Diff for "BashFAQ/020"

How can I find and safely handle file names containing newlines, spaces or both?

First and foremost, to understand why you're having trouble, read Arguments to get a grasp on how the shell understands the statements you give it. It is vital that you grasp this matter well if you're going to be doing anything with the shell.

The preferred method to deal with arbitrary filenames is still to use find(1):

find ... -exec command {} \;

or, if you need to handle filenames en masse:

find ... -exec command {} +

xargs is rarely ever more useful than the above, but if you really insist, remember to use -0:

# Requires GNU/BSD find and xargs
find ... -print0 | xargs -r0 command

# Never use xargs without -0 or similar extensions!

Use one of these unless you really can't.

Another way to deal with files with spaces in their names is to use the shell's filename expansion (globbing). This has the disadvantage of not working recursively (except with zsh's extensions or bash 4's globstar), but if you just need to process all the files in a single directory, it works fantastically well.

For example, this code renames all the *.mp3 files in the current directory to use underscores in their names instead of spaces:

# Bash/ksh
for file in ./*\ *.mp3; do
  mv "$file" "${file// /_}"
done

For more examples of renaming files, see FAQ #30.

Remember, you need to quote all your Parameter Expansions using double quotes. If you don't, the expansion will undergo WordSplitting (see also argument splitting and BashPitfalls). Also, always prefix globs with "./"; otherwise, if there's a file with "-" as the first character, the expansions might be misinterpreted as options.

Another way to handle filenames recursively involves using the -print0 option of find (a GNU/BSD extension), together with bash's -d option for read:

# Bash
unset a i
while IFS= read -r -d $'\0' file; do
  a[i++]="$file"        # or however you want to process each file
done < <(find /tmp -type f -print0)

The preceding example reads all the files under /tmp (recursively) into an array, even if they have newlines or other whitespace in their names, by forcing read to use the NUL byte (\0) as its line delimiter. Since NUL is not a valid byte in Unix filenames, this is the safest approach besides using find -exec. IFS= is required to avoid trimming leading/trailing whitespace, and -r is needed to avoid backslash processing. In fact, $'\0' is actually the empty string (bash doesn't support passing NUL bytes to commands even built-in ones) so we could also write it like this:

# Bash
unset a i
while IFS= read -r -d '' file; do
  a[i++]="$file"
done < <(find /tmp -type f -print0)

So, why doesn't this work?

# DOES NOT WORK
unset a i
find /tmp -type f -print0 | while IFS= read -r -d '' file; do
  a[i++]="$file"
done

Because of the pipeline, the entire while loop is executed in a SubShell and therefore the array assignments will be lost after the loop terminates. (For more details about this, see FAQ #24.)

CategoryShell