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README.md

Bash

Resources

Tools

Example

#!/usr/bin/env bash
set -Eeuo pipefail
# set -x # print each command before executing

readonly VAR1="foo"

function help(){
  cat > /dev/stdout << END
  Some help.
END
}

 #!/bin/bash
while getopts "h?vxy:z" opt
do
  case "$opt" in
  h|\?) 
          help
          exit 0
          ;;
  v) VERBOSE=1 ;;
  x) echo '-x';;
  y) echo "-y ${OPTARG}";;
  z) echo '-z';;
  esac
done

if [[ ${VERBOSE} -gt 0 ]]
then
  set -x fi
fi

# todo

if [[ 1 -lt 2 ]] 
then
  echo 'c' 
elif [[ 1 -gt 2 ]]; then
  echo 'b' 
else
  echo 'c' 
fi

for f in $(ls *.scala)
do
  echo "$f"
done

while [[ true ]]
do 
  echo "forever"
done

case "$A" in 
a) echo "a";;
b) echo "b";;
b*) echo "b*";;
*) echo "something else";;
esac

Pitfalls

  • variable assignment must NOT have space around the = -- otherwise, it's 3 space-delimited arguments! e.g., "FOO=foo"

  • not block-scoped -- variables that are assigned are in-scope for the rest of the script

  • export allows sub-processes (shells?) to see the value, or run cmds with "FOO=foo some-cmd"

  • set +o history to not write commands to history (good for passwords and keys)--> +o means off, lol

  • VARIABLES are usually ALL CAPS

  • reference in strings as "a${SOMEVAR}b" -- double quotes interpolate, single quotes do not

  • put one test in each [...] and logical those, e.g., ! [ ... ], [ ... ] && [ ... ]

  • = and == are the same in test

Other

  • readonly A=a ==> readonly vars
  • local A=a ==> by default vars are global, this makes A local to the function it's defined in
  • clean Bash shell ==> env -i bash --noprofile --norc
  • $IFS defines the default separator for things, e.g. \t\n

Tests

  • = and == are the same

  • use [[ ${A} = 'a']] for conditionals -- [ ${A} = 'a'] fails if ${A} is undefined

  • [ is a builtin that does test

  • [[ is a completely different builtin that does test but handles non-existent variables correctly

  • The trick [ "x$UNDEFINED" = "x" ] should use [[ w/o the x

  • -z ${A} true if empty string

  • -e ${A} true if A exists as a file

  • -d ${A} true if A is a directory

  • integer comparisons should use form like -lt or put double-parens instead of [[ --> (( 1 < 2))

With glob pattern matching:

[[ $a == z* ]]   # True if $a starts with an "z" 
[[ $a == "z*" ]] # literal match

[ $a == z* ]     # globbing and word splitting
[ "$a" == "z*" ] # literal match

What's the difference between .bash_profile and .bashrc?

https://blog.flowblok.id.au/2013-02/shell-startup-scripts.html

  • Interactive login shell -- ~/.bash_profile
  • Interactive non-login shell, it reads and executes commands from ~/.bashrc

Add to ~/.bash_profile:

if [ -f ~/.bashrc ]; then
	. ~/.bashrc
fi

Prelude

Start scripts with:

#!/usr/bin/env bash
set -Eeuo pipefail
set -x # print each command before exec
  • -E : fire ERR traps correctly
  • -e : exit immediately on command failure. Append '|| true' or execute in conditional to disable.
  • -u : unset variables trigger failure
  • -o pipefail : don't prevent piped results from masking non-zero exit code
  • -x : print each command before execution

Static analysis

run a command in a loop

while [[ true ]]; do
	time {cmd}
done

Default variable values

FOO=${VARIABLE:-default} 
VARIABLE=${1:-DEFAULTVALUE}

output from running another command

result=$(ls)

splitting with regexes

p=/foo/bar/file1 path=$( echo ${p%/} ) file=$( echo ${p##/} )

shopt -o nounset
declare -rx       FILENAME=payroll_2007-06-12.txt

# Splits
declare -rx   NAME_PORTION=${FILENAME%.*}     # Left of .
declare -rx      EXTENSION=${FILENAME#*.}     # Right of .
declare -rx           NAME=${NAME_PORTION%_*} # Left of _
declare -rx           DATE=${NAME_PORTION#*_} # Right of _
declare -rx     YEAR_MONTH=${DATE%-*}         # Left of _
declare -rx           YEAR=${YEAR_MONTH%-*}   # Left of _
declare -rx          MONTH=${YEAR_MONTH#*-}   # Left of _
declare -rx            DAY=${DATE##*-}        # Left of _

clear

echo "  Variable: (${FILENAME})"
echo "  Filename: (${NAME_PORTION})"
echo " Extension: (${EXTENSION})"
echo "      Name: (${NAME})"
echo "      Date: (${DATE})"
echo "Year/Month: (${YEAR_MONTH})"
echo "      Year: (${YEAR})"
echo "     Month: (${MONTH})"
echo "       Day: (${DAY})"

Comparison Operators

Comparison Ops in the Advanced Bash-Scripting Guide

Integer comparison: -eq -ne -gt -ge -lt -le < <= > >=

String comparison: = == != < > -z (== '') -n (!= '')

File test: -e exists

Heredoc

substituted $MYVAR

cat > script.sh << END
  #!/bin/bash
  echo $MYVAR
END

literal $MYVAR (END in quotes)

cat > script.sh << 'END'
  #!/bin/bash 
  echo $MYVAR
END

Scratch

exec 5>&1
FF=$(echo aaa|tee >(cat - >&5))
echo $FF
if [ -z "${MY_VAR:-}" ]; then
  echo "MY_VAR was not set"
fi
function prt
{
  echo "[$(date +"%Y-%m-%d %H:%M:%S")] - $1"
}

if [ -z "$(aws s3 ls ${HUED_URI})" ]; then
    exit 1
fi

last exit code

$?

pushd dirs popd !! previous command !-n nth last cmd !?string

find . -name MultiPro* -exec svn log {} ;

sed -n '1h;1!H;${g;s/search/replace/;p;}'

perl -pi.bak -e 's/(12</foo>\n)42(</bar>)/${1}33${2}/' text.txt

find deid_files -type f -name '*.json' \
-print0 | xargs -0 python replay.py --uri=http://localhost:3004 \
for d in */; do
    zip -r Albemarle_Board_of_Supervisors_Minutes_${d::-1}.zip $d 
done

find . -name '*.pdf' -print0 | sort -z | xargs -0 -L1 sh -c 'echo "<li><a href=\"${0:2}\">${0:2}</a></li>"' > index.html

Glob

  • * - zero or more characters (equivalent to regex .* • ? - a single character (equivalent to regex .) • [abz] - a b or z • [a-z] - range of a to z

Array

  • array -- arr=("a" "b" "c")
  • access -- echo "${arr[0]}"
  • length -- echo {#arr[@]}

Process Substitution

make a virtual file out of command output

diff <(cmd1) <(cmd2)

also >()

Subshell

(
  ls
) >> out