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README.md
257
README.md
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@ -130,6 +130,135 @@ $USER current username
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$HOSTNAME current hostname
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```
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## Variable
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[[back to top](#handy-bash-one-liners)]
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##### Variable substitution within quotes
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```bash
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# foo=bar
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echo "'$foo'"
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#'bar'
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# double/single quotes around single quotes make the inner single quotes expand variables
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```
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##### Get the length of variable
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```bash
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var="some string"
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echo ${#var}
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# 11
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```
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##### Get the first character of the variable
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```bash
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var=string
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echo "${var:0:1}"
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#s
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# or
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echo ${var%%"${var#?}"}
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```
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##### Remove the first or last string from variable
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```bash
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var="some string"
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echo ${var:2}
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#me string
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```
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##### Replacement (e.g. remove the first leading 0 )
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```bash
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var="0050"
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echo ${var[@]#0}
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#050
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```
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##### Replacement (e.g. replace 'a' with ',')
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```bash
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{var/a/,}
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```
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##### Replace all (e.g. replace all 'a' with ',')
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```bash
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{var//a/,}
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```
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```bash
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#with grep
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test="god the father"
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grep ${test// /\\\|} file.txt
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# turning the space into 'or' (\|) in grep
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```
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##### To change the case of the string stored in the variable to lowercase (Parameter Expansion)
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```bash
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var=HelloWorld
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echo ${var,,}
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helloworld
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```
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##### Expand and then execute variable/argument
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```bash
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cmd="bar=foo"
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eval "$cmd"
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echo "$bar" # foo
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```
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## Math
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[[back to top](#handy-bash-one-liners)]
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##### Arithmetic Expansion in Bash (Operators: +, -, *, /, %, etc)
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```bash
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echo $(( 10 + 5 )) #15
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x=1
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echo $(( x++ )) #1 , notice that it is still 1, since it's post-incremen
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echo $(( x++ )) #2
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echo $(( ++x )) #4 , notice that it is not 3 since it's pre-incremen
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echo $(( x-- )) #4
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echo $(( x-- )) #3
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echo $(( --x )) #1
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x=2
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y=3
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echo $(( x ** y )) #8
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```
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##### Print out the prime factors of a number (e.g. 50)
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```bash
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factor 50
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# 50: 2 5 5
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```
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##### Sum up input list (e.g. seq 10)
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```bash
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seq 10|paste -sd+|bc
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```
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##### Sum up a file (each line in file contains only one number)
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```bash
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awk '{s+=$1} END {print s}' filename
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```
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##### Column subtraction
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```bash
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cat file| awk -F '\t' 'BEGIN {SUM=0}{SUM+=$3-$2}END{print SUM}'
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```
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##### Simple math with expr
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```bash
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expr 10+20 #30
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expr 10\*20 #600
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expr 30 \> 20 #1 (true)
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```
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##### More math with bc
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```bash
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# Number of decimal digit/ significant figure
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echo "scale=2;2/3" | bc
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#.66
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# Exponent operator
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echo "10^2" | bc
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#100
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# Using variables
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echo "var=5;--var"| bc
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#4
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```
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## Grep
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[[back to top](#handy-bash-one-liners)]
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@ -947,134 +1076,6 @@ case $type in
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esac
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```
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## Variable
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[[back to top](#handy-bash-one-liners)]
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##### Variable substitution within quotes
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```bash
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# foo=bar
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echo "'$foo'"
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#'bar'
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# double/single quotes around single quotes make the inner single quotes expand variables
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```
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##### Get the length of variable
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```bash
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var="some string"
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echo ${#var}
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# 11
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```
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##### Get the first character of the variable
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```bash
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var=string
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echo "${var:0:1}"
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#s
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# or
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echo ${var%%"${var#?}"}
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```
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##### Remove the first or last string from variable
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```bash
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var="some string"
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echo ${var:2}
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#me string
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```
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##### Replacement (e.g. remove the first leading 0 )
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```bash
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var="0050"
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echo ${var[@]#0}
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#050
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```
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##### Replacement (e.g. replace 'a' with ',')
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```bash
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{var/a/,}
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```
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##### Replace all (e.g. replace all 'a' with ',')
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```bash
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{var//a/,}
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```
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```bash
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#with grep
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test="god the father"
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grep ${test// /\\\|} file.txt
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# turning the space into 'or' (\|) in grep
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```
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##### To change the case of the string stored in the variable to lowercase (Parameter Expansion)
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```bash
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var=HelloWorld
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echo ${var,,}
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helloworld
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```
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##### Expand and then execute variable/argument
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```bash
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cmd="bar=foo"
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eval "$cmd"
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echo "$bar" # foo
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```
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## Math
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[[back to top](#handy-bash-one-liners)]
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##### Arithmetic Expansion in Bash (Operators: +, -, *, /, %, etc)
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```bash
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echo $(( 10 + 5 )) #15
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x=1
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echo $(( x++ )) #1 , notice that it is still 1, since it's post-incremen
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echo $(( x++ )) #2
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echo $(( ++x )) #4 , notice that it is not 3 since it's pre-incremen
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echo $(( x-- )) #4
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echo $(( x-- )) #3
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echo $(( --x )) #1
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x=2
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y=3
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echo $(( x ** y )) #8
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```
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##### Print out the prime factors of a number (e.g. 50)
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```bash
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factor 50
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# 50: 2 5 5
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```
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##### Sum up input list (e.g. seq 10)
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```bash
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seq 10|paste -sd+|bc
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```
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##### Sum up a file (each line in file contains only one number)
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```bash
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awk '{s+=$1} END {print s}' filename
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```
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##### Column subtraction
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```bash
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cat file| awk -F '\t' 'BEGIN {SUM=0}{SUM+=$3-$2}END{print SUM}'
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```
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##### Simple math with expr
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```bash
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expr 10+20 #30
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expr 10\*20 #600
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expr 30 \> 20 #1 (true)
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```
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##### More math with bc
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```bash
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# Number of decimal digit/ significant figure
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echo "scale=2;2/3" | bc
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#.66
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# Exponent operator
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echo "10^2" | bc
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#100
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# Using variables
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echo "var=5;--var"| bc
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#4
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```
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## Time
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[[back to top](#handy-bash-one-liners)]
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