Tag: sysadmin

What shell am I in?

When you connect to a system, whether directly on the system’s keyboard or through a remote connection you will automatically start in your default shell. The default shell was originally assigned to you when your account was created.

To find out what shell you are currently using we can use the echo command:

$ echo $SHELL
/bin/bash

In this command we are using the echo command to examine the value of the environment variable $SHELL. This variable was set by the system when we started this command line session and shows the full path to the shell we were assigned at login. Here are some common shells you might see:

  • /bin/sh – Bourne shell
  • /bin/bash – Bourne Again shell
  • /bin/csh – C shell
  • /bin/ksh – Korn shell
  • /bin/tcsh – TC shell
  • /bin/zsh – Z shell

Shell binaries are also commonly found in the /usr/local/bin directory. Consult your system administrator if you’re having trouble finding your shell binaries.

Oracle Shell ScriptingFor more information like this check out my book Oracle Shell Scripting, only $34.95 from Rampant TechPress.

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Oracle Shell Scripting, another review

Another review for my book Oracle Shell Scripting: Linux and UNIX Programming for Oracle has shown up on Amazon!

Prashant wrote:

This book has helped me impress my colleagues and boss..I originally browsed through it at Border’s (and found myself sitting on the ground with a notepad scrambling to copy as much as possible)..of course, then I realized I had to have it, so I bought it online..I knew the publisher was a trustworthy source because I’m always using Don Burleson’s DBA tips online.. this author’s approach is easy-to-follow and concise; yet it’s a thorough guide that is like a catalyst for your own creativity…it has made me look forward to extracting the power of the shell.

It’s a lot better than parsing through thick UNIX encyclopedias or cycling through fragmented online material..as an OCP 10g/9i DBA, I still feel like there are not enough practical day-to-day guides like this one for junior/mid/senior-level administrators, since over half our work is directly/indirectly connected to the shell.

Thanks for the great review Prashant! After all the work that goes into a book like this it’s great to know that it’s helping people. That’s what it’s all about, after all.

Oracle Shell ScriptingFor more information like this check out my book Oracle Shell Scripting, only $34.95 from Rampant TechPress.

Buy it now!


Color ls output and permission denied

In my post from earlier this week on removing the color output from the ls command I mentioned that in some circumstances the color-coded output can cause permissions errors. Cloned Milkmen asked that I explain why this would be.

I was hoping someone would ask that. In Linux and UNIX if the execute flag is not set on a directory, as in the example below, the ls --color=tty command will return Permission denied.

$ ls -ld ls_test/
dr-------- 4 jonemmons pg222211 4096 Nov 12 20:07 ls_test/
$ ls ----color=tty ls_test/
ls: ls_test/test1: Permission denied
ls: ls_test/test2: Permission denied
ls: ls_test/test3: Permission denied
dir1 dir2
$ ls ls_test/
dir1 dir2 test1 test2 test3

The execute flag is funny on directories. The main effect is that it will inhibit a user’s ability to cd into a directory, but it will also keep you from being able to get information out of the file’s inode (the inode contains information about when a file was created, modified, accessed etc.) Since ls requires information from the inode to properly color-code files in a directory it will fail if the execute bit is not set. An ls -l will fail for the same reason.

So thanks to Cloned Milkmen for asking the question. If you’re looking for some eclectic and distracting reading, check out his blog The Synthetic Librarian.

unix, linux, directory, permission, security, sysadmin, system administration

Get rid of color ‘ls’ output in Linux

Color ls outputMany popular varieties of Linux use a “feature” which causes the ls command output to show files, directories, links, etc. all in different colors. I guess some people prefer this, but I find it at best annoying, and at worst illegible. Specifically the color-coding of symbolic links tend to show in such a light color that it is often impossible to read.

The color output is accomplished by adding the --color=tty or a similar option to the ls command. This is typically accomplished by creating an alias to ls in either the user’s profile or in one of the system-wide profiles.

alias ls='ls --color=tty'

My personal preference is to remove this line from any system-wide configuration files (such as /etc/profile) and allow users to set it in their own profile if preferred. If you don’t have the desire or ability to make this change universally than a user can easily disable the color output by using the unalias command:

unalias ls

This can either be added to the user’s configuration file (e.g. the .profile or .bash_profile in their home directory), or you can just type unalias ls anytime to disable color ls output for the rest of the current shell session. This can be especially useful to turn off the color output when you’re working on someone else’s system.

Color-coded ls output can cause permissions errors in some circumstances, so in my opinion it is best left off, but if you’re stuck with it then it’s nice to know how it can be disabled when necessary.

ls, sysadmin, system administration, linux, shell, bash, sh, UNIX

Built-in shell scripting variables

UNIX and Linux shells provide an abundance of useful built-in information that can be referenced in globally available variables. In order to see the information provided in a shell, the set command can be run as demonstrated below.

Here’s a partial output of the set command:

$ set
BASH=/bin/bash
BASH_VERSINFO=([0]="2" [1]="05b" [2]="0" [3]="1" [4]="release" [5]="i386-redhat-linux-gnu")
BASH_VERSION='2.05b.0(1)-release'
GROUPS=()
G_BROKEN_FILENAMES=1
HISTFILE=/home/tclark/.bash_history
HISTFILESIZE=1000
HISTSIZE=1000
HOME=/home/tclark
HOSTNAME=appsvr.mytec.com
OSTYPE=linux-gnu
PATH=/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/tclark/bin
...
PS1='[\u@\h \W]\$ '
PS2='> '
PS4='+ '
PWD=/home/tclark
SHELL=/bin/bash
SHLVL=1
SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass
SSH_CLIENT='206.107.231.178 1379 22'
SSH_CONNECTION='206.107.231.178 1379 192.168.15.105 22'
SSH_TTY=/dev/pts/0
SUPPORTED=en_US.UTF-8:en_US:en
TERM=vt100
UID=503
USER=tclark
_=clear

The contents of a shell variable can be displayed by using the echo command and prefacing the variable name with a dollar sign as demonstrated below. Shell variables are referenced using all capital letters.

$ echo $TERM
vt100
$ echo $USER
tclark
$ echo $HOSTNAME ... $LOGNAME
appsvr.mytec.com ... tclark

There are also some special built-in variables that can be useful when creating shell scripts. Some of them are listed in the table below.

Built-in Variable Description
$# The total number of arguments passed to a shell script on the command line.
$* All arguments passed to the shell script.
$0 The command (script) invoked on the command line.
$1 – $9 The first through ninth arguments passed to the shell script from the command line.

These variables are provided by the shell and the names should not be used for other variables.

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linux, unix, system administration, sysadmin