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Shell Expansion

Types of Shell Expansions

  1. Brace Expansion {}
  2. Tilde Expansion ~
  3. Variable Expansion $VAR
  4. Command Substitution command or $(command)
  5. Arithmetic Expansion $((expression))
  6. Filename (Pathname) Expansion / Globbing * ? []
  7. Process Substitution <(command) and >(command)

1. Brace Expansion {}

Use Expression Expands to
Comma-Separated {item1,item2,item3} item1 item2 item3
Number sequence {1..5} 1 2 3 4 5
Character Sequence {a..e} a b c d e
Num Sequence with step {1..10..2} 1 3 5 7 9
Reverse Num Sequence {5..1} 5 4 3 2 1
Char Sequence with Step {a..z..5} a f k p u z
Nested brace expansion {a,b}{1,2} a1 a2 b1 b2
Triple Nesting {A,B}{1,2}{x,y} A1x A1y A2x A2y B1x B1y B2x B2y
Combined brace expansion file{1,2} file1 file2
Brace with Parenthesis /home/{user1,user2}/docs /home/user1/docs /home/user2/docs

2. Tilde Expansion ~

Expression Equivalent to Expansion
~ \$HOME /home/user
~user /home/user
~root /root
~+ \$PWD
~- \$OLDPWD
~user/Document /home/user/Document

3. Variable Expansion $VAR

If Variable declared as VAR='VALUE'

Variable Expression Variable Expansion
\$VAR VALUE
\${VAR} VALUE
\${VAR:-NEWVAL} NEWVAL #( Variable value is not assigned)
\${VAR:=NEWVAL} NEWVAL #(Variable value will be assigned)
\${VAR:+NEWVAL} NEWVAL #(New Var value is printed if VAR is non empty)
\${VAR:?NEWVAL} NEWVAL #(New Var value is printed if var is empty prints to STD Err)
\${VAR:2} LUE Leaves first 2 chars and prints
\${VAR:2:2} LU Leaves first 2 and prints next 2
\${#VAR} 5 Prints VAR length
\${!VAR} Prints variables variable

Consider the variable filename=archive.tar.gz

Var expression VAR Expansion
\${filename#*.} tar.gz Remove shortest length of chars before pattern match
\${filename##*.} gz Remove longest length of chars before pattern match
\${filename%.*} archive.tar Remove shortest char after the pattern match
\${filename%%.*} archive Remove longest char after the pattern match

4. Command Execution $(command)

echo "This are home directory content: $(ls $HOME)"

5. Arithmetic Expansion $((arithmetic expressions))

a=2, b=3 , c=5

Operation Expression Output
Add \$((a+a)) 5
Sub \$((a-b)) -3
Multiply \$((a*b)) 6
Divide \$((a/b)) 1
Reminder \$((c%b)) 2

6. File Globbing

  • ?(pattern) → zero or one occurrence
  • *(pattern) → zero or more
  • +(pattern) → one or more
  • @(pattern) → exactly one
  • !(pattern) → anything except
Pattern Description
* Matches any number of characters
? Matches exactly one character
[abc] Matches one of the characters a, b, c
[a-z] Matches one character in range
[!a] Matches anything except a
{a,b,c} Expands to each item
{1..5} Expands to range
** Recursive match with globstar
!(p) Anything but pattern