Defining Variables#

By default all variables are local and the can just be defined by an assignment. It is important to leave no whitespace around the =. This would be a syntactic error. To define a global variable it has to be exported, either direct in an assignment or later by invoking export VARNAME. The export can be undone with export -n:

MY_LOCAL_VAR=123            # defining a local variable
export MY_GLOBAL_VAR=123    # defining a global variable
export MY_LOCAL_VAR         # this now also a global variable
export -n MY_LOCAL_VAR      # now it is local again

If not otherwise stated, all variables are treated as strings. E.g. MY_LOCAL_VAR is not interpreted as an integer. It is rarely needed but variables and their attributes can be declared explicitly:

declare FOO=bar             # equivalent to FOO=bar
declare -i N=100            # N is now defined as integer
declare -x MY_GLOBAL_VAR    # equivalent to export MY_GLOBAL_VAR

Sometime it is useful do delete a variable. An assignment VAR="" or even VAR= is not enough. This just assigns an empty string to VAR, but VAR is still defined. The command unset VAR undefines VAR.

Note

bash provides a special way to inherit or inject variables to its child processes’ environments without setting them for the parent process. If variables are set on the same line they are inherited as environment to the child but do not alter the environment of the parent (i.e. the current shell) LC_ALL=C MY_VAR=120 ./my_prog In the previous example the (existing) environment variable LC_ALL is not altered and (the non-existent variable) MY_VAR does not become an environment variable in the current shell, but both will be seen as environment from my_prog.

Inspecting Variables#

The attributes (including the scope) and the value of a variable can be checked with declare -p, this command displays an equivalent definition of the variable using the declare built-in command.

declare -p MY_LOCAL_VAR
declare -- MY_LOCAL_VAR="123"  # -- can be ignored

declare -p MY_GLOBAL_VAR
declare -x MY_GLOBAL_VAR="123" # -x indicates a global variable

There are several ways to inspect variables, some are listed in the following table.

Command

Effect

env

Print all environment variables

printenv

Print all environment variables

set

Print all shell variables (including functions)

declare -x

Print all environment variables declarations

export

Print all environment variables declarations

Using Variables#

Variables are referenced by their name preceding a $ symbol. There is no distinction in local and global variables. bash will replace $VAR by its value (or empty string if VAR is undefined). Variables can be used inside strings and other expressions. They can even be used as commands themselves.

NAME=World
echo "Hello$NAME!"    # prints/echoes "HelloWorld!"

OPTIONS="-l"
COMMAND="ls"
echo Now running $COMMAND $OPTIONS  # echoes "Now running ls -l"
$COMMAND $OPTIONS                   # runs ls -l

Special variables#

Bash maintains a set of special variables. They are read-only and very useful in scripting.

Variable

Meaning

$?

Exit code of the last command

$$

The pid of the current shell

$0

The name of the current shell or shell script

$n

The n-th positional argument for the current shell script. For \(n \geq 10\) the form ${n} makes the expression unambiguous

$#

The number of arguments for the current shell script

The exit code is useful to check for errors in a shell script. For example in the snippet below a directory foo is created. But this may fail, for example because the directory already exists, in this case $? will be equal to 1 and the script can react accordingly. If mkdir foo succeeds, $? will be equal to 0.

mkdir foo               
if [ $? -neq 0 ] ; then   # if "$? is not equal to 0" then
   # error handling

To illustrate the other special variables assume the following shell script my_script.sh:

#!/bin/bash

echo "Hello, this is $0"    # print out a greeting
if [ $# -neq 2 ] ; then     # if $# is not equal to 2 
    echo "I need exactly 2 arguments"
    exit 1                  # exit with exit code 1
fi
echo "My arguments are $1 and $2"
exit 0                      # exit with exit code 0

The example invocations show the values of the special variables. In particular $0 includes

./my_script.sh foo bar
Hello, this is ./my_script.sh
My arguments are foo and bar

some_path/my_script.sh foo bar baz
Hello, this is some_path/my_script.sh
I need exactly 2 arguments