Special Variables

These variables have special meaning in the shell.

Source: src/scripting/variables.f90, src/execution/executor.f90

Positional Parameters

$@ vs $*

set -- "arg one" "arg two" "arg three"

# Quoted $@ preserves word boundaries
for arg in "$@"; do
    echo "Arg: $arg"
done
# Arg: arg one
# Arg: arg two
# Arg: arg three

# Quoted $* joins with first char of IFS
for arg in "$*"; do
    echo "Arg: $arg"
done
# Arg: arg one arg two arg three

Manipulating Parameters

shift                         # Remove $1, shift others down
shift 3                       # Remove $1-$3
set -- "new" "args"          # Replace all parameters

Status Variables

Exit Status $?

true
echo $?                       # 0

false
echo $?                       # 1

ls /nonexistent 2>/dev/null
echo $?                       # 2 (or similar)

Background PID $!

sleep 100 &
echo "Started sleep with PID $!"
kill $!

Current Shell $$

echo "This shell's PID: $$"
echo $$ > /var/run/myscript.pid

Shell Options $-

echo $-                       # e.g., "himBHs"
set -x
echo $-                       # Now includes 'x'

Shell Information

echo "Running fortsh $FORTSH_VERSION"
echo "Nested $SHLVL levels deep"
echo "User ID: $UID"

Path Variables

echo "Current: $PWD"
echo "Previous: $OLDPWD"
cd -                          # Goes to $OLDPWD

Prompt Variables

PS1 Escape Sequences

PS1='\u@\h:\w\$ '             # user@host:/path$

History Variables

export HISTSIZE=10000
export HISTCONTROL=ignoredups:erasedups

IFS (Input Field Separator)

Controls word splitting:

IFS=$' \t\n'                  # Default: space, tab, newline

IFS=:
echo $PATH | tr ':' '\n'      # Split PATH

IFS=$'\n'
lines=($(cat file))           # Split on newlines only

Random and Sequences

echo $RANDOM                  # e.g., 12345
echo "Line $LINENO"           # Current line
echo "Running for $SECONDS seconds"

Regex Captures

After [[ string =~ regex ]]:

if [[ "v1.2.3" =~ ^v([0-9]+)\.([0-9]+)\.([0-9]+)$ ]]; then
    echo "Major: ${BASH_REMATCH[1]}"   # 1
    echo "Minor: ${BASH_REMATCH[2]}"   # 2
    echo "Patch: ${BASH_REMATCH[3]}"   # 3
fi

Readonly Variables

These cannot be modified:

readonly PPID UID EUID HOSTNAME SHELLOPTS FORTSH_VERSION