BASH

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BASH – Leaving a line after Cat

If you are writing a script that involve cat and if you wish to leave a line after “cat”, do the following

cat /usr/local/lic_lmstat_log/abaqus_lmstat.log ; echo
.....
.....
Users of tfluid_int_ccmp:  (Total of 128 licenses issued;  Total of 0 licenses in use)

Users of tfluid_int_fluent:  (Total of 128 licenses issued;  Total of 0 licenses in use)
[user1@node1 ~]$
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Shortening BASH Commands (Part 1)

I had a casual read on the book “Bash Idioms” by Carl Albing. I scribbled what I learned from what stuck me the most. There are so much more. Please read the book instead.

Lesson 1: “A And B” are true only if both A and B are true…..

Example 1: If the cd command succeeds, then execute the “rm -Rv *.tmp” command

cd tmp && rm -Rv *.tmp

Lesson 2: If “A is true”, “B is not executed” and vice versa.

Example 2: Change Directory, if fail, put out the message that the change directory failed and exit

cd /tmp || { echo "cd to /tmp failed" ; exit ; }

Lesson 3: When do we use the [ ] versus [[ ]]?

I learned that the author advises BASH users to use the [[ ]] unless when avoidable. The Double Bracket helps to avoid confusing edge case behaviours that a single bracket may exhibit. If however, the main goal is portability across various platform to non-bash platforms, single quota may be advisable.

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Change Runlevel of Rocky Linux 8 without rebooting

You may use the command “telinit” to change the SysV system runlevel without the need to reboot. You will need root access to run the command

Use 1: Single User Mode

# telinit S

Use 2: To go to Graphical.Target

# telinit 5

Use 3: To go to Multi-User.Target

# telinit 3

Use 4: To Reload daemon configuration. This is equivalent to systemctl daemon-reload.

# telinit q

To verify, you can use the command systemctl get-default

# systemctl get-default
graphical.target
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Using Bash alias to keep things simple

Alias are very useful tools to create shorthand pseudonyms to run the command you want without typing the whole thing.

Display all alias names

Typing alias the whole command will 

$ alias
alias cp='cp -i'
alias egrep='egrep --color=auto'
alias fgrep='fgrep --color=auto'
alias grep='grep --color=auto'
alias l.='ls -d .* --color=auto'
alias ll='ls -l --color=auto'
alias ls='ls --color=auto'
alias mv='mv -i'
alias rm='rm -i'
alias xzegrep='xzegrep --color=auto'
alias xzfgrep='xzfgrep --color=auto'
alias xzgrep='xzgrep --color=auto'
alias zegrep='zegrep --color=auto'
alias zfgrep='zfgrep --color=auto'
alias zgrep='zgrep --color=auto'

Useful Alias to consider

alias ls='ls --color=auto'
alias egrep='egrep --color=auto'
alias fgrep='fgrep --color=auto'
alias grep='grep --color=auto'
alias mv='mv -i'
alias rm='rm -i'
alias cp='cp -i'

Remove alias

Just use the command unalias 

unalias ll
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Using NMCLI to manage Network on Rocky Linux 8

Point 1: View all the saved connections

# nmcli connection show
ens1f0     XXXX-XXXX-XXXX-XXXX-XXXX  ethernet  ens1f0
ens1f1     YYYY-YYYY-YYYY-YYYY-YYYY  ethernet  ens1f1
ens10f0    ZZZZ-ZZZZ-ZZZZ-ZZZZ-ZZZZ  ethernet  --
ens10f1    AAAA-AAAA-AAAA-AAAA-AAAA  ethernet  --

Point 2a: Stop Network

You can use the command “nmcli connection down ssid/uuid". For example 

# nmcli connection down XXXX-XXXX-XXXX-XXXX-XXXX
Connection 'eno0' successfully deactivated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/3)

Point 2b: Start Network

You can use the command “nmcli connection up ssid/uuid". For example 

# nmcli connection up XXXX-XXXX-XXXX-XXXX-XXXX
Connection 'eno0' successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/3)

Point 3: Device Connection

To check the Device status

# nmcli dev status
ens1f0  ethernet  connected     ens1f0
eno1f1  ethernet  connected     ens1f1
eno10f0  ethernet  disconnected  --
eno10f1  ethernet  disconnected  --

Point 4: List all Device 

# nmcli device show
GENERAL.DEVICE:                         ens1f0
GENERAL.TYPE:                           ethernet
GENERAL.HWADDR:                         XX:XX:XX:XX:XX:XX
GENERAL.MTU:                            1500
GENERAL.STATE:                          100 (connected)
GENERAL.CONNECTION:                     ens1f0
GENERAL.CON-PATH:                       /org/freedesktop/NetworkManager/ActiveConnection/2
WIRED-PROPERTIES.CARRIER:               on
IP4.ADDRESS[1]:                         192.168.0.1
IP4.GATEWAY:                            192.168.0.254
IP4.ROUTE[1]:                           dst = 0.0.0.0/0, nh = 192.168.0.254, mt = 101
IP4.ROUTE[2]:                           dst = 198.168.0.0/19, nh = 0.0.0.0, mt = 101
IP6.ADDRESS[1]:                         xxxx::xxxx:xxxx:xxxx:xxxx/64
IP6.GATEWAY:                            --
IP6.ROUTE[1]:                           dst = fe80::/64, nh = ::, mt = 1024

GENERAL.DEVICE:                         eno1f1
GENERAL.TYPE:                           ethernet
GENERAL.HWADDR:                         94:6D:AE:9B:76:1C
GENERAL.MTU:                            1500
GENERAL.STATE:                          100 (connected)
GENERAL.CONNECTION:                     eno1f1
GENERAL.CON-PATH:                       /org/freedesktop/NetworkManager/ActiveConnection/4
WIRED-PROPERTIES.CARRIER:               on
IP4.ADDRESS[1]:                         192.168.200.201/19
IP4.GATEWAY:                            --
IP4.ROUTE[1]:                           dst = 192.168.192.0/19, nh = 0.0.0.0, mt = 102
IP6.ADDRESS[1]:                         fe80::966d:aeff:fe9b:761c/64
IP6.GATEWAY:                            --
IP6.ROUTE[1]:                           dst = fe80::/64, nh = ::, mt = 1024

Point 5: Start and Stop Device 

# nmcli con down ens1d1
# nmcli con up ens1d1

References:

  1. nmcli command in Linux with Examples
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Using Find to Search Better

Basic Use of FIND

If you are looking to find a file, one of the most common tools is Find. Here is a recap.

OFILE TYPEDESCRIPTION
1type -fLimits search results to files only
2type -dLimits search results to directories only
3type -lLimits search results to symbolic links only

For example, search for a case-insensitive file named “hello.mov”

$ find $HOME -type -iname "Hello.mov"

Parameters

NOPARAMETERSDESCRIPTION
1-namePerform a case-sensitive search for “files”
2-inamePerform a case-insensitive search for “files”
3size +nMatches files of size larger than size n
4size -nMatches files of size smaller than size n
5-mtime nMatches files or directories whose contents were last modified n*24 hours ago
6-atime nMatches files last access n*24 hours ago

For example, search for all case-insensitive files with the extension *mov 2 days ago

$ find $HOME -type -iname "*.mov" -mtime 2

Operators

S/NOOPERATOREXPLANATION
1-andMatch for both sides of the operators
2-orMatch for either test of the operators
3-noteDon’t match the test of the operators

For example, search for all files with Hello*, but excl ude pdf and jpg

$ find \( -name "Hello*" -mtime 2 \) -and -not \( -iname "*.jpg" -or -iname "*.pdf" \)

When using the () to combine tests, remember to escape the (\) brackets. You will need to leave a space after you open and close the brackets

find -type f -iname "*.mov" -exec chmod +x {} \;

The first part find -type f -iname”*.mov” will not be explained….. Executed commands must end with \; (a backslash and semi-colon) and may use {} (curly braces) as a placeholder for each file that the find command locates.

References:

  1. Linux Format – March Edition
  2. Use the Unix find command to search for files
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How to check Disk Usage

Checking whether the root partition has run out of inodes. Use the command. If it shows 100%, there are many small files. Perhaps, do look for some of these files at /tmp

df -i
Filesystem                                      Inodes        IUsed        IFree IUse% Mounted on
/dev/mapper/centos-root                        9788840       320849      9467991    4% /
devtmpfs                                      70101496          560     70100936    1% /dev
tmpfs                                         70105725            8     70105717    1% /dev/shm
tmpfs                                         70105725         1581     70104144    1% /run
.....
.....

You may want to check which directories is using the most space with the commands below

% du -hx -d 1 |sort -h
1.3M    ./Espresso-BEEF
4.9M    ./NB07
8.3M    ./Gaussian2
31M     ./Gaussian
65M     ./MATLAB
478M    ./Abaqus
647M    ./pytorch-GAN
10G     ./COMSOL
12G     .

-h argument produces the human-readable output
-x restricts the search to the current directory
-d 1 is the summary for each directory
sort -h produces human-readable output and the directories with the largest usage will appear at the bottom of the list.

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chsh -s /bin/tcsh and you (user) don’t exist error

Sometimes, you are a non-root user and you wish to change shell and you have an error

$ chsh -s /bin/tcsh
chsh you (user xxxxxxxxx) don't exist

This error occurs when the userID and Passowrd is using LDAP or Active Directory so there is no local account in the /etc/passwd where it first looks to. I used Centrify where we can configure the Default Shell Environment on AD. But there is a simple workaround if you do not want to bother your system administrator

First check that you have install tcsh. I have it!

$ chsh -l
/bin/bash
/bin/cdax/bash
/bin/cdax/csh
/bin/cdax/ksh
/bin/cdax/rksh
/bin/cdax/sh
/bin/cdax/tcsh
/bin/csh
/bin/ksh
/bin/rksh
/bin/sh
/bin/tcsh
/sbin/nologin
/usr/bin/bash
/usr/bin/cdax/bash
/usr/bin/cdax/dzsh
/usr/bin/cdax/sh
/usr/bin/dzsh
/usr/bin/sh
/usr/sbin/nologin
/usr/bin/tmux

Next Step: Check your current shell

$ echo "$SHELL"
/bin/bash

Step 3: Write a simple .profile file

$ vim ~/.profile
if [ "$SHELL" != "/bin/tcsh" ]
then
    export SHELL="/bin/tcsh"
    exec /bin/tcsh -l    # -l: login shell again
fi

Step 4: In your .bashrc, just add the “source ~/.profile”

# .bashrc

# Source global definitions
if [ -f /etc/bashrc ]; then
        . /etc/bashrc
fi

source ~/.profile

Source the .bashrc again

$ source ~/.bashrc