Linux Basic — Iterating Through Lists

Iterating Through Lists

You have arrays. You have command output. You have files in a directory. Now you need to loop over them safely — which sounds trivial, but classic shell scripts get this wrong constantly and break on filenames with spaces, newlines, or shell metacharacters.

This chapter is the cure.

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What You Will Learn

By the end of this chapter you will be able to:

• Loop over arrays with the correct quoting
• Loop over directory entries with globs, not ls
• Stream command output line-by-line with while read
• Use glob patterns including ** and extglob
• Handle arbitrary filenames with find … -print0 | xargs -0 or mapfile -d ''

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Topics in This Chapter

#	Topic	Description
1	Looping Array Elements	for x in "${arr[@]}" and index loops
2	Looping Directory Listings	globs, hidden files, empty matches
3	Looping Command Output	while read vs subshell pitfalls
4	Glob Patterns	*, ?, […], **, extglob
5	find & xargs	NUL-safe iteration for any filename
6	Practice Exercises	Hands-on tasks

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The Iteration Decision Tree

What am I looping over?
│
├── A Bash array       → for x in "${arr[@]}"
├── Files in cwd       → for f in *           (use a glob, never ls)
├── Recursive files    → find -print0 | mapfile -d ''   OR   shopt -s globstar; for f in **/*
├── Lines from a file  → while IFS= read -r line; do … done < file
├── Lines from a cmd   → while IFS= read -r line; do … done < <(cmd)
├── Tokens from a str  → IFS=… read -ra arr <<< "$str"
└── Numbers / range    → for ((i=0; i<n; i++)) …       OR   for i in {1..10}

Memorise these five forms and 95% of “iterate over X” questions answer themselves.

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The Forbidden Idiom

# ❌ NEVER do this
for f in $(ls); do …
for f in $(find . -type f); do …

Both word-split on $IFS and glob-expand the result. Any filename with a space, tab, glob char (*, ?, [), or newline breaks the loop. Use a glob, mapfile, or find -print0.

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Next → Looping Array Elements