Bash: Shell parameter expansion

Substring of a variable

The substring of a variable can be extracted with ${var:start} or ${var:start:end}:

#!/bin/bash

zeroToNine=0123456789

echo ${zeroToNine:0}
# 0123456789

echo ${zeroToNine:7}
# 789

echo ${zeroToNine:5:4}
# 5678

echo ${zeroToNine: -3}
# 789

echo ${zeroToNine: -6: 2}
# 45

echo ${zeroToNine: -4:-1}
# 678

array=(0 1 2 3 4 5 6 7 8 9)
echo ${array[@]:4:3}
# 4 5 6

Github repository about-Bash, path: /expansion/shell-parameter/substring

A substring can also be realized with

expr $var $start $len

Replace/change parts of the variable value

${var/pattern/replacement} returns the value of the variable with pattern replaced by replacement.

The variable itself does not change.

#!/bin/bash

var="foo bar baz"
echo ${var/a*/REPLACED}
# foo bREPLACED

echo $var
# foo bar baz

Github repository about-Bash, path: /expansion/shell-parameter/replace

Indirection

${!var_name} evaluates to the value of the variable whose name is $var_name.

num_1=one
num_2=two
num_3=three
num_4=four
num_5=five

the_num=num_2

echo "The value of \$$the_num is ${!the_num}"
# The value of $num_2 is two

Github repository about-Bash, path: /expansion/shell-parameter/indirect

TODO

# Assignment might occur in the form ${VAR=VALUE} or ${VAR:=VALUE}

var1=foo
nullVar1=

# Substitute if null or undefined {

#    first batch: with colon {
#   (assign if not set or null)

echo ${var1:=abc}
# foo
echo $var1
# foo

echo ${nullVar1:=def}
# def 
echo $nullVar1
# def

echo ${unset1:=ghi}
# ghi
echo $unset1
# ghi

# }

echo

#    second batch: without colon {
#   (assign if not set)

var2=bar
nullVar2=


echo ${var2=jkl}
# bar
echo $var2
# bar

echo ${nullVar2=mno}
#
echo $nullVar2
#

echo ${unset2=pqr}
# pqr
echo $unset2
# pqr

# }

# }

Github repository about-Bash, path: /expansion/shell-parameter/assign_unset_and_or_null_variables

#!/bin/bash

car_one=Mercedes
car_two=BMW
car_three=Porsche

for v in ${!car*}; do
  echo $v
done
# car_one
# car_three
# car_two

Github repository about-Bash, path: /expansion/shell-parameter/common_prefix

#!/bin/bash

# mnemonic: # is on the left, % on the right side of the $ key.
# hence # deletes from the left, % from the right side.

var="foo bar baz"

# delete shortest possible match from the left
echo ${var#*a}
# r baz

# delete longest possible match from the left
echo ${var##*a}
# z

# delete shortest possible match from the right
echo ${var%a*}
# foo bar b

# delete longest possible match from the right
echo ${var%%a*}
# foo b

Github repository about-Bash, path: /expansion/shell-parameter/delete

# Substitution might occur in the form ${VAR-SUBST} or ${VAR:-SUBST}

var1=foo
nullVar1=

# Substitute if null or undefined {

#    first batch: with colon {
#   (substitue if set and non-null)

echo ${var1:+abc}
# abc

echo ${nullVar1:+def}
# 

echo ${unset1:+ghi}
# 

# }

echo ----------


#    second batch: without colon {
#   (substitue if set)

var2=foo
nullVar2=

echo ${var2+jkl}
# jkl

echo ${nullVar2+mno}
# mno

echo ${unset2+pqr}
#

# }

# }

Github repository about-Bash, path: /expansion/shell-parameter/substitute_set_and_or_non-null_variables

# Substitution might occur in the form ${VAR-SUBST} or ${VAR:-SUBST}

var=foo
nullVar=

# Substitute if null or undefined {

#    first batch: with colon {
#   (substitue if not set or null)

echo ${var:-abc}
# foo

echo ${nullVar:-def}
# def 

echo ${unset:-ghi}
# ghi

# }
#    second batch: without colon {
#   (subsitue if not set)

echo ${var-jkl}
# foo

echo ${nullVar-mno}
#

echo ${unset-pqr}
# pqr

# }

# }


# The following line is written to std err
exit_program=${unset?Exit the program because \'unset\' is not set}
echo "This line is not printed..."

Github repository about-Bash, path: /expansion/shell-parameter/substitute_unset_and_or_null_variables

Bash: Shell parameter expansion

Substring of a variable

Length of a variable

Replace/change parts of the variable value

Indirection

TODO