Variables & Expansion

Hash shell supports environment variable expansion in commands and arguments.

Basic Syntax

$VAR - Simple Expansion

#> export NAME=Julio
#> echo Hello $NAME
Hello Julio

${VAR} - Braced Expansion

Use braces when concatenating:

#> export PRE=test
#> echo ${PRE}ing
testing

#> echo $PREing
# Tries to expand $PREing (undefined)

Special Variables

$? - Exit Code

#> true
#> echo $?
0

#> false
#> echo $?
1

$$ - Process ID

#> echo $$
12345

$0 - Shell Name

#> echo $0
hash

Variable Name Rules

Valid names:

• Start with letter or underscore
• Contain letters, numbers, underscores
• Case-sensitive

$HOME       # Valid
$MY_VAR     # Valid
$_private   # Valid
$123        # Invalid (starts with number)
$my-var     # Invalid (hyphen)

Escaping

Prevent expansion with backslash:

#> echo \$HOME
$HOME

#> echo Price: \$5
Price: $5

Quote Handling

Double Quotes - Variables Expand

#> export NAME=Julio
#> echo "Hello $NAME"
Hello Julio

Single Quotes - No Expansion

#> echo 'Hello $NAME'
Hello $NAME

Common Patterns

Building Paths

#> export PROJECT=myapp
#> export BUILD_DIR=/tmp/builds
#> mkdir -p ${BUILD_DIR}/${PROJECT}

PATH Manipulation

#> export PATH=$HOME/bin:$PATH

Configuration

# In .hashrc
export EDITOR=vim
export PROJECT_DIR=~/projects

alias edit='$EDITOR'
alias proj='cd $PROJECT_DIR'

Undefined Variables

Expand to empty string:

#> unset UNDEFINED
#> echo "Value: $UNDEFINED"
Value:

Limitations

Current limitations (may be added):

• ${VAR:-default} - Default value
• ${VAR:=value} - Assign default
• ${#VAR} - Length
• ${VAR%pattern} - Remove suffix
• ${VAR#pattern} - Remove prefix