bash-scripting.md

bash scripting

• string
  • delete empty files
  • skip first N lines
  • extract substring
  • check if string contains another
  • references
• files
  • loop through files arguments
  • loop through file lines
  • loop over find results
• file age in seconds/months
• control
• OR condition
  • function with arguments
• math
  • decimal numeric test using bc

string

delete empty files

awk 'NF'

skip first N lines

tail -n +<N+1>

extract substring

expr substr STR begin len ( begin : 1-based index )

#                    amp
# pos 1-based index:123456
echo $(expr substr "sample" 2 3)

results in "amp"

check if string contains another

• using wildcard

if [[ "$str" == *"portion"* ]]; then echo "it contains"; fi

• using regex

if [[ "$str" =~ .*"portion".* ]]; then echo "it contains"; fi

references

• arrays
• cheat sheet

files

loop through files arguments

• Use "$@" to grab pathfilenames passed as argument
• will work with "file with space" names

#!/bin/bash

for i in "$@"; do
	echo "file [$i]"
done

loop through file lines

readarray -t myarr < <(cat filename.txt)
for line in "${myarr[@]}"; do echo "line = [$line]"; done

notes:

• cat filename.txt | readarray -t myarr would not work because readarray (builtin cmd) will be in a subshell and couldn't access given stdin
• array size is "${#myarr[@]}"
• "${myarr[i]}" is the zero-based i-th item

loop over find results

follow example search folders from current dir limiting depth 1th level and executes a control body foreach result

readarray -t myarr < <(find . -maxdepth 1 -type d -exec echo "{}" \;)

for line in "${myarr[@]}"; do
    echo "do something with [$line]"
done

file age in seconds/months

# args: $1=pathfilename
# returns: fileage in seconds
function fileagesecs()
{
        echo "$(( $(date +%s) - $(date +%s -r "$1") ))"
}

# args: $1=pathfilename
# returns: fileage in months (w/decimals)
function fileagemonths()
{
        echo "$(echo "$(fileagesecs $1) / (365 / 12 * 60 * 60 * 24)" | bc -l)"
}

echo "test filename age:"
echo "$(fileagesecs filename) secs"
echo "$(fileagemonths filename) months"

control

OR condition

if [[ EXPR1 || EXPR2 ]]; then echo "or satisfied"; fi

function with arguments

note:

• $@ variable MUST double quoted to avoid string space splitting
• $0 is the executing script filename
• $1 is the first argument

function test()
{
	another_process "$@"
}

test "sample 1" "sample 2"

math

decimal numeric test using bc

a="12.1875"

if (( $(echo "$a > 12.187" | bc -l) )); then
	echo "[$a] is great than 12.187"
fi

if (( ! $(echo "$a > 12.1876" | bc -l) )); then
	echo "[$a] is not great than 12.1876"
fi