Globbing for only files in Bash

Globbing for Only Files in Bash: A Comprehensive Guide

Learn how to effectively use Bash globbing patterns to select only files, excluding directories, symlinks, and other file types, ensuring precise command execution.

Bash globbing is a powerful feature that allows you to specify sets of filenames using wildcard characters. While commonly used to match both files and directories, there are often scenarios where you need to target only regular files. This article will guide you through various techniques to achieve precise file-only globbing in Bash, enhancing the reliability and safety of your shell scripts and command-line operations.

Understanding Basic Globbing Behavior

By default, glob patterns like * or *.txt will match both files and directories that fit the pattern. This can be problematic when commands are designed to operate exclusively on files, such as cat, grep, or mv (when moving to a new name, not into a directory). Executing such commands on directories can lead to errors, unexpected behavior, or even data loss.

mkdir my_dir
touch file1.txt file2.log

# This will list both files and the directory
ls *

# This will attempt to 'cat' the directory, resulting in an error
cat *

Default globbing behavior matching files and directories

Using find for Robust File Selection

For more complex or robust scenarios, especially when dealing with subdirectories or specific file properties, the find command is often the most reliable tool. It offers extensive filtering capabilities that go beyond simple globbing.

# Find only regular files in the current directory
find . -maxdepth 1 -type f

# Find only regular files and execute a command on them
find . -maxdepth 1 -type f -exec ls -l {} \;

# Using xargs for efficiency with many files
find . -maxdepth 1 -type f -print0 | xargs -0 cat

Using find to select only regular files

flowchart TD
    A["Start: Need to process files only"] --> B{Globbing or `find`?}
    B -->|Simple case, current dir| C["Use `GLOBIGNORE` or `extglob`"]
    B -->|Complex, subdirs, specific types| D["Use `find` command"]
    C --> C1["Set `GLOBIGNORE='*:*'`"]
    C --> C2["Enable `extglob` and use `!(*/)`"]
    C1 --> E["Process files"]
    C2 --> E["Process files"]
    D --> D1["Specify `-type f` for files"]
    D --> D2["Combine with `-maxdepth`, `-name`, etc."]
    D1 --> E["Process files"]
    D2 --> E["Process files"]
    E --> F["End: Files processed safely"]

Decision flow for selecting file-only globbing methods

Bash Globbing Options for File-Only Selection

Bash provides several built-in mechanisms to refine globbing behavior. These are generally more efficient for simple cases within the current directory compared to external commands like find.

Method 1: Using GLOBIGNORE

The GLOBIGNORE shell variable allows you to specify a colon-separated list of patterns to exclude from globbing matches. To exclude directories, you can set GLOBIGNORE to match anything ending with a slash, which typically denotes a directory.

mkdir dir1 dir2
touch fileA.txt fileB.log

# Set GLOBIGNORE to exclude directories
GLOBIGNORE='*:*'

# Now, '*' will only match files
ls *

# Unset GLOBIGNORE to revert to default behavior
unset GLOBIGNORE

Using GLOBIGNORE to exclude directories

Method 2: Using extglob with Pattern Matching

Bash's extglob option enables extended pattern matching operators, similar to regular expressions. This allows for more sophisticated exclusions directly within the glob pattern.

mkdir folder1 folder2
touch doc1.pdf doc2.txt

# Enable extended globbing
shopt -s extglob

# Match everything EXCEPT directories (patterns ending with /)
ls !(*/*)

# Another common pattern to exclude directories
ls !(*/*|*/)

# Disable extended globbing
shopt -u extglob

Using extglob to exclude directories from glob matches

Method 3: Looping with Conditional Checks

For maximum control and clarity, especially within scripts, iterating through all matches and performing a conditional check (-f for regular file) is a very reliable method.

mkdir test_dir
touch test_file.txt

for item in *;
do
  if [ -f "$item" ]; then
    echo "Processing file: $item"
    # Your command here, e.g., cat "$item"
  fi
done

Looping through glob matches and checking if they are regular files