Shell Wildcards – How rm/ls Work with [0-9]

lsshellwildcards

I am learning the shell commands and came across the short tags eg.[0-9],[[:digit:]] etc.. As a proof of concept i tried deleting all the files with the rm command(i know its not a good practise but i am trying to understand how things work),like this

rm [0-9].txt

there were two files in the directory 0.txt and 9.txt and it deleted the files 0.txt and 9.txt .I guessed that the expression [0-9] is expanded and then read as 0.txt 1.txt 2.txt …. However when you try only
rm 5.txt,and the file does not exist, an error is thrown..

someone please tell me how the shorthand commands work when used with rm or ls.

Best Answer

This is called globbing (link to bash documentation, but this is not specific to bash).

When you ran rm [0-9].txt, the shell expanded [0-9].txt to the list of files present in the directory that matched that pattern. Then that list is passed as an argument to rm (each file as a separate argument).
So no, the shell didn't expand it to 0.txt 1.txt ... 9.txt, it looks at the files that matched.

Why you run just rm 5.txt, there is no glob pattern to expand, so the argument is passed as-is to rm, which notices that the file doesn't exist and complains.

Try something else: same command rm [0-9].txt, but in a directory that doesn't have any file that matches the pattern. You'll notice rm complains again, but this time it will say:

rm: cannot remove '[0-9].txt': No such file or directory

This is what happens (by default anyway) if a glob pattern doesn't match anything: the shell doesn't expand it and leaves it untouched.

POSIX reference for this sort of pattern matching: Pattern Matching Notation.

Related Solutions

Bash – Assignment to variables

Don't use backticks for command substitutions: the expansion rules are tricky and not completely consistent across shells. Use $(…) instead; it has exactly the same meaning, but an intuitive and portable syntax. You don't need to use any special quoting; the matching close parenthesis determines the end of the command and that's it. In the rare case when the command starts with an opening parenthesis, put a space to avoid confusion with arithmetic expansion (e.g.. variable=$( (foo; bar) || qux )).

count=$(echo $foo | sed 's/./&\
/g' | sort | uniq -c | sort -n -r | head -1 | awk '{print $1}')

count=$(echo $foo | sed 's/./&\n/g' | sort | uniq -c | sort -n -r | awk 'NR==1 {print $1}')

Shell – Triple dot wildcards

This wildcard exists in ksh93, bash ≥4.3 (≥ 4.0 if there are no symbolic link to directories in your tree) and zsh. It's spelled **. In ksh93, it needs to be activated first with set -o globstar. In bash, it needs to be activated first with shopt -o globstar.

ls -l src/**/foobar/**/*.java

This won't do to make the copy, though. The target of cp is a single directory, cp doesn't do any wildcard matching. You can't use a single cp command to drop files in different places.

You can use rsync instead. Pass it the root of the source tree and the root of the destination tree, and define include and exclude rules to copy only the files you want and the directories leading to them. Rsync will copy empty directories as well if they match the pattern, you can make it delete them afterwards with --prune-empty-dirs.

rsync --include='**/foobar/**/*.java' --include='**/' \
      --exclude='*' --prune-empty-dirs \
      src/ trusted/src/main/java/

Another tool you can use is pax. Note that pax is standard in that it is defined by POSIX (unlike rsync), but some Linux distributions omit it from the default installation (it's always available as a package however). The approach is similar to rsync: include .java files, exclude the rest; the syntax is a bit stranger: you specify a pattern replacement, which can be the original to include a file and not rename it, or an empty replacement to exclude a file. Leading directories are automatically created in the destination as necessary.

pax -rw -s '/\.java$/&/' -s '/.*//' src/* trusted/src/main/java/

Best Answer

Related Solutions

Bash – Assignment to variables

Shell – Triple dot wildcards

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