[csh] Simple loops in csh or tcsh

If you are stuck with a c-shell and you want to print (or execute a command) for an increment, say of 5,  in a variable (for example, $x), then do either of the following:

#!/bin/csh
# Exammple: 1
foreach x (`seq 1 5 20`)
     echo $x
end
#EOF

#!/bin/csh
# Example: 2
@ x = 1
while ($x <= 20)
     echo $x
@ x += 5
end
#EOF

They both print the numbers 1 to 20 in steps of 5. If you want to use it in the command line, then you need to type each line (of course, except  #!/bin/csh and #EOF) and hit enter .

If you know all the possible elements (numbers, characters or strings) of the array passed on to x for each foreach, then you may  choose to use the following

#!/bin/csh
# Example: 3
foreach x (1 2 9 40)
     echo $x
end
#EOF

when the elements are 1, 2, 9 and 40; or

#!/bin/csh
# Example: 4
foreach x (*.cpp)
     echo $x
end
#EOF

when the elements are all the filenames ending with .cpp in the current directory.

Update: The second option (Example: 2) spits out some syntax error message when used in the command line (in the same way as mentioned above). However, it works fine in a script file.

[bash] Test if a variable is an integer using BASH script

#!/bin/bash
echo "Enter an integer; I'll test if it's really a one"
read VARIABLE
while [ -z "$VARIABLE" ]; do # Trap empty strings (but there's really no need!)
   echo "Please enter something instead of just hitting ENTER. Thank you!"
    read VARIABLE
done
#
if [ "$VARIABLE" -eq "$VARIABLE" >& /dev/null ]; then # test if integer
    echo "'$VARIABLE' is indeed an integer"
else
     echo "'$VARIABLE' is not an integer"
 fi
 echo "Good bye!"

Save the above in a file, say testint.sh, make it executable and test in the command line. The if statement does an arithmetic comparison which complains a lot if $VARIABLE is not an integer (that’s why there is a redirection!).

[bash] Introductory BASH scripting for scientists

These are based on the type of work I am interested in, i.e. scientific programming: you won’t see much about character strings in this post since we are mostly interested in numbers!

These can all be directly typed in the BASH prompt. Semicolons basically indicate the start of a new statement: you may put them in separate lines; and in that case, do not put the semicolon between the statements typed in different lines. The dollar sign ($) at the beginning of the command line indicates the command prompt and it is not a part of the command!

1. The basic
Print the sum of two integers:

$ echo $((2+3))
The two pairs of parentheses interpret them as numbers. You can also use square brackets (just a single pair):

$ echo $[2+3].

For floating point numbers use bc.

Variable substitutions may be done as
$ num1=10; num2=20; echo $(($num1+$num2))
(no space between the equality sign and the variable/number)

2. Conditional block: arithmetic conditional operators are eq, lt, le, gt and ge:

$ i=10; j=20; if [ $i -lt $j ]; then echo "$i<$j? True"; fi
$ i=$RANDOM;j=$RANDOM; if [ $i -lt $j ]; then echo "i=$i is less than j=$j"; else echo "i=$i is NOT less than j=$j";fi
($RANDOM equals a random number at each invocation)

Notice the  spaces between the square brackets and the expression within it.

3. Array:
Assignment:

$ arr=(10 11 12)
which is the same as

$ arr[0]=10; arr[1]=11; arr[2]=12

Following constructs show how you can actually access the array (link)

$ echo ${arr[2]}  # The 3rd element of the array
$ echo ${arr[*]}  # All of the items in the array
$ echo ${!arr[*]} # All of the indexes in the array
$ echo ${#arr[*]} # Number of items in the array
$ echo ${#arr[0]} # Length of item zero


4. For loop

$ for (( c=1; c<=5; c++ )); do echo "c = $c"; done            # C-style
$ for c in 1 2 3 4 5; do echo "c = $c"; done                  # List
$ arr=(1 2 3 4 5); for c in ${arr[*]}; do echo "c = $c"; done # Explicit array
$ for c in {1..5}; do echo "c = $c"; done                     # 1 thru' 5 with unit intervals
$ for c in {0..10..2}; do echo "c = $c"; done                 # 0 thru' 10 with interval=2


5. While and until loops
$ c=0; while [ $c -lt 10 ]; do echo "c = $c"; let c++; done
You may replace “let c++” by “((c++))”.

$ c=20; until [ $c -lt 10 ]; do echo "c = $c"; let c--; done

6. User input
Use read command to read in user input, e.g.
$ echo "Enter two integers"; read num1 num2; echo "$num1+$num2=$((num1+num2))"
In the second echo statement, when num1 and num2 are not enclosed within two pairs of parentheses they are treated as character strings, whereas in the last part the double parentheses cause them to be treated as numbers.

N.B. For the above loops and blocks, a line break between do or then and the statement that immediately follows it is allowed.

Reference: link.