Reading and Writing

Beyond listing files, the operator’s everyday I/O is putting data in and getting data out. The same set of commands covers tiny configs, terabyte logs, and binary blobs because Linux treats all of them as byte streams against a path.

Reading

Form	Effect
cat FILE	print contents
less FILE	pager
head -n N FILE / tail -n N FILE	first / last N lines
tail -f FILE	follow as the file grows
read VAR < FILE	read a single line into a shell variable
mapfile -t LINES < FILE	read all lines into a Bash array
while IFS= read -r line; do ...; done < FILE	safe line loop

Writing (redirections)

Form	Effect
cmd > FILE	write stdout, truncating
cmd >> FILE	append stdout
cmd 2> FILE	write stderr, truncating
cmd 2>> FILE	append stderr
cmd > FILE 2>&1	stdout + stderr → file
cmd &> FILE	same (Bash short form)
cmd > /dev/null 2>&1	discard everything
cmd | tee FILE	write and keep going on stdout
cmd | tee -a FILE	append + tee
cmd < FILE	feed file as stdin
cmd <<< "string"	here-string as stdin
: > FILE	truncate FILE to zero bytes

Heredocs

$ cat <<EOF > /etc/myapp/config.yml
$ server:
  $ port: 8080
  $ host: $HOSTNAME
$ EOF

$ cat <<'EOF' > script.sh
$ echo $LITERAL
$ EOF

$ tr a-z A-Z <<-EOF
    $ hello
    $ world
$ EOF

Writing without a process

Form	Effect
echo "text" > FILE	simple write
printf '%s\n' "$x" > FILE	safer for arbitrary data
: > FILE	create or empty a file in one syscall

Process substitution

Form	Effect
diff <(cmdA) <(cmdB)	treat command output as files
cmd > >(filter1) 2> >(filter2)	send streams through filters

Atomic writes

A common safe-write pattern. Write to a temp file in the same directory, then rename.

$ cat > "$f.tmp" <<EOF
$ contents
$ EOF
$ mv "$f.tmp" "$f"

mv (rename) within the same filesystem is atomic; readers either see the old file or the new one, never a half-written one.

Editing in place

Form	Effect
sed -i 's/old/new/g' FILE	in-place substitution (GNU)
sed -i '' 's/old/new/g' FILE	BSD / macOS (note empty arg)
perl -pi -e 's/old/new/g' FILE	portable alternative
ex -s -c '%s/old/new/g|wq' FILE	POSIX-correct in-place edit

Splitting and joining

Form	Effect
split -l 1000 big.csv chunk_	split by line count
split -b 100M big.bin part_	split by byte count
cat part_* > big.bin	reassemble

Useful tools

Form	Effect
dd if=SRC of=DST bs=4M status=progress	raw block copy
cp --reflink=auto	CoW copy on Btrfs / XFS / APFS
rsync -aP src/ dst/	preserve everything, with progress
shred -u FILE	overwrite then delete (limited use on CoW / journaled filesystems)

References

• man 1 cat, man 1 less, man 1 head, man 1 tail, man 1 read, man 1 mapfile (reading file contents).

• man 1 tee, man 1 echo, man 1 printf, man 1 truncate (writing without a process).

• man 1 sed, man 1 perl, man 1 ex (in-place editing).

• man 1 split, man 1 dd, man 1 cp, man 1 rsync, man 1 shred (bulk move and copy).

• man 1 fallocate, man 1 mv (atomic write pattern).

• Files for the file-as-byte-stream model these tools build on.

• Archives for compressed and bundled forms.

• Encryption for sealing the byte stream before it lands on disk.

• Standard I/O for the redirection mechanics behind >, >>, |, and heredocs.