Bash
Time the script
start=$(date +%s.%N)
# some important stuff
end=$(date +%s.%N)
runtime=$( echo "$end - $start" | bc -l )
Read stdin into a variable
INPUT_VAR=$(</dev/stdin)
This allows something like:
echo 'hello world' | ./script.sh
Running commands in parallel
Use GNU parallel:
mplayer [--gnu] ::: file1.mp4 file2.mp4
Rename files from uppercase to lowercase
for i in *; do mv $i `echo $i | tr [:upper:] [:lower:]`; done
Find Latest modified file
find . -type f -printf '%T@ %p\n' | sort -n | tail -1 | cut -f2- -d" "
- %T@ gives you the modification time like a unix timestamp
- sort -n sorts numerically
- tail -1 takes the last line (highest timestamp)
- cut -f2 -d" " cuts away the first field (the timestamp) from the output
Se also: mail
Variable expansion from string
Access script parameters by iteration
$ for parameter in $(eval echo "{1..$#}"); do
echo "Number: ${parameter}"
echo "Value: ${!parameter}"
done
Trim whitespaces
var=" abc "
# remove leading whitespace characters
var="${var#"${var%%[![:space:]]*}"}"
# remove trailing whitespace characters
var="${var%"${var##*[![:space:]]}"}"
echo "===$var==="
# remove both (simpler)
var="$(echo $var | xargs)"
Set environment
# see `man set`
set -eo pipefail
exit on command non-zero status
set -e # or
set -o errexit
Error on unset variables
set -u # or
set -o nounset
History
ls *.jp
^jp^jpg # Replaces only the first occurence
!!:s/jp/jpg/ # Replaces only the first occurence
!ls:s/jp/jpg/ # most recent ls command, only first occurence
Test for remote open ports
nc -z <host> <port>
echo $?
For a quick interactive check
$ nc -z -v -w5 <host> <port>
...
Add or remove trailing slash
# Add
STR="/i/am/a/path"
length=${#STR}
last_char=${STR:length-1:1}
[[ $last_char != "/" ]] && STR="${STR}"; :
echo "${STR}" # => /i/am/a/path/
# Remove
STR="/i/am/a/path/"
length=${#STR}
last_char=${STR:length-1:1}
[[ $last_char == "/" ]] && STR="${STR:0:length-1}"; :
echo "${STR}" # => /i/am/a/path
or short for removing:
STR=${STR%/}
VI bindings in bash
# file: ~/.bashrc
set -o vi
Readline VI Editing Mode Cheat Sheet
Variablesubstition in find
To assign the find placeholder {} to a variable, you need to do the following:
# _ = $0
# {} = $1 # read `man bash`
$ find . -iname "*.xml" -exec bash -c 'echo "$1"' _ {} \;
Variable substituion
# Removs -._ from the string
echo ${string//[-._]/}
Commandline parsing
#!/usr/bin/env bash
POSITIONAL=()
while [[ $# -gt 0 ]]; do
key="$1"
case $key in
-e|--extension)
EXTENSION="$2"
shift # past argument
;;
-s|--searchpath)
SEARCHPATH="$2"
shift # past argument
;;
-l|--lib)
LIBPATH="$2"
shift # past argument
;;
--default)
DEFAULT=YES
;;
*) # unknown option
POSITIONAL+=("$1") # save it in an array for later
;;
esac
shift # past argument/value
done
set -- "${POSITIONAL[@]}" # restore positional parameters
echo FILE EXTENSION = "${EXTENSION}"
echo SEARCH PATH = "${SEARCHPATH}"
echo LIBRARY PATH = "${LIBPATH}"
echo DEFAULT = "${DEFAULT}"
echo "Number files in SEARCH PATH with EXTENSION:" $(ls -1 "${SEARCHPATH}"/*."${EXTENSION}" | wc -l)
if [[ -n $1 ]]; then
echo "Last line of file specified as non-opt/last argument:"
tail -1 "$1"
fi
Logg input/output
Log the input and output of a session:
script <filename>
Script full path
# Method A
SCRIPTPATH="$( cd $(dirname '$0') ; pwd -P )"
# Method B
SCRIPT_PATH=$(dirname "$(realpath -s '$0')")
Exit script on key press
while true; do
read -t 1 -N 1
case "$k" in
$'\x09') # TAB
exit 0
;;
$'\x7f') # Back-Space
exit 0
;;
$'\x01') # Ctrl+A
exit 0
;;
$'\x1b') # ESC
exit 0
esac
done
Version comparing
#!/bin/bash
# Return values
# 0 = '='
# 1 = '>'
# 2 = '<'
vercomp () {
if [[ $1 == $2 ]]
then
return 0
fi
local IFS=.
local i ver1=($1) ver2=($2)
# fill empty fields in ver1 with zeros
for ((i=${#ver1[@]}; i<${#ver2[@]}; i++))
do
ver1[i]=0
done
for ((i=0; i<${#ver1[@]}; i++))
do
if [[ -z ${ver2[i]} ]]
then
# fill empty fields in ver2 with zeros
ver2[i]=0
fi
if ((10#${ver1[i]} > 10#${ver2[i]}))
then
return 1
fi
if ((10#${ver1[i]} < 10#${ver2[i]}))
then
return 2
fi
done
return 0
}
Exiting
Scripts
# Syntax
exit <errorcode>
# Example
exit 1
Functions
# Syntax
return <errorcode>
# Example
return 1
Help text
Include a header like this
#!/bin/bash
###
### my-script — does one thing well
###
### Usage:
### my-script <input> <output>
###
### Options:
### <input> Input file to read.
### <output> Output file to write. Use '-' for stdout.
### -h Show this message.
###
This function will print out all lines pre-prended with three octothorpes.
#
# Help function
help() {
sed -rn 's/^### ?//;T;p' "$0"
}
Tipps and tricks
Repeat until success
$ while ! cat missingfile
do
echo waiting for missingfile
sleep 10
done
$ until cat missingfile
do
echo waiting for missingfile
sleep 10
done
Loop through array
## declare an array variable
declare -a arr=("element1" "element2" "element3")
## or
declare -a arr=("element1"
"element2" "element3"
"element4"
)
## now loop through the above array
for i in "${arr[@]}"
do
echo "$i"
# or do whatever with individual element of the array
done
# You can access them using echo "${arr[0]}", "${arr[1]}" also
Decimal octal interpration
When facing the error
Value too great for base (error token is "08")
or something similar, you basically using octal numbers instead of decimal numbers. You can define the base of the variable you're using like this:
${variable#0}
DU - Disk usage output formation
format the output of du and sort the files by size:
dusk() { du -sk "$@" | sort -rn | sed -E ':a; s/([[:digit:]]+)([[:digit:]]{3})/\1,\2/; ta' | awk -F'\t' '{printf "%10s %s\n",$1,substr($0,length($1)+2)}';}
dusk *
Performance measuring
While time provides some basic measurement for efficency comparison, a more suited tool is hyperfine which can run
several commands and compare their average speed:
hyperfine [--warmup 10] --shell bash "$command1" "$command2"