Linux

Let's see, and please do not execute this "as is" since I am pulling this out of my butt. Assuming directory name is "dir1", the file to be renamed is "crunk", and the new name is "chunk", then... (I assumed bash)

find dir1 -type f -name 'crunk' -exec bash -c 'mv "$0" "${0/crunk/chunk}"' {} \;

You can do that with the find command. Also xargs is a good one to know for stuff like this, but when commands have the capacity to work on a list already like find, you don't need xargs. You can use find -exec rename -n with a regex perhaps. I'm not a linux system currently and can't test a snippet, but I'll check back later if the question is still open.

| is usually my solution.

Quoting variables in scripts is my other go to before I actually start tracing what went wrong.

Two ways:

1. Say you want to rename the files Sevilla_big{number}.jpg to Sevilla_small{number}.jpg:

in bash, using the extglob option:

 shopt -s extglob
for f in pics/**/Sevilla_big*.jpg
do 
     mv "${f}" "${f/big/small/}"
done

(If you want to type it in a single line, you need to add semicolons as separators.)

The '"' are only needed if file names contain whitespaces.

2. Using find:

   find . -name 'Sevilla.*big.jpg' > t

   now, you edit the file named "t" with Emacs or vim so that you duplicate the file names, modify them as you want, and copy them using the copy-rectangle command right to the collumn with the original names. (Vim also offers this functionality, it is super useful to learn!) Delete any names of files you don't want to change. save the result to t.

Then, in bash:

 <t while read a b; do mv $a $b; done

Alternatively, you could also insert the mv command as a first collumn in t, save t and do:

source t

This variant wouldn't work with spaces in filenames.

3. Bonus option:

Use the mmv command

Here is a different approach involving loops instead. This allows for greater control over what happens. Note in this script the rename() function just echos the command without any effect. This is just a demonstration, so you can experiment with the arguments and edit the script first.

If you have only one argument that is then used to search with find, no slashes in search term is allowed. I didn't want complicate the script further and left it as it is.

#!/usr/bin/env bash

# Rename files
#
# This script has 3 ways to input filenames.
#
# If only one argument is given, then its used as search term to match
# filenames. If more than one argument
#
# is given, then instead searching for files it will rename all given
# filenames.
#
# If no filename is given, then list of files will be read from stdin pipe. In
# that case combine it with a tool like find.
#
# Change the below function rename() to set what to do with each old filename.
# The default is to echo, so change that first.
#
#
# Examples:
#
#   renameclean.sh '*txt'
#
#   renameclean.sh 'dir1/random?file.txt' 'dir2/subdir/anotherfile.zip'
#
#   find dir -type f -name '*.txt' | renameclean.sh

rename() {
    old="${1}"
    new="${old}"

    # Special characters such as ?, * or " need a backslash to match them
    # literally.
    new="${new/\?/REPLACEMENT}"
    new="${new/\*/REPLACEMENT}"
    new="${new/\"/REPLACEMENT}"

    echo mv -- "${old}" "${new}"
}

# If no argument is given, then read list from stdin.
if [ "${#}" == 0 ]; then
    while read -r file; do
        rename "${file}"
    done
# If one argument is given, then use it as a search name.
elif [ "${#}" == 1 ]; then
    find . -type f -name "${1}" -print0 |
        while read -r -d $'\0' file; do
            rename "${file}"
        done
# If more than one argument is given, then use all arguments as filenames to
# rename.
else
    for file in "${@}"; do
        rename "${file}"
    done
fi

Are you maybe looking for mmv (multimove)?

It's hard to tell what you actually want to do.

But you should not need to use grep to finde specific files. Just bash's own abilities should be enough.

For example */*a* would go into every directory and look for files with "a" in them.

For more complex operations find can also be used.