Loops & Reading Files Line-by-Line

for loops, while loops, and the robust while IFS= read -r line idiom

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Bash iterates over lists of words, numeric ranges, and file contents. Reading files line-by-line requires the robust while IFS= read -r line pattern.

The Canonical File Reading Loop

for item in "${array[@]}"; do ... done.

for i in {1..5}; do ... done.

CRITICAL IDIOM: while IFS= read -r line; do ... done < file.txt.

IFS= prevents stripping leading/trailing whitespace.

-r prevents backslash character escapes from being mangled.

Reading Configuration Line-by-Line

Try It Yourself »

#!/usr/bin/env bash

# Reading a file safely line-by-line
while IFS= read -r line || [[ -n "$line" ]]; do
    # Skip empty lines and comment lines
    if [[ -z "$line" || "$line" =~ ^# ]]; then
        continue
    fi
    echo "Processing record: $line"
done < "servers.txt"

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Test Yourself With Exercise

+15 XP

Why should you always include `-r` when reading a file with `read -r line` in Bash?

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