers*.txt" ]]; then echo success ; else echo fail ; fi

But the wildcard doesn't work in this case. I'm writing this script to allow for the user to have different iterations of the source file name, which must begin with aduser and have the .txt file extension.

I believe you need to not quote the RHS of the =~ operator. — Etan Reisner
– Etan Reisner, Commented Aug 21, 2014 at 1:42

Community · Accepted Answer · 2023-10-11 22:00:10Z

8

In bash, you can get the first seven characters of a shell variable with:

${sourceFile:0:7}

and the last four with:

${sourceFile:${#sourceFile}-4}

Armed with that knowledge, simply use those expressions where you would normally use the variable itself, something like the following script:

arg=$1
shopt -s nocasematch
i7f4="${arg:0:7}${arg:${#arg}-4}"
if [[ "${i7f4}" = "adusers.txt" ]] ; then
    echo Okay
else
    echo Bad
fi

You can see it in action with the following transcript:

pax> check.sh hello
Bad

pax> check.sh addUsers.txt
Bad

pax> check.sh adUsers.txt
Okay

pax> check.sh adUsers_new.txt
Okay

pax> check.sh aDuSeRs_stragngeCase.pdf.gx..txt
Okay

edited Oct 11, 2023 at 22:00

CommunityBot

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answered Aug 21, 2014 at 1:40

paxdiablo

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1 Comment

Dmitry Alexandrov Over a year ago

That works undoubtedly. But why? Why not just use wildcard matching?

Dmitry Alexandrov · Accepted Answer · 2014-08-21 03:12:44Z

6

=~ operator requires regexp, not wildcard. == accepts wildcards, but they should not be quoted:

if [[ "$sourceFile" == adUsers*.txt ]]; then echo success; else echo fail; fi

You may use a regexp too of course, but it would be a bit overkill:

if [[ "$sourceFile" =~ ^adUsers.*\.txt$ ]]; then echo success; else echo fail; fi

Please note that regexp is open by default (a == .*a.*) while glob is closed (a != *a*).

edited Aug 21, 2014 at 3:12

answered Aug 21, 2014 at 2:58

Dmitry Alexandrov

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1 Comment

Jesse Over a year ago

The RegEx makes it applicable to more situations

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