Bash
Article by on June 1, 2012, last modified on August 2, 2014
About
Bash is a UNIX shell that is awesome. Bash is a "command processor" that allows you to type text commands to a computer, or even a remote computer using programs like SSH. Bash stands for "Bourne-Again SHell", the Bourne shell (sh) being a predecessor and competitor to Bash.
Debugging
Run a Harmless Non-Zero Exit Status Command
Let's say you are writing a bash script and want to handle the case where a command returns a non-zero exit status, you can test that case using a harmless command that returns a non-zero exit status. For example,
cat nosuchfile 2>/dev/null
if [ $? -ne 0 ]
then
echo "It failed!"
fi
Then, once you have written the code for that case, you can simply replace the command with the one you actually want to use.
The idea came from an irrelevant comment on StackOverflow.
Control Structures
This is the best reference for "if" statements: http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html
if [ -z "$1" ] || [ "$a" == "$b" ]
Screen
How to run a command in the background?
As a StackOverflow comment notes, you can use the screen command:
$ screen
Then type whatever command you want to run.
$ /usr/bin/java -jar /usr/local/bin/selenium-server-standalone-2.21.0.jar &
Then hit CTRL+D or type exit and it will return you back to the previous "screen".
List of Colors
A great article on ArchLinux.org lists all the bash colors, but doesn't list their RGB equivalents. However, Edward Livingston-Blade published a list of colors and their bash code, but not the bash syntax. He notes that the colors aren't the same for every environment since they aren't defined as RGB colors by the system. He also lists some great bash snippets to show the colors. Nonetheless, here is a list of colors in a table, which I find convenient:
| Color | Bash Syntax | RGB | Color |
|---|---|---|---|
Regular Fonts |
|||
Black |
\e[0;30m |
#000000 |
|
Red |
\e[0;31m |
#AA0000 |
|
Green |
\e[0;32m |
#00AA00 |
|
Yellow |
\e[0;33m |
#AA5500 |
|
Blue |
\e[0;34m |
#0000AA |
|
Purple |
\e[0;35m |
#AA00AA |
|
Cyan |
\e[0;36m |
#00AAAA |
|
White |
\e[0;37m |
#AAAAAA |
|
Bold Fonts |
|||
Black |
\e[1;30m |
#000000 |
|
Red |
\e[1;31m |
#FF5555 |
|
Green |
\e[1;32m |
#55FF55 |
|
Yellow |
\e[1;33m |
#FFFF55 |
|
Blue |
\e[1;34m |
#5555FF |
|
Purple |
\e[1;35m |
#FF55FF |
|
Cyan |
\e[1;36m |
#55FFFF |
|
White |
\e[1;37m |
#FFFFFF |
|
Underlined Fonts |
|||
Black |
\e[4;30m |
#000000 |
|
Red |
\e[4;31m |
#AA0000 |
|
Green |
\e[4;32m |
#00AA00 |
|
Yellow |
\e[4;33m |
#AA5500 |
|
Blue |
\e[4;34m |
#0000AA |
|
Purple |
\e[4;35m |
#AA00AA |
|
Cyan |
\e[4;36m |
#00AAAA |
|
White |
\e[4;37m |
#AAAAAA |
|
Backgrounds |
|||
Black |
\e[40m |
#000000 |
|
Red |
\e[41m |
#AA0000 |
|
Green |
\e[42m |
#00AA00 |
|
Yellow |
\e[43m |
#AA5500 |
|
Blue |
\e[44m |
#0000AA |
|
Purple |
\e[45m |
#AA00AA |
|
Cyan |
\e[46m |
#00AAAA |
|
White |
\e[47m |
#AAAAAA |
|
Other |
|||
Text Reset |
\e[0m |
References
https://wiki.archlinux.org/index.php/Color_Bash_Prompt
https://bbs.archlinux.org/viewtopic.php?id=51818
http://linuxgazette.net/issue51/livingston-blade.html
http://webhome.csc.uvic.ca/~sae/seng265/fall04/tips/s265s047-tips/bash-using-colors.html
Dot Files
Many people have their "dot files" listed on GitHub. Here is a link to mine: https://github.com/josephdpurcell/dotfiles.
Create a custom prompt: http://bashrcgenerator.com/
Shortcuts
http://www.howtogeek.com/?post_type=post&p=44997
The above link has a very amusing command:
$ cat /dev/urandom | hexdump -C | grep "ca fe"
Use Ouptput of Previous Command as Argument
Let's say you want to use the output of a previous command as an argument for the current command. You can use the magical double bang, as a StackOverflow comment shows. Here is an example:
$ which git /opt/local/bin/git $ `!!` status # On branch master nothing to commit (working directory clean)
Or, maybe you are looking for a file to edit, you may do something like:
$ sudo find /etc/* -name "php.ini" /etc/php.ini $ vim `!!`
There are a number of ways to accomplish this same idea, as is discussed in a StackOverflow thread.
Pipes and Redirects
Options for Piping and Redirecting
First, read Peteris Krumins' article "Bash One-Liners Explained, Part III: All about redirections". Then, in the future you can use this for reference:
Option 1: >
$ cat file.txt > file2.txt
Option 2: tee
$ cat file.txt | tee file.txt
http://linux.101hacks.com/unix/tee-command-examples/
Option 3: mkfifo
"Using Named Pipes (FIFOs) with Bash" (Linux Journal)
"Introduction to Named Pipes" (Linux Journal)
Option 4: Named Pipes
http://www.hollenback.net/NamedPipesInBash
Switching stderr to stdout
Use 2>&1, i.e.
$ find / -name "file.txt" 2>&1
http://stackoverflow.com/questions/818255/in-the-bash-shell-what-is-21
Piping stderr and stdout to file
Use &>, i.e.
$ find / -name "file.txt" &> output.txt
http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO-3.html
Further Reading
- http://www.catonmat.net/blog/bash-one-liners-explained-part-three/
- http://www.linuxjournal.com/content/working-stdin-and-stdout
Arrays
http://www.linuxjournal.com/content/bash-arrays
Loop Through an Array
http://www.cyberciti.biz/faq/bash-for-loop-array/
http://www.cyberciti.biz/faq/bash-iterate-array/
http://www.thegeekstuff.com/2010/06/bash-array-tutorial/
Loop Through Files in a Directory
http://www.cyberciti.biz/faq/bash-loop-over-file/
http://www.cyberciti.biz/faq/unix-loop-through-files-in-a-directory/
Splitting a String as an Array
Say you have a string called $str and it is delimited by an underscore, you can split it apart and access it as an array using the following method:
local arr=(`echo $str | tr "_" "\n"`)
local x1=${flag[0]}
local x2=${flag[1]}
echo $x1
echo $x2
For more examples and other methods of splitting an array, see "bash: split string to array" post on Short IT Recipes.
String Manipulation
http://tldp.org/LDP/abs/html/string-manipulation.html
Handling Inputs
Inputs are space delimited. Each input is then saved as $1, $2, $3, ..., $n. The array of all inputs is $@. However, if you want a full blown CLI, you will want to use a tool to handle these parameters. The most common is getopt and getopts.
Colin Zwiebel has a concise list of relevant links: http://clogin.posterous.com/great-bash-scripting-getopts-example. They are worth going through. I found the following article the most helpful: http://techpatterns.com/forums/about1138.html.