Expansion and Substitution in Unix-like Operating Systems
Mohamed Yahia
Posted on October 12, 2023
Pathname expansion
expanding a pathname, how do we do that?
if we do echo 2 * 3
nuclearegg69@zenbook-f13:~/testRen$ echo 2 * 3
2 a apple b banana c cactus inNano2Ren test2 3
I am presented with those what appears to be file names of current directory, all of them, the 2 and 3 are present at both ends but the asterisk * got replaced/expanded into those file names
This one of the wildcard characters that we can use for expansion. The following list contain some more:
- * asterisk:
- it matches 0 or more characters
- ex: *.txt matches file.txt and 0.txt and .txt
- ? question mark:
- matches any one character
- ex: blue?.txt matches blue1.txt and bluez.txt but doenst match blue12.txt
- [] square brackets or range wilcards:
- they match a range of characters inside them
- ex: [rbc]at matches rat, bat or cat
- ex: [a-h]* matches any name that starts with a to h (case sensitivity matters so only small letters a to h)
- ex: [0-9]* matches any name that starts with a number
Arithmetic expansion
nuclearegg69@zenbook-f13:~$ echo $((5+5))
10
syntax: $((arithmetic_expression))
Command substitution
nuclearegg69@zenbook-f13:~/testRen$ echo today is $(date)
today is Thu Oct 12 11:50:06 +03 2023
syntax: $(command)
Changing-behavior Quotes
nuclearegg69@zenbook-f13:~/testRen$ a=5
nuclearegg69@zenbook-f13:~/testRen$ echo $a
5
nuclearegg69@zenbook-f13:~/testRen$ echo "$a"
5
nuclearegg69@zenbook-f13:~/testRen$ echo "\$a"
$a
nuclearegg69@zenbook-f13:~/testRen$ echo '$a'
$a
nuclearegg69@zenbook-f13:~/testRen$ echo "$((a+4))"
9
nuclearegg69@zenbook-f13:~/testRen$ echo '$((a+4))'
$((a+4))
nuclearegg69@zenbook-f13:~/testRen$ echo 1..9
1..9
nuclearegg69@zenbook-f13:~/testRen$ echo {1..9}
1 2 3 4 5 6 7 8 9
nuclearegg69@zenbook-f13:~/testRen$ echo "{1..9}"
{1..9}
nuclearegg69@zenbook-f13:~/testRen$ echo ~
/home/nuclearegg69
nuclearegg69@zenbook-f13:~/testRen$ echo "~"
~
nuclearegg69@zenbook-f13:~/testRen$ echo '~'
~
nuclearegg69@zenbook-f13:~/testRen$ echo *
a apple b banana c cactus inNano2Ren test2
nuclearegg69@zenbook-f13:~/testRen$ echo '*'
*
nuclearegg69@zenbook-f13:~/testRen$ echo "*"
*
nuclearegg69@zenbook-f13:~/testRen$ echo "`ls`"
a
apple
b
banana
c
cactus
inNano2Ren
test2
nuclearegg69@zenbook-f13:~/testRen$ echo '`ls`'
`ls`
Based on the above commands, we can deduce that:
1- double quotes: they disable all substitutions except those that involve a $ (dollar sign), \ (backslash), ` (backtick)
2- single quotes: disable all substitutions.
💖 💪 🙅 🚩
Mohamed Yahia
Posted on October 12, 2023
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