Special Shell Parameters

In shell scripting, specific variables provide information about the execution environment. These can be referenced with or without braces (e.g., $var vs ${var}).

File Test Operaotrs

These operators are used within conditional statements (like if [ ... ]) to check file properties.

# Check existence
if [ -e "$config_file" ]; then
  echo "File exists."
fi

# Check file type
[ -f "$target" ]   # Regular file?
[ -d "$target" ]   # Directory?
[ -L "$target" ]   # Symbolic link?
[ -b "$target" ]   # Block special file?

# Check attributes
[ -s "$target" ]   # File exists and size > 0
[ -r "$target" ]   # Readable?
[ -w "$target" ]   # Writable?
[ -x "$target" ]   # Executable?
[ -O "$target" ]   # Owned by current user?

# Compare timestamps
if [ "/var/log/app.log" -nt "/var/log/app.log.bak" ]; then
  echo "Current log is newer."
fi

if [ "/var/log/app.log" -ot "/var/log/app.log.bak" ]; then
  echo "Current log is older."
fi

Arithmetic and Logical Operators

Operators used for integer comparisons and string logic.

# Integer Comparisons
[ $val1 -eq $val2 ]  # Equal
[ $val1 -ne $val2 ]  # Not equal
[ $val1 -lt $val2 ]  # Less than
[ $val1 -gt $val2 ]  # Greater than
[ $val1 -le $val2 ]  # Less than or equal
[ $val1 -ge $val2 ]  # Greater than or equal

# Logical Operators
[ -z "$string" ]     # True if string is empty
[ $cond1 -a $cond2 ] # Logical AND (within single brackets)
[ $cond1 -o $cond2 ] # Logical OR (within single brackets)

# Control flow short-circuits
command1 && command2  # Run command2 only if command1 succeeds
command1 || command2  # Run command2 only if command1 fails

Process Control and Utilities

Managing script behavior and signals.

# Trap signals to handle interrupts (e.g., Ctrl+C)
# This script ignores SIGINT, SIGQUIT, and SIGTSTP
trap - INT QUIT TSTP

# Setting positional parameters manually
set alpha beta gamma

echo $1  # Output: alpha
echo $2  # Output: beta
echo $3  # Output: gamma