Shell by Example: For Loops

Shell by Example: For Loops POSIX

	`#!/bin/sh`
The `for` loop iterates over a list of items. It’s one of the most commonly used control structures in shell scripting.
Basic for loop over a list of words:	`for fruit in apple banana cherry; do echo "I like $fruit" done`
Loop over files in a directory using globbing:	`echo "Shell scripts in current directory:" for file in *.sh; do # Check if glob matched anything [ -e "$file" ] \|\| continue echo " - $file" done`
Loop over command output using command substitution:	`echo "Users with login shells:" for user in $(cat /etc/passwd \| cut -d: -f1 \| head -5); do echo " - $user" done`
POSIX sh lacks range syntax for ‘for’, so use ‘while’ instead:	`echo "Counting to 5:" i=1 while [ $i -le 5 ]; do echo " $i" i=$((i + 1)) done`
Bash Bash provides a C-style for loop with cleaner numeric iteration:	`for ((i=1; i<=5; i++)); do echo " $i" done`
Bash also supports brace expansion for ranges:	`for n in {1..5}; do echo " $n" done`
Step values work too: {1..10..2} gives 1,3,5,7,9
You can also use seq if available:	`echo "Using seq:" for n in $(seq 1 3); do echo " Number: $n" done`
Loop over positional parameters:	`set -- first second third for arg in "$@"; do echo "Argument: $arg" done`
Loop over lines in a file (safer than for loop):	`echo "Reading lines:" while IFS= read -r line \|\| [ -n "$line" ]; do echo " Line: $line" done << 'EOF' First line Second line Third line EOF`
The loop variable persists after the loop ends.	`for letter in a b c; do : # do nothing done echo "Last letter was: $letter"`

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