Shell by Example: Writing Files

Shell by Example: Writing Files POSIX

	`#!/bin/sh`
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):	`echo "Hello, World!" >/tmp/output.txt echo "Created file with: $(cat /tmp/output.txt)"`
Append with >> (adds to end of file):	`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`
Write multiple lines with here-document:	`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`
Here-document with variable expansion:	`name="Alice" cat >/tmp/template.txt <<EOF Hello, $name! Today is $(date +%Y-%m-%d) Your home is $HOME EOF echo "Template with variables:" cat /tmp/template.txt`
Write without newline using printf:	`printf "No newline here" >/tmp/nonewline.txt printf " - continued\n" >>/tmp/nonewline.txt cat /tmp/nonewline.txt`
Redirect stdout and stderr:	`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)"`
Discard output:	`echo "Discarding output:" ls /nonexistent 2>/dev/null # Discard stderr echo " Stderr discarded (no error shown)"`
Tee - write to file and stdout simultaneously:	`echo "Using tee:" echo "This goes to both" \| tee /tmp/tee.txt echo "File contains: $(cat /tmp/tee.txt)" echo "Append with tee -a:" echo "Appended line" \| tee -a /tmp/tee.txt >/dev/null cat /tmp/tee.txt`
Write from pipeline:	`echo "Pipeline to file:" seq 1 5 \| grep -v 3 >/tmp/pipeline.txt cat /tmp/pipeline.txt`
Atomic write (write to temp, then rename):	`atomic_write() { tmpfile=$(mktemp) echo "Important data" >"$tmpfile" mv "$tmpfile" "$1" } atomic_write /tmp/atomic.txt echo "Atomic write result: $(cat /tmp/atomic.txt)"`
Write with file descriptor:	`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`
Append with file descriptor:	`exec 4>>/tmp/fd.txt echo "Appended line" >&4 exec 4>&- cat /tmp/fd.txt`
Create file if not exists, don’t overwrite:	`if [ ! -f /tmp/nooverwrite.txt ]; then echo "New content" >/tmp/nooverwrite.txt fi echo "Safe write (no overwrite): $(cat /tmp/nooverwrite.txt)"`
Truncate file to empty:	`echo "content" >/tmp/truncate.txt : >/tmp/truncate.txt # or > /tmp/truncate.txt echo "After truncate, size: $(wc -c </tmp/truncate.txt) bytes"`
Bash Bash provides noclobber option to prevent overwriting:	`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:	`echo "Multiple files" \| tee /tmp/file1.txt /tmp/file2.txt >/dev/null echo "File1: $(cat /tmp/file1.txt)" echo "File2: $(cat /tmp/file2.txt)"`
Write binary data:	`printf '\x48\x65\x6c\x6c\x6f' >/tmp/binary.txt echo "Binary write: $(cat /tmp/binary.txt)"`
Write with specific permissions:	`( umask 077 echo "Secret" >/tmp/secret.txt ) ls -l /tmp/secret.txt \| awk '{print "Permissions:", $1}'`
Process substitution for writing (bash): diff <(echo “a”) <(echo “b”)
Write configuration file:	`write_config() { cat >"$1" <<EOF # Configuration file # Generated on $(date) host=localhost port=8080 debug=false EOF } write_config /tmp/app.conf echo "Config file:" cat /tmp/app.conf`
Cleanup temporary files:	`rm -f /tmp/output.txt /tmp/append.txt /tmp/multiline.txt rm -f /tmp/template.txt /tmp/nonewline.txt /tmp/stdout.txt rm -f /tmp/stderr.txt /tmp/both.txt /tmp/out.txt /tmp/err.txt rm -f /tmp/tee.txt /tmp/pipeline.txt /tmp/atomic.txt rm -f /tmp/fd.txt /tmp/nooverwrite.txt /tmp/truncate.txt rm -f /tmp/file1.txt /tmp/file2.txt /tmp/binary.txt rm -f /tmp/secret.txt /tmp/app.conf echo "File writing examples complete"`

« Reading Files | File Paths »