bash.md

Variables

• $0 - The name of the Bash script.
• $1 - $9 - The first 9 arguments to the Bash script. (As mentioned above.)
• $# - How many arguments were passed to the Bash script.
• $@ - All the arguments supplied to the Bash script.
• $* - All the arguments supplied to the Bash script seperated by the IFS.
• $? - The exit status of the most recently run process.
• $$ - The process ID of the current script.
• $USER - The username of the user running the script.
• $HOSTNAME - The hostname of the machine the script is running on.
• $SECONDS - The number of seconds since the script was started.
• $RANDOM - Returns a different random number each time is it referred to.
• $LINENO - Returns the current line number in the Bash script.

Array

• create an array from multiline variables

Suites="Clubs
Diamonds
Hearts
Spades"

suite=($Suites)                # Read into array variable.
num_suites=${#suite[*]}        # Count how many elements.
${suite[$((RANDOM%num_suites))]} # Access random index

Arithmetic

find the length

a='Hello World'
echo ${#a} # 11

If Statements

• square brackets if [ ] represent the test command so can man test to see options

! EXPRESSION	        The EXPRESSION is false.
-n STRING	            The length of STRING is greater than zero.
-z STRING	            The lengh of STRING is zero (ie it is empty).
STRING1 = STRING2	    STRING1 is equal to STRING2
STRING1 != STRING2	  STRING1 is not equal to STRING2
INTEGER1 -eq INTEGER2	INTEGER1 is numerically equal to INTEGER2
INTEGER1 -gt INTEGER2	INTEGER1 is numerically greater than INTEGER2
INTEGER1 -lt INTEGER2	INTEGER1 is numerically less than INTEGER2
-d FILE	              FILE exists and is a directory.
-e FILE	              FILE exists.
-r FILE	              FILE exists and the read permission is granted.
-s FILE	              FILE exists and it's size is greater than zero (ie. it is not empty).
-w FILE	              FILE exists and the write permission is granted.
-x FILE	              FILE exists and the execute permission is granted.

cut

cut -d' ' -f 2,3 data.txt # each line of file, split on the whitespace and take the 2 and 3rd column

wc - count words / lines

wc -l data.txt # count number of lines in the file

sed

sed -ie 's/ow/aagh/g' # match globally and update file
sed 's#/a/b/c/#/d/e/f/#' # use a different delimeter
sed '11,$ d' ~/temp.txt # delete from 11th line to end of file
sed "${NUM}q;d" file # print the $NUM line of file. q will quit at that line, d will delete everything up until that line

awk

awk '/root/ {print $1, $9;}' temp.txt # match the regex for "root" and print the first and ninth column
ls -la | awk '$10 ~/^\.s/ {gsub(/sample/, "cool"); print;}' # match the 10th column to dotfile and substitute within the matched line

File Descriptors

• A file descriptor is a non-negative number that is used to access a file or stream.
• the file descriptor for stdin, stdout and stderr is 0, 1, and 2 respectively

So now if we want to redirect our stderr to the file we can do this:

$ ls /fake/directory 2> peanuts.txt
You should see just the stderr messages in peanuts.txt.

Now what if I wanted to see both stderr and stdout in the peanuts.txt file? It's possible to do this with file descriptors as well:

$ ls /fake/directory > peanuts.txt 2>&1
This sends the results of ls /fake/directory to the peanuts.txt file and then it redirects stderr to the stdout via 2>&1. The order of operations here matters, 2>&1 sends stderr to whatever stdout is pointing to. In this case stdout is pointing to a file, so 2>&1 also sends stderr to a file. So if you open up that peanuts.txt file you should see both stderr and stdout. In our case, the above command only outputs stderr.

There is a shorter way to redirect both stdout and stderr to a file:

$ ls /fake/directory &> peanuts.txt
Now what if I don't want any of that cruft and want to get rid of stderr messages completely? Well you can also redirect output to a special file call /dev/null and it will discard any input.

$ ls /fake/directory 2> /dev/null