Performance evaluation
Runtime
One of the methods for scpecifying the runtime of an application (Program?) is to use SECONDS valriable.
SECONDS=0; sleep 3; echo $SECONDS;
Condition
Conditional structure
The general structure of a conditional statement in Bash is as follows:
val=14 if test $val -eq 14 ; then echo "exit value of a command" elif (( $val + 6 > 20 )); then echo "Mathematical expressions" elif [ $val -gt 10 ] ; then echo "single bracket (POSIX standard)" elif [[ $val -gt 10 ]]; then echo "double bracket (Bash and some others)" else echo "else condition" fi
Comparison conditions
File Operators
The conditional operators for checking the status of files are visible in the table below:
| Operator | Example | Description |
|---|---|---|
-e | -e $fileName | Checks if a file exists. |
-d | -d $directoryName | Checks if a file exists and it is a directory. |
-f | -f $fileName | Checks if a file exists and it is a regular file. |
-h | -h $linkName | Checks if a file exists and it is a symbolic link. |
-r | -r $fileName | Checks if a file is readable. |
-W | -W $fileName | Checks if a file is writable. |
-X | -X $fileName | Checks if a file is executable. |
-s | -S $fileName | Checks if size of a file is non-zero. |
String Operators
Conditional operators for comparing strings is visible in the table below:
| Operator | Example | Description |
|---|---|---|
-z | -z $str | Check if the length of the string is zero. |
-n | -n $str | Check if the length of the string is not zero. |
== | $str1 == $str2 | Check if the two strings are the same. |
!= | $str1 != $str2 | Check if the two strings are not the same. |
> | $str1 > $str2 | Check if str1 is lexicographically greater than str2. |
< | $str1 < $str2 | Check if str1 is lexicographically less than str2. |
Arithmatic Operators
Arithmatic comparisons can be performed using the following operators:
| Operator | Example | Description |
|---|---|---|
-eq | $int1 -eq $int2 | Check if int1 and int2 are equal. |
-ne | $int1 -ne $int2 | Check if int1 and int2 are not equal. |
-gt | $int1 -gt $int2 | Check if int1 is greater than int2. |
-ge | $int1 -ge $int2 | Check if int1 is greater than or equal to int2. |
-lt | $int1 -lt $int2 | Check if int1 is less than int2. |
-le | $int1 -le $int2 | Check if int1 is less than or equal to int2. |
Conditional execution
If execution of the first command was successfull, the second command will be executed:
true && echo "this message should show"
If a command encounters an error, another command will be executed:
false || echo "this message should show"
If the previous command or function returned an error code, the next command will be executed:
false [[ $? -ne 0 ]] && echo "this message should show"
Loops
The following two loops have the same functionality:
for i in `seq 1 10` do echo "\$i is $i" done for s in foo bar baz do echo $s done for (( i = 0; i < 10; i++ )) do echo "\$i is $i" done
Working with arrays
array=(zero one two three) echo "${array[0]}" # shows the first element echo "${array[-1]}" # shows the last element echo "${array[@]}" # shows all elements echo "${array[@]:1:2}" # shows two elements (:2), starting from element with index of 1 (which is the 2nd element (:1)) array[4]='fifth element' # inserts an element at index 4 echo "${array[4]}" array+=('sixth') # appends an element to array unset -v 'array[2]' # deletes element with index of 2 from array array=("${array[@]}") # re-indexing an array echo "${#array[@]}" # shows the length of array
To add an element to index 3 of an array, use the following code:
array=("${array[@]:0:2}" '2.5' "${array[@]:2}") echo "${array[2]}" #output: new echo "${array[@]}"
Extract an array from a string(?):
theStr="Zero,One,Two" array=(${theStr//,/ }) echo ${array[0]} echo ${array[2]}
Reading elements of an array:
array=(1 2 3 4) # foreach loop for y in "${array[@]}"; do # act on $y echo "$y" done # for loop with index for ((idx=0; idx < ${#array[@]}; ++idx)); do # act on ${array[$idx]} echo "${array[$idx]}" done
Recieving the output of a command as an array, and then reading that array using Foreach loop(?):
array=($(tr ',' ' ' <<<"0,1,2,3,4")) for y in "${array[@]}"; do echo "$y" done
Filling an array based on the output of a command:
while read -r; do #array+=("$REPLY") # Array append array[$i]="$REPLY" # Assignment by index let i++ # Increment index done < <(seq 1 10) # command substitution echo ${array[@]} # output: 1 2 3 4 5 6 7 8 9 10
Working with Map (Associative Array)
The first step is to create a map(?):
declare -A mp # declare (This line is mandatory) mp[hello]=world # insert mp["a long key"]=v # insert with a long key echo ${mp[hello]} # access echo "${!mp[@]}" # access all keys echo "${mp[@]}" # access all values echo "${#mp[@]}" # count of elements in a map for key in "${!mp[@]}"; do # Iterate Over map echo "Key: ${key}" echo "Value: ${mp[$key]}" done unset mp # destroy a map/array
working with file:
Reading a file line by line and converting it into an array:
IFS=$'\n' read -r -a arr < file
Reading an entire file until reaching an empty line:
while read line || [ -n "$line" ]; do echo $line done < file
Processing input parameters
If no parameter is passed to for loop, it will be executed on the input file. The following two loops have the same results:
for arg; do echo arg=$arg done for arg in "$@"; do echo arg=$arg done
The following code from stackoverflow demonstrates various use casess for utilizing getopts for processing input parameters:
#!/bin/bash
usage() { echo "Usage: $0 [-s <45|90>] [-p <string>]" 1>&2; exit 1; }
while getopts ":s:p:" o; do
case "${o}" in
s)
s=${OPTARG}
((s == 45 || s == 90)) || usage
;;
p)
p=${OPTARG}
;;
*)
usage
;;
esac
done
shift $((OPTIND-1))
if [ -z "${s}" ] || [ -z "${p}" ]; then
usage
fi