Several cheat-sheets of different topics in .md format. Checkout the Github pages version.
Only stderr
2>/dev/null
Only stdout
1>/dev/null
Both stderr and stdout
2> /dev/null 1>&2
Check if input is empty
if [ -z $1 ]
then
...
fi
Check number of input arguments
if [ "$#" -ne 1 ]; then
echo "Illegal number of parameters"
fi
Check if a file exists:
exists=$( 2>/dev/null ls ${MAINDIR}/${TFFILE} )
if [ -z "${exists}" ]
then
echo "File ${MAINDIR}/${TFFILE} does not exists"
else
...
fi
Compare two integers:
if (( a > b )); then
...
fi
# or
if [ "$a" -gt "$b" ]; then
...
fi
Declare arrays
x=();
y=("anr" "123" "asd");
Access to specific elements
echo "${y[0]}" "${y[2]}" "${y[-1]}" "${y[-2]}"
Access to all the elements
echo "${y[@]}"
Loop over all the elements
for i in "${y[@]}"
do
...
done
also, to loop over integer values:
for i in {1..32}
do
...
done
or in one line
for i in {1..90}; do; echo "" > file-$i.txt ; done
Append an element to existing array
array+=new_element
Create a new array copying an existing one
newArray=( "${oldArray[@]}" )
Print the number of elements of an array
echo "${#array[@]}"
Associative arrays, or arrays with keys-value pairs, example of dynamic creation from file:
declare -A REPOSITORIES
for rpt in `cat "${repositories_file}" | grep -v '#'`; do
rpt_name=`echo ${rpt} | awk -F: '{print $1}'`
rpt_flag=`echo ${rpt} | awk -F: '{print $2}'`
AAT_REPOSITORIES+=(["${rpt_name}"]="${rpt_flag}")
done
Another example:
declare -a myArray
myArray[a]=123
myArray[b]=343
myArray+=([c]=345 [c]=054)
Access the keys:
for key in "${!array[@]}"; do
...
done
echo "${!array[@]}
Access the values:
echo ${myArray[a]}
echo ${myArray[@]}
Example of declaring a function called “fun”
fun () {
echo "Hello World! My name is $1 and I like $2"
}
so that, we invoke it as follows:
fun "Peter" "bash programming"
Function that splits a string and stores elements splitted in an array
func_splitter() {
str=$1
delimiter=$2
s=$str$delimiter
array=();
while [[ $s ]]; do
array+=( "${s%%"$delimiter"*}" );
s=${s#*"$delimiter"};
done;
}
Call it to split a string using a specific delimiter, like -, as follows
func_splitter "$string" "$delimiter"
echo "${array[@]}"
Simply split a string in two parts
partLeft=${string%%"$delimiter"*}
partRight=${string#*"$delimiter"}
Examples
var1=$(echo ls -lrt)
var2=$(cat file.log)
Check if a string contains only alphanumeric and {“-”, “_”} characters.
if [[ "${ENVNAME}" =~ ^[a-zA-Z0-9_-]+$ ]]
then
echo "${ENVNAME}"
else
echo "${ERRMSSGA}"
exit
fi
Read a string line by line:
#!/bin/bash
while IFS= read -r line
do
echo "$line"
done <<< "${string}"
Read a file line by line:
#!/bin/bash
while IFS= read -r line
do
echo "$line"
done < "${pathToFile}"
Replace a string substring by a new one
string="a new new test"
substring="new"
newone="old"
echo "${string/$substring/$newone}"
Replace all occurrences in a string of a substring by a new one
message='The secret key is 431241'
echo "${message//[0-9]/X}"
# prints 'The secret key is XXXXX'
Remove white spaces
var=$(echo "${var}//[:blank:]]/")
Remove all control characters:
var=${var//[ $'\001'-$'\037']}
Transform string to integer after removing control characters
expr $string
num1=$((string + 0))
num2=$((stra + strb))
Modify using patterns
string=str1_str2_str3.str4.str5
echo ${string##*_} # removes largest occurence of pattern at the begginning of string
echo ${string#*_} # removes smallest occurence of pattern at the begginning of string
echo ${string}
echo ${string%.*} # removes smallest occurence of pattern at the end of string
echo ${string%%.*} # removes largest occurence of pattern at the end of string
echo ${string/.*/repl01} # replaces first occurrence of pattern in string
echo ${string//.*/repl01} # replaces all occurrences of pattern in string
script_name=`basename "$0"`
At the root of the repository, create a unique dummy file.
For example, if the repository is named “etl”, the dummy file could be .etl.dir.
Next, in the script code, the repository absolute path can be found with:
repo_dir=`find ${HOME} -name ".etl.dir" -type f -print -quit`
repo_dir=${script_dir%/*}
Replace all “word” occurencies by “new” in all file “FILE”
sed -i -e "s/word/new/g" FILE
Remove lines from a file that contain a substring
while read line; do
[[ ${line} != *"RISK SCORE"* ]] && echo "$line" >> ${temp}
done < $outputData
Store current date-time stamp in a variable
dtimestamp=$( echo "$(date +'%Y%m%d-%H_%M_%S_%3N')" )
Add timestamp to echo’s with a log format:
timestamp () {
echo "[`date +'%Y-%m-%d %H:%M:%S'`]"
}
echo "$(timestamp) Some log message."
Verify number of occurrences in a set of files:
files=(
"./*.csv"
)
for file in ${files}
do
I103=$(cat ${file} | grep -c '{2:I103')
O103=$(cat ${file} | grep -c '{2:O103')
nI103=$((I103+0))
nO103=$((O103+0))
ntotalMT103=$((nI103+nO103))
total=$(cat ${file} | grep -c '\$')
ntotal=$((total+0))
if [ ${ntotal} -eq ${ntotalMT103} ]
then
echo
echo ${file}
echo ' 'number of 103 occurrences in file: ${ntotalMT103}
echo ' 'total number of symbol occurrences in file: ${ntotal}
fi
done
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