how to check wheather the user exists or not??

**Mohan1289** · November 3rd, 2012

hi how can we check weather the user exists or not??

do we have to check the /home directory??

what i want is

echo "Enter username"
read u
if [ "$u" exists "/home/$u"]; then

echo "User Exists"

is it ok??? and how can i check whether the directory at home directory exists or not??

is checking in home directory enough??? or do i have to check /etc/group file??

or do i have to check something else??

**spjackson** · November 3rd, 2012

To check whether a directory exists, you want:

Code:

if [ -d "/home/$u"]; then

but that is not necessarily a good test for whether the user exists.

First, not all users have their home directory in /home. It is common practice for normal user accounts on Linux to be created with /home/$username to be their home directory, but this does not have to be the case. Special users, e.g. root, nobody, mysql, postgres, www-data, have their home directories elsewhere. The name of the home directory does not have to be the same as the username, but for normal user accounts, this is usually the case.

Second, there is nothing to prevent you having a directory in /home for a user who does not exist. Indeed, you can remove a user but leave their home directory intact.

You will get a definitive answer by grepping /etc/passwd or for example:

Code:

/usr/bin/id $u > /dev/null 2>&1
if [ $? -eq 0 ] ; then
   echo valid user
fi

However, this does not tell you where their home directory is. Maybe try:

Code:

user_home=$(sh -c "echo ~$u")
if [ -d "$user_home" ] ; then
    blah blah
fi

**Mohan1289** · November 3rd, 2012

Originally Posted by spjackson

To check whether a directory exists, you want:

Code:

if [ -d "/home/$u"]; then

but that is not necessarily a good test for whether the user exists.

First, not all users have their home directory in /home. It is common practice for normal user accounts on Linux to be created with /home/$username to be their home directory, but this does not have to be the case. Special users, e.g. root, nobody, mysql, postgres, www-data, have their home directories elsewhere. The name of the home directory does not have to be the same as the username, but for normal user accounts, this is usually the case.

Second, there is nothing to prevent you having a directory in /home for a user who does not exist. Indeed, you can remove a user but leave their home directory intact.

You will get a definitive answer by grepping /etc/passwd or for example:

Code:

/usr/bin/id $u > /dev/null 2>&1
if [ $? -eq 0 ] ; then
   echo valid user
fi

However, this does not tell you where their home directory is. Maybe try:

Code:

user_home=$(sh -c "echo ~$u")
if [ -d "$user_home" ] ; then
    blah blah
fi

Oh so /home directory is not reliable for determining whether the user exists or not.

what is the use of -d in the if condition?? what does it represent?

**Lars Noodén** · November 3rd, 2012

Originally Posted by Mohan1289

Oh so /home directory is not reliable for determining whether the user exists or not.

what is the use of -d in the if condition?? what does it represent?

The -d is a test to check whether a files both exists and is a directory.

Probably the definitive way to check if a user exists is to look in /etc/passwd

Code:

#!/bin/sh

echo "Enter username"
read u
if grep -q "^$u:" /etc/passwd ; then
  echo "User Exists"
fi

**Mohan1289** · November 3rd, 2012

Originally Posted by Lars Noodén

The -d is a test to check whether a files both exists and is a directory.

Probably the definitive way to check if a user exists is to look in /etc/passwd

Code:

#!/bin/sh

echo "Enter username"
read u
if grep -q "^$u:" /etc/passwd ; then
  echo "User Exists"
fi

why you used ^$u: what does it do??

-q for silent or quite but what's the use of ^ ??
it means it searches for the username??

**Lars Noodén** · November 3rd, 2012

The password file has a specific format. There is one record (user) per line and several fields per line. The fields are delimited by colons (:) The username happens to be the first field, so it starts at the beginning of the line.

If we don't match for the whole field, if you search for the user 'fred', you will also get the users 'alfred' and 'freddy'

grep uses regular expressions to find patterns in a file. The pattern we need to tell grep to look for is the username, held in $u, that begins at the beginning of the line (^) and continues to the end of the field, which is marked by a colon (:) The resulting pattern is /^$u:/ Look around for tutorials on "regular expressions" to find one that matches your level and interest.