Useful bash flags for scripts verbose trace expand error exit

Useful flags:
set -v # "verbose" - echo commands

set -x # "xtrace" - like verbose but expands commands

set -e # "errexit" - abort script at first error

set -u # "nounset" - strict mode: undefined variable causes an exit

Long format:

set -o verbose

Bash "strict mode":
#!/bin/bash
set -euo pipefail
IFS=$'\n\t'

(source, source)

Easily add colours to bash scripts

RED="$(tput setaf 1)"
GREEN="$(tput setaf 2)"
YELLOW="$(tput setaf 3)"
WHITE="$(tput setaf 7)"
RESET=$WHITE

echo "${RED}An error occurred${RESET}"
echo "${YELLOW}Warning: Be careful${RESET}"
echo "${GREEN}Everything is groovy${RESET}"

(source)

Old, naive way:

GREEN="\[\033[32m\]"
YELLOW="\[\033[33m\]"
RESET="\[\033[0m\]"

Bash script must run as root

# This script must be run as root

if [ "$(id -u)" != "0" ]; then
    echo "This script must be run as root" 1>&2
    exit 1
fi

Debug bash scripts

bashdb script.sh

Automate an FTP script

in the ftp script put.pl:
#!/bin/bash
HOST="example.com"
HOMEDIR="/"
ftp -i $HOST <<-EOF
passive
cd $HOMEDIR
put $1
quit
EOF
echo "I put the file "$1

in ~/.netrc:

machine example.com
    login [username]
    password [password]

to run:
put.pl [filename]

Run an SQL script

• (a) pipe it into the program:  mysql < script.sql
• (b) source it from within mysql:  source script.sql

See also batch commands

Combine multiple lines in multiple files into a single spreadsheet row

for k in $(ls *.xml); do echo "$(perl -e'print "$ARGV[0]\t"' $k)""$(head -14 $k | egrep 'h:shortdesc|dc:title' | perl -lne'($t)=$_=~m{([^<]+)} if ! $t; ($d)=$_=~m{([^<]+)<} if ! $d;END{print "$d\t$t"}')"; done

Bash profile settings

~/.bash_profile gets called when you log in for the first time. It's your user-specific settings.

~/.bashrc gets called when you start a new shell. Only interactive shells and user scripts read this file.

Bash menu

Try this menu script for bash scripts:

while getopts ":u:a:s:v" options; do

  case $options in

    u ) uname=$OPTARG;;

    a ) attrs=$OPTARG;;

    s ) searchattr=$OPTARG;;

    v ) att=ALL;;

    h ) echo $USAGE;;

    \? ) echo $USAGE

         exit 1;;

    * ) echo $USAGE

          exit 1;;

  esac

done

echo uname = $uname

Bash variable comparisons

...bash logic...

Note that integer and string comparison use a different set of operators.

integer comparison

-eq

is equal to

if [ "$a" -eq "$b" ]

-ne

is not equal to

if [ "$a" -ne "$b" ]

-gt

is greater than

if [ "$a" -gt "$b" ]

-ge

is greater than or equal to

if [ "$a" -ge "$b" ]

-lt

is less than

if [ "$a" -lt "$b" ]

-le

is less than or equal to

if [ "$a" -le "$b" ]

<

is less than (within double parentheses)

(("$a" < "$b"))

<=

is less than or equal to (within double parentheses)

(("$a" <= "$b"))

>

is greater than (within double parentheses)

(("$a" > "$b"))

>=

is greater than or equal to (within double parentheses)

(("$a" >= "$b"))

string comparison

=

is equal to

if [ "$a" = "$b" ]

==

is equal to

if [ "$a" == "$b" ]

This is a synonym for =.

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