Deprecated Syntactic Sugar in C++ for Minimum and Maximum functions
The g++ compiler does not support
`<?' and `>?'
which used to be the minimum and maximum operators any more and the user should use
`std::min' and `std::max'
instead.
Yearly Archives: 2011
List the contents of a tar or tar.gz file
List the contents of a tar file
<code>tar -tvf file.tar</code>
List the contents of a tar.gz file
<code>tar -ztvf file.tar.gz</code>
List the contents of a tar.bz2 file
<code>tar -jtvf file.tar.bz2</code>
Options:
-t List the contents of an archive
-v Verbose mode
-z Use gzip so that you can process a compressed (.gz) tar file
-j Use bzip2, use to decompress .bz2 files
-f filename Use archive file called filename
Linux: Check if a User or a Group Exists 2
You can find out if user exists by searching in the /etc/passwd file using the following command:
egrep -i "^useraccount:" /etc/passwd
The above command will print the matching record from /etc/passwd if the user exists or nothing if the user does not exist.
The ^ symbol is used to make sure there is no characters before the username and the : character is used as the delimiter in the file (which indicates the end of the username). By wrapping the username with these characters we are sure that if we matched a record, we matched the correct record with the full username.
A very simple way to use this code in a script is by utilizing the $? (question mark) variable. The question mark variable contains the exit status of the last command that executed. Specifically, egrep will return 0 if there was a match or else it will return a a positive number (usually 1).
Taking advantage of this behavior, after executing the above command, we check the $? variable to see the result with an if statement.
egrep -i "^useraccount:" /etc/passwd; if [ $? -eq 0 ]; then echo "User Exists" else echo "User does not exist -- Invalid Username" fi
You can also find out if a group exists by searching in the /etc/group file. Similar to the approach we showed before, we can check if a group exists using the following:
egrep -i "^groupname" /etc/group; if [ $? -eq 0 ]; then echo "Group Exists" else echo "Group does not exist -- Invalid Group name" fi
gpasswd
gpasswd group gpasswd -a useraccount groupname gpasswd -d useraccount groupname gpasswd -R groupname gpasswd -r groupname gpasswd [-A useraccount,...] [-M useraccount ,...] groupname
gpasswd is used to administer the /etc/group file (and /etc/gshadow file if compiled with SHADOWGRP defined).
System administrator can use:
-A option to define group administrator(s)
-M option to define members and has all rights of group administrators and members.
Group administrator can use:
-a to add users
-d to delete users.
Administrators can use:
-r option to remove group password. When no password is set only group members can use newgrp to join the group.
-R disables access to the group through newgrp command.
gpasswd called by a group administrator with group name only prompts for the group password. If password is set the members can still newgrp without a password, non-members must supply the password.
