[bash] Conditional operators in BASH

-a file

True if file exists.

-b file

True if file exists and is a block special file.

-c file

True if file exists and is a character special file.

-d file

True if file exists and is a directory.

-e file

True if file exists.

-f file

True if file exists and is a regular file.

-g file

True if file exists and its set-group-id bit is set.

-h file

True if file exists and is a symbolic link.

-k file

True if file exists and its “sticky” bit is set.

-p file

True if file exists and is a named pipe (FIFO).

-r file

True if file exists and is readable.

-s file

True if file exists and has a size greater than zero.

-t fd

True if file descriptor fd is open and refers to a terminal.

-u file

True if file exists and its set-user-id bit is set.

-w file

True if file exists and is writable.

-x file

True if file exists and is executable.

-O file

True if file exists and is owned by the effective user id.

-G file

True if file exists and is owned by the effective group id.

-L file

True if file exists and is a symbolic link.

-S file

True if file exists and is a socket.

-N file

True if file exists and has been modified since it was last read.

file1 -nt file2

True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.

file1 -ot file2

True if file1 is older than file2, or if file2 exists and file1 does not.

file1 -ef file2

True if file1 and file2 refer to the same device and inode numbers.

-o optname

True if shell option optname is enabled. The list of options appears in the description of the -o option to the set builtin (see The Set Builtin).

-z string

True if the length of string is zero.

-n string

string

True if the length of string is non-zero.

string1 == string2

True if the strings are equal. ‘=’ may be used in place of ‘==’ for strict posix compliance.

string1 != string2

True if the strings are not equal.

string1 < string2

True if string1 sorts before string2 lexicographically in the current locale.

string1 > string2

True if string1 sorts after string2 lexicographically in the current locale.

arg1 OP arg2

OP is one of ‘-eq’, ‘-ne’, ‘-lt’, ‘-le’, ‘-gt’, or ‘-ge’. These arithmetic binary operators return true if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively. Arg1 and arg2 may be positive or negative integers.

Reference: Copied directly from here.

[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.