I/O and Redirection

Control input/output streams, compose here-docs, and use process substitution.

Streams

• stdin (0), stdout (1), stderr (2)

Redirecting output/input

echo "hi" > out.txt       # overwrite
echo "again" >> out.txt    # append
cmd < in.txt               # stdin from file
cmd > out.txt 2> err.txt   # split stdout/stderr
cmd &> all.txt             # both to same file (bash)

Pipelines

producer | filter | consumer

Exit status is that of the last command; use set -o pipefail to fail on any stage.

Here-strings and here-docs

cat <<< "$var"           # here-string (stdin from variable)

cat <<'EOF' > script.sh    # literal (no expansion)
#!/usr/bin/env bash
echo "Hello"
EOF

cat <<EOF | sed 's/x/y/g'
line x
EOF

Process substitution

diff <(sort a.txt) <(sort b.txt)

Creates named pipes for commands that expect file paths.

File descriptors

exec 3> log.txt
echo "log" >&3
exec 3>&-

Open/close custom FDs for advanced routing.

Summary

• Redirect stdin/stdout/stderr, use here-docs/strings, and process substitution for flexible pipelines