Shell by Example: Reading Files POSIX 

#!/bin/sh

Shell provides several ways to read file contents. This example covers common file reading patterns.

Create a sample file for demonstration:

 

cat >/tmp/sample.txt <<'EOF'
Line 1: Hello
Line 2: World
Line 3: Shell
Line 4: Scripting
Line 5: Example
EOF

Read entire file with cat:

 

echo "Using cat:"
cat /tmp/sample.txt

Read file into a variable:

 

content=$(cat /tmp/sample.txt)
echo "File content in variable:"
echo "$content"

Read first N lines with head:

 

echo "First 2 lines (head):"
head -n 2 /tmp/sample.txt

Read last N lines with tail:

 

echo "Last 2 lines (tail):"
tail -n 2 /tmp/sample.txt

Read specific line range:

 

echo "Lines 2-4 (sed):"
sed -n '2,4p' /tmp/sample.txt

Read file line by line with while loop:

 

echo "Reading line by line:"
while IFS= read -r line; do
  echo "  -> $line"
done </tmp/sample.txt

IFS= prevents leading/trailing whitespace stripping. -r prevents backslash interpretation.

Process lines with line numbers:

 

echo "With line numbers:"
n=1
while IFS= read -r line; do
  echo "  $n: $line"
  n=$((n + 1))
done </tmp/sample.txt

Read file and handle last line without newline:

 

printf "no newline" >/tmp/nolf.txt
echo "Reading file without final newline:"
while IFS= read -r line || [ -n "$line" ]; do
  echo "  [$line]"
done </tmp/nolf.txt
rm /tmp/nolf.txt

Read specific fields from each line:

 

cat >/tmp/data.txt <<'EOF'
alice:30:engineer
bob:25:designer
carol:35:manager
EOF

echo "Reading colon-separated fields:"
while IFS=: read -r name age job; do
  echo "  $name is a $age year old $job"
done </tmp/data.txt

Read file into array (bash-specific shown later):

Check if file exists before reading:

 

read_safe() {
  if [ ! -f "$1" ]; then
    echo "Error: File not found: $1" >&2
    return 1
  fi
  cat "$1"
}

read_safe /tmp/sample.txt >/dev/null && echo "File read successfully"
read_safe /nonexistent 2>&1

Read from stdin:

 

echo "Enter your name (or type 'Alice'):"
name="Alice" # Simulated input
echo "Hello, $name"

Read with timeout (bash/ksh feature): read -t 5 input # Wait 5 seconds max

Read a single character: read -n 1 char # Bash-specific

Read with prompt: read -p “Enter value: ” value # Bash-specific

Count lines, words, characters:

 

echo "File statistics:"
wc /tmp/sample.txt | while read -r lines words chars filename; do
  echo "  Lines: $lines"
  echo "  Words: $words"
  echo "  Chars: $chars"
done

Read binary files (not recommended for text processing):

 

echo "Binary file info:"
ls -l /bin/sh | cut -d' ' -f5-

Read compressed files:

 

echo "Test" | gzip >/tmp/test.gz
echo "Compressed file content:"
zcat /tmp/test.gz 2>/dev/null || gzip -dc /tmp/test.gz
rm /tmp/test.gz

Read from URL (using curl or wget):

 

# content=$(curl -s https://example.com)
# content=$(wget -qO- https://example.com)
Bash

Bash provides mapfile/readarray for reading into arrays:

 

mapfile -t lines < /tmp/sample.txt
echo "Number of lines: ${#lines[@]}"
echo "First line: ${lines[0]}"

Read into array with process substitution:

 
mapfile -t users < <(cut -d: -f1 /etc/passwd | head -3)

Read file descriptor directly:

 

echo "Using file descriptor:"
exec 3</tmp/sample.txt
read -r first_line <&3
echo "  First line: $first_line"
read -r second_line <&3
echo "  Second line: $second_line"
exec 3<&- # Close file descriptor

Read with grep filtering:

 

echo "Lines containing 'Line':"
grep "Line" /tmp/sample.txt

Read random line:

 

echo "Random line:"
shuf -n 1 /tmp/sample.txt 2>/dev/null \
  || awk 'BEGIN{srand()} {lines[NR]=$0} END{print lines[int(rand()*NR)+1]}' /tmp/sample.txt

Cleanup

 

rm -f /tmp/sample.txt /tmp/data.txt

echo "File reading examples complete"

« Environment Variables | Writing Files »

by Jose Diaz-Gonzalez | inspired by Go by Example | source