TopolearnVariables hold paths, names and settings in every script and are the first thing you expand.
Assign with name=value and no spaces around `=`: name = value would run a command called name. Read a variable with $name, or ${name} when the name must be separated from following text, as in ${name}_backup.
Every bash variable is a string unless you declare otherwise; count=5 stores the text 5, and arithmetic happens only inside $(( )). Referencing a variable that was never set silently gives an empty string, which is why typos are dangerous (strict mode, later, fixes that). Remove a variable with unset name and protect one with readonly NAME=value; assigning to a read-only variable is an error that aborts a non-interactive script.
Bash maintains several special parameters:
$?exit status of the last command,$$PID of the shell,$0script name,$1,$2... positional arguments,$#how many,$RANDOMa random integer,$PWDthe current directory,$HOMEyour home.
Convention: lower-case names for your own script variables and UPPER_CASE for environment variables and constants, so you never clobber PATH or HOME by accident. Values with spaces must be quoted when assigned: title="My report".
Example
name="Ada" # no spaces around =
greeting="Hello, $name"
echo "$greeting" # Hello, Ada
echo "${name}_backup" # Ada_backup (braces delimit the name)
count=5
echo "$count + 1" # 5 + 1 (just text)
echo "$(( count + 1 ))" # 6 (arithmetic expansion)
unset name
echo "[$name]" # [] (unset expands to empty)
readonly PI=3.14
declare -p PI # declare -r PI="3.14"
echo "PID $$, last status $?"Remember
- name=value with no spaces; read it with "$name" or "${name}"
- Unset variables expand to an empty string, silently
- Use lower-case names for script variables, UPPER_CASE for environment ones
Watch out for
- Writing
x = 5: bash runs a command named x with arguments = and 5.