Bash scripting tips - Anuradha Chowdhary

Understanding `[` and `[[`

Square brackets are a shorthand notation for performing a conditional test. [ and [[ are commands in Unix.

[ is a builtin command, while [[ is a keyword

    $ type -a [
    [ is a shell builtin
    [ is /usr/bin/[

    $ type -a [[
    [[ is a shell keyword

[ is same as test command. Two statements below have same outcome:

    if [ "$myvar" = 'success' ]; then ...
    if test "$myvar" = 'success'; then ...

[[ is bash’s improvement to the [ command. It has more syntactical features:

handles string comparison without quotes
has =~ operator for regex if [ $myvar =~ ^suc.*s$ ]
can use ||, &&, > and <

Use log function

    err() {
      echo "[$(date +'%Y-%m-%dT%H:%M:%S%z')]: $*" >&2
    }

    err() {
      echo "[$(date +'%Y-%m-%dT%H:%M:%S%z')]: $*" 
    }


    log "doing_something"

    if ! do_something; then
      err "Unable to do_something"
    fi

Redirect all logs to file

    readonly LOG_FILE="~/mylogs/script.log"
    touch $LOG_FILE

    # Open standard out at `$LOG_FILE` for write.
    exec 1>$LOG_FILE

    # standard error ends up going to wherever standard out goes.
    exec 2>&1

Above does not redirect output for subporocesses.

    (
      my_function
      my_script
    ) &>$LOG_FILE

Concatenate String

#!/usr/bin/env bash

# method 1
foo="I like"
foo+=" Bash Scripting."

# method 2
foo="I like"
bar="Bash Scripting."
foobar="$foo $bar"

# method 3
foo="Writ"
foo="${foo}ing Bash Scripting is fun."

Default value for variable

    name="${name:-achowdhary}""

Parentheses () vs. Braces {}

Parentheses cause the commands to be run in a subshell, and braces cause the commands to be grouped together but not in a subshell.

User Input

#!/bin/bash

read -p "Enter your name [Anuradha]: " name
name=${name:-Anuradha}
echo $name

Understanding [ and [[