Changing a user’s default Shell to bash v5

sdiff

So be careful running editing a bash script that may be currently executing. It could execute an invalid command, or do something very surprising.

readonly TUX=penguinpower

function foo { local -n data_ref=$1 echo ${data_ref[a]} ${data_ref[b]} } declare -A data data[a]="Fred Flintstone" data[b]="Barney Rubble" foo data

How To Create a Sudo User on Ubuntu

echo "scale=2; 2/3" | bc .

break : . continue eval exec exit export readonly return set shift trap unset

(set -f; IFS=:; printf "%s\n" $PATH)

From Bash to Zsh

zsh will be the default login shell for new accounts, and even then, you can select bash instead

Apple replaced Bourne Again SHell with Z shell for licensing reasons

One thing to consider is that getting used to this being enabled in your profile may result in some confusion if you run into a situation where your personalized profile configuration isn't applied (rebuilt machine, shell scripts which may run on other machines, etc). There's some benefit to sticking close to defaults. This is definitely a conservative viewpoint, however.

ps f

It’s worth noting that first line of the script starts with #!. It is a special directive which Unix treats differently.

When you execute commands in non login shell like ssh server command or scp file server:~ or sudo(without -i) or su (without -l) it will execute ~/.bashrc

open a login shell which sources ~/.bash_profile

The point of the .bashrc file is that it sets the shell up to be more convenient for interactive users. Helpful alias, pretty colors, useful prompts, common environment variables, that sort of thing. And some of these conveniences could break non-interactive scripts.

The main benefit I can see to having .bashrc sourced when running a (non-interactive) remote command is that shell functions can be run. However, most of the commands in a typical .bashrc are only relevant in an interactive shell

I discovered that remote shells are treated differently. While non-interactive Bash shells don’t normally run ~/.bashrc commands at start-up, a special case is made when the shell is Invoked by remote shell daemon:

This has the consequence that if the .bashrc contains any commands that print to standard output, file transfers will fail, e.g, scp fails without error.

COMMAND EXECUTE BASHRC -------------------------------- bash -c foo NO bash foo NO foo NO rsh machine ls YES (for rsh, which calls `bash -c') rsh machine foo YES (for shell started by rsh) NO (for foo!) echo ls | bash NO login NO bash YES

If you want happy cow messages when you login change your bash_profile.

AFAIK, the right way to enable un-hindered scp is less about which conditional for stdout in your ~/.bashrc script, and more about simply restricting screen output to the ~/.bash_profile script. At least that is how it works for my distro (CentOS.) Edit for clarity: Put only lines in your ~/.bashrc file as required by "all" remote conections

So, though normally bash would not run ~/.bashrc for a non-interactive shell, with ssh it does so anyway.

For those (like me) wondering why is the space needed, man bash has this to say about it: > Note that a negative offset must be separated from the colon by at least one space to avoid being confused with the :- expansion.

将错误IP放到数组里面判断是否ping失败三次

#!/bin/bash  
IP_LIST="192.168.18.1 192.168.1.1 192.168.18.2"
for IP in $IP_LIST; do
    NUM=1
    while [ $NUM -le 3 ]; do
        if ping -c 1 $IP > /dev/null; then
            echo "$IP Ping is successful."
            break
        else
            # echo "$IP Ping is failure $NUM"
            FAIL_COUNT[$NUM]=$IP
            let NUM++
        fi
    done
    if [ ${#FAIL_COUNT[*]} -eq 3 ];then
        echo "${FAIL_COUNT[1]} Ping is failure!"
        unset FAIL_COUNT[*]
    fi
done

获取随机8位字符串：

方法1：
# echo $RANDOM |md5sum |cut -c 1-8
471b94f2
方法2：
# openssl rand -base64 4
vg3BEg==
方法3：
# cat /proc/sys/kernel/random/uuid |cut -c 1-8
ed9e032c

获取随机8位数字：

# echo $RANDOM |cksum |cut -c 1-8
23648321
方法2：
# openssl rand -base64 4 |cksum |cut -c 1-8
38571131
方法3：
# date +%N |cut -c 1-8
69024815

注意事项

You do that using backticks: echo World > file.txt

Shell Utilities for QC

Process Substitution

${FUNCNAME[@]}

while IFS='=' read -r col1 col2 do echo "$col1" echo "$col2" done <testprop.properties

In Bash you quite often need to check to see if a variable has been set or has a value other than an empty string. This can be done using the -n or -z string comparison operators.

;

Perl script in bash's HereDoc

Associative arrays in bash

.

strings $PWD/bin/myapp | egrep '\.gcda$'

A shell script is a file of executable commands that has been stored in a text file. When the file is run, each command is executed.

Counting number of lines

-print0

What is missing is a space between the $( and the following (, to avoid the arithmetic expression syntax. The section on command substitution in the shell command language specification actually warns for that: