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I have come across a script that is supposed to make adding a user to a linux system easier. I am fairly new to bash scripts so i am trying to figure out what each line of said script is doing. Now i am trying to find out what this part of the script means "^$username" what is the function of the ^ before the variable?

for some more context here is the part of the script this variable is in:

if [ $(id -u) -eq 0 ]; then read -p "Enter username : " username read -s -p "Enter password : " password egrep "^$username" /etc/passwd >/dev/null
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  • Reading the section on regular expressions in the grep manual will enlight you. Commented Nov 24, 2016 at 10:32

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It's a literal ^. In this case, it's being passed to egrep, which takes extended regular expressions, a kind of way of specifying a pattern to match in a given string (kind of like globbing/wildcards, but much more powerful). ^ just means "This has to be at the start of the string/line". So in this case, it'll match every line in /etc/passwd that starts with the contents of the "username" variable.

(Note there's a potential bug here, because if there's a user rich and a user richard, typing the username rich will also match richard.)

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  • Thanks @Muzer the following has removed the potential bug: if [ $? -eq 0 ]; then echo "$username exists!" exit 1 Commented Nov 24, 2016 at 10:39
  • @a.smith, if you only added checking of error code of grep, then it didn't solve a bug Muzer was talking about. You won't be able to add user 'rich', when you have a user 'richard'. Regexp is too wide here. Think about using "^$username:" - colon is used to separate fields in /etc/passwd, so it would match whole field as a username. Commented Nov 24, 2016 at 12:00
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It's part of a regular expression and just means "at beginning of line".

For a nice list on regular expressions, take a look at grymoire.

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