I have a series of files named KLLxxx (the x represents numbers). They're text files but have no file extension to them. Just files that say KLL followed by three digits. I want them all to end with the file extension .fastq. Such as KLL123 -> KLL123.fastq.

So I tried using this command

mv KLL* KLL*.fastq

But I keep on getting the error message:

mv: target 'KLL067.fastq' is not a directory

I know about the rename command function, but I've never used Perl before so I don't know how to approach this. I know this is a basic question, but this is my first time trying out Linux.

3 Answers 3


prename... rename with Perl expressions. The following command takes the list of files (KLL*), and for each one, it grabs the entire name of KLL plus three numbers (KLL\d{3})) and saves that into the special named $1 variable. the s at the beginning of the expression means 'substitute' what's between the first two // with what is between the 2nd set, which is the full filename in $1 and append .fastq to it.

prename 's/(KLL\d{3})/$1.fastq/' KLL*
  • Use 's/(KLL\d{3})$/$1.fastq/' - otherwise it will rename KLL* files that have already been renamed, or with extra chars after the 3 digits.
    – cas
    Commented Nov 6, 2015 at 23:23

Running the following find command in the directory you are looking at will do the trick:

find . -name 'KLL[0-9][0-9][0-9]' -exec sh -c 'mv $0 $0.fastq' {} \;

Which will add the extension .fastq to any file name starting with KLL followed by three digits.

  • 3
    There's no need for the sh invocation: "-exec mv {} {}.fastq" by itself works and doesn't spin up a whole new shell for each file. And, of course, weird things will happen if any of the KLL files have "weird" characters in their names (eg., spaces, tabs, slashes).
    – minnmass
    Commented Nov 6, 2015 at 16:56
  • 2
    ... just something for the OP to keep in mind when their requirement changes next week is all.
    – minnmass
    Commented Nov 6, 2015 at 19:32

many possible solutions here, this should be one:

for f in KLL*; do mv "$f" "$f.fastq"; done
  • 1
    for f in KLL[0-9][0-9][0-9] Commented Nov 6, 2015 at 21:06

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