Shell Operators

Command Chaining

Operator	Name	Behavior
A ; B	Semicolon	Run B after A regardless of A’s exit status
A && B	AND	Run B only if A succeeds (exit code 0)
A || B	OR	Run B only if A fails (non-zero exit code)
A | B	Pipe	Send A’s stdout as B’s stdin

# Always run both
mkdir /tmp/work ; cd /tmp/work

# Only continue if previous succeeds
apt update && apt upgrade -y

# Fallback on failure
ping -c1 host1 || ping -c1 host2

# Chain with both
./build.sh && echo "Success" || echo "Failed"

---

Output Redirection

Operator	Purpose
>	Redirect stdout to file (overwrite)
>>	Redirect stdout to file (append)
2>	Redirect stderr to file
2>>	Redirect stderr to file (append)
&>	Redirect both stdout and stderr to file
2>&1	Redirect stderr to wherever stdout is going
< file	Use file as stdin

# Save command output to file
echo "hello" > output.txt

# Append to a log
echo "entry" >> log.txt

# Discard errors
find / -name "*.conf" 2>/dev/null

# Capture everything
./script.sh &> all-output.txt

# Redirect stderr to stdout then pipe both
./script.sh 2>&1 | grep "error"

---

Background & Job Control

Operator	Purpose
&	Run command in background
Ctrl+Z	Suspend foreground job
bg	Resume suspended job in background
fg	Bring background job to foreground
jobs	List background jobs
nohup	Run command immune to hangups (survives logout)
disown	Detach a running job from the shell

# Run in background
./long-task.sh &

# Check background jobs
jobs -l

# Survive logout
nohup ./server.sh &

---

Subshells & Grouping

Syntax	Purpose
(A; B)	Run A and B in a subshell (own environment)
{ A; B; }	Run A and B in the current shell (grouped)
$(command)	Command substitution — capture output
`command`	Command substitution (legacy syntax)

# Subshell — cd doesn't affect parent
(cd /tmp && ls)

# Grouping — redirect output of multiple commands
{ echo "header"; cat data.txt; } > combined.txt

# Command substitution
files=$(ls *.txt)
echo "Today is $(date +%Y-%m-%d)"

---

Test & Conditional

Syntax	Purpose
[[ condition ]]	Bash conditional test
$?	Exit code of last command (0 = success)
test expression	POSIX conditional test

# Check if file exists
[[ -f /etc/passwd ]] && echo "exists"

# Check exit code
grep -q "root" /etc/passwd
echo $?  # 0 if found, 1 if not

# String comparison
[[ "$USER" == "root" ]] && echo "running as root"

---

Special Variables

Variable	Meaning
$?	Exit code of last command
$$	Current shell’s PID
$!	PID of last background command
$0	Name of the script
$1–$9	Positional arguments
$#	Number of arguments
$@	All arguments (as separate words)
$*	All arguments (as one word)

---

Links

• Bash Reference Manual — Shell Operations
• Advanced Bash-Scripting Guide