Linux System Administration

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Chapter 10: Scripting

cbarrel:x:502:502:Creighton Barrel:/home/cbarrel:/bin/bash

cmaharias:x:503:503:C Maharias:/home/cmaharias:/bin/bash

pgasquette:x:504:504:Papa Gasquette:/home/pgasquette:/bin/bash

bfrapples:x:505:505:Bob Frapples:/home/bfrapples:/bin/bash

The colon-separated fields are:

• Account name

• Encrypted password, or

x if /etc/shadow is used

• User ID (uid)

• Group ID (gid)

• Full name or description

• Home directory

• Shell

We’re interested in the fifth field (full name or description). In the ancient Unix

scrolls, this was called the gecos field, for reasons that were obsolete even then. The

name persists, and it’s useful to know.

Script Versions

We’ll start each of the following sections with a minimal script that searches for a

string anywhere in the /etc/passwd file and prints the matching line. We know this is

too broad, but we want to get the script working before we get too fancy.

Next, we’ll split the input lines into fields and restrict the pattern matching to the

gecos field that contains our users’ names.

Then we’ll further restrict the search to lines in which the value of the uid field is

greater than 500. In our case, normal user IDs start at 501, so this will exclude sys-

tem accounts and other automatons.

By this point we’ll be pretty tired of the previous steps, so we’ll look for some tools

that can do some of this work for us.

The bash script

Most languages provide function ...