Tutorial: A bit of Bash

--D. Thiebaut (talk) 11:03, 26 November 2013 (EST)

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A quick review of some useful Bash commands that can be used on the command line, or that can be included in bash scripts

Exploration

A good reference on using a shell can be found here: http://linuxcommand.org/.

Here's a recommended path through this on-line tutorial; just click on each link, read the description, and try it out on the linux server (aurora.smith.edu very likely) that you are connected to.

• What Is "The Shell"?
• Navigation
• Looking Around
• (A Guided Tour --skip for later)
• Manipulating Files
• Working With Commands
• I/O Redirection (very important)
• Expansion (stop at arithmetic expansion)
• Permissions (very important)
• (Job Control --skip for later)

Challenge 1

• Create three files in your Linux account
• The first one should be called file1.txt and reside in the path ~/labs/lab1/. Its permissions should be "rwx------". It should contain the text "Mary had a little lamb."
• The second one should be called hello.asm and reside in ~/hws/hw1/. Its permissions should be "rw-rw-rw-". It should contain the "hello world" program we saw in class.
• The third should be the executable version of hello.asm, and reside in the ~/ directory.

Creating a bash script

• first line should contain name of bash shell

#! /bin/bash

• make the script executable

chmod +x scriptname

Variables

• Declaration

file="hello.txt"
file=`ls -1 *.txt | head -1`     # get the first .txt file in the directory

• In expressions: put a $ in front of it!

URL="http://cs.smith.edu/dftwiki/images/"
file="toto.png"
pathFile=$URL$file
file="alpha"
pathFile=$URL${file}.png      # add braces if string needs to be concatenated to variable

Script Parameters

• assume doit.sh is a script and it is called as follows:

 doit.sh 8 2000 50

• To get the arguments inside the script:

NP=$1    # will get 8
N=$2     # will get 2000
M=$3     # will get 50

• To test if the number of parameters is 3 inside the script:

if [ "$#" -ne 3 ]; then
    echo "syntax: doit.sh NP N M"
    exit
fi

Displaying strings and various quantities

Getting the length of a string variable

name="toto.txt"
echo "length of $name = ${#name}"      #will output 8

Getting extension and name (without extension) of a file

fullPath="/usr/local/bin/toto.txt"
fileName="${fullPath##*/}"
echo "file name with extension = $fileName"

extension="${fullPath##*.}"
fileName="${fileName%.*}"

echo "fullPath = $fullPath"
echo "fileName without extension = $fileName"
echo "extension = $extension"

For Loops

• loop from 1 to 10

for i in 1 2 3 4 5 6 7 8 9 10 ; do 
    echo $i
done

or

for i in {1..10} ; do 
    echo $i 
done

or

for i in {1..10} ; do echo $i; done

• loop through strings

for file in toto.txt tata.txt tutu.txt titi.txt ; do
     echo $file
done

• loop through all the files in a directory

dir="/enwiki/0/00"
for file in $dir/* ; do
    echo "$file"
done

or

dir="/enwiki/0/00"
for file in `ls -1 $dir` ; do
    echo $file
done

Sort lists

Alphabetically

• to generate a sorted list of the files on the Desktop:

ls Desktop | sort

Numerically

• to sort a list of numbers.

for i in 10 1 3 5 6 2 4 7 9 ; do echo $i ; done | sort -n

• to remove replicated numbers

for i in 10 1 10 3 3 5 6 3 2 4 7 9 6 6 6 5 ; do echo $i ; done | sort -n | uniq