Shell by Example: Writing Files

Shell by Example: Writing Files POSIX + Bash

Shell provides several ways to write data to files using redirection operators. This example covers common file writing patterns. Basic write with `>` (overwrites existing file):	Edit `#!/bin/sh echo "Hello, World!" >/tmp/output.txt echo "Created file with: $(cat /tmp/output.txt)"`
	Output: Created file with: Hello, World!

Append with >> (adds to end of file):	Edit `#!/bin/sh echo "First line" >/tmp/append.txt echo "Second line" >>/tmp/append.txt echo "Third line" >>/tmp/append.txt echo "Appended file:" cat /tmp/append.txt`
	Output: Appended file: First line Second line Third line

Write multiple lines with here-document:	Edit `#!/bin/sh cat >/tmp/multiline.txt <<'EOF' This is line 1 This is line 2 This is line 3 EOF echo "Here-document file:" cat /tmp/multiline.txt`
	Output: Here-document file: This is line 1 This is line 2 This is line 3

Here-document with variable expansion:	Edit `#!/bin/sh name="Alice" cat >/tmp/template.txt <<EOF Hello, $name! Today is $(date) Your home is $HOME EOF echo "Template with variables:" cat /tmp/template.txt`
	Output: Template with variables: Hello, Alice! Today is Wed Jan 1 10:00:00 UTC 2025 Your home is /root

Write without newline using printf:	Edit `#!/bin/sh printf "No newline here" >/tmp/nonewline.txt printf " - continued\n" >>/tmp/nonewline.txt cat /tmp/nonewline.txt`
	Output: No newline here - continued

Redirect stdout and stderr:	Edit `#!/bin/sh echo "Redirect stdout (1) and stderr (2):" # Stdout only to file ls /tmp >/tmp/stdout.txt 2>/dev/null # Stderr only to file ls /nonexistent 2>/tmp/stderr.txt # Both to same file ls /tmp /nonexistent >/tmp/both.txt 2>&1 # Both to different files ls /tmp /nonexistent >/tmp/out.txt 2>/tmp/err.txt echo " Stderr captured: $(cat /tmp/stderr.txt)"`
	Output: Redirect stdout (1) and stderr (2): Stderr captured: ls: cannot access '/nonexistent': No such file or directory

Discard output:	Edit `#!/bin/sh echo "Discarding output:" ls /nonexistent 2>/dev/null # Discard stderr echo " Stderr discarded (no error shown)"`
	Output: Discarding output: Stderr discarded (no error shown)

Tee - write to file and stdout simultaneously:	Edit `#!/bin/sh echo "Using tee:" echo "This goes to both" \| tee /tmp/tee.txt echo "File contains: $(cat /tmp/tee.txt)" echo "Append with tee -a:" echo "Appended line" \| tee -a /tmp/tee.txt >/dev/null cat /tmp/tee.txt`
	Output: Using tee: This goes to both File contains: This goes to both Append with tee -a: This goes to both Appended line

Write from pipeline:	Edit `#!/bin/sh echo "Pipeline to file:" seq 1 5 \| grep -v 3 >/tmp/pipeline.txt cat /tmp/pipeline.txt`
	Output: Pipeline to file: 1 2 4 5

Atomic write (write to temp, then rename):	Edit `#!/bin/sh atomic_write() { tmpfile=$(mktemp) echo "Important data" >"$tmpfile" mv "$tmpfile" "$1" } atomic_write /tmp/atomic.txt echo "Atomic write result: $(cat /tmp/atomic.txt)"`
	Output: Atomic write result: Important data

Write with file descriptor:	Edit `#!/bin/sh exec 3>/tmp/fd.txt echo "Line 1" >&3 echo "Line 2" >&3 exec 3>&- # Close the file descriptor echo "File descriptor write:" cat /tmp/fd.txt`
	Output: File descriptor write: Line 1 Line 2

Append with file descriptor:	Edit `#!/bin/sh exec 4>>/tmp/fd.txt echo "Appended line" >&4 exec 4>&- cat /tmp/fd.txt`
	Output: Appended line

Create file if not exists, don’t overwrite:	Edit `#!/bin/sh if [ ! -f /tmp/nooverwrite.txt ]; then echo "New content" >/tmp/nooverwrite.txt fi echo "Safe write (no overwrite): $(cat /tmp/nooverwrite.txt)"`
	Output: Safe write (no overwrite): New content

Truncate file to empty:	Edit `#!/bin/sh echo "content" >/tmp/truncate.txt : >/tmp/truncate.txt # or > /tmp/truncate.txt echo "After truncate, size: $(wc -c </tmp/truncate.txt) bytes"`
	Output: After truncate, size: 0 bytes

Bash Bash provides noclobber option to prevent overwriting:	Edit `#!/bin/bash set -o noclobber echo "test" >existingfile.txt # Fails if file exists echo "test" >\|existingfile.txt # Force overwrite set +o noclobber`

Write to multiple files at once:	Edit `#!/bin/sh echo "Multiple files" \| tee /tmp/file1.txt /tmp/file2.txt >/dev/null echo "File1: $(cat /tmp/file1.txt)" echo "File2: $(cat /tmp/file2.txt)"`
	Output: File1: Multiple files File2: Multiple files

Write binary data:	Edit `#!/bin/sh printf '\x48\x65\x6c\x6c\x6f' >/tmp/binary.txt echo "Binary write: $(cat /tmp/binary.txt)"`
	Output: Binary write: Hello

Write with specific permissions:	Edit `#!/bin/sh ( umask 077 echo "Secret" >/tmp/secret.txt ) stat -c 'Permissions: %A' /tmp/secret.txt 2>/dev/null \|\| stat -f 'Permissions: %Sp' /tmp/secret.txt`
	Output: Permissions: -rw-------

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