Shell

Variables

• $PS1: prompt
• $HOME: base directory
• $IFS: separator
• $0: name of the script
• $1, $2: arguments
• $*: list of arguments - truncated with first char of $IFS
• $@: list of arguments - as separated args
• $#: number of arguments
• $?: last return code

man bash can explain a lot

Variable escaping

#!/bin/sh
x="hello world"

# a variable without quote can be two separate arguments
printf '%s\n' $x
# hello
# world

# a variable with quote will be one argument (exception exists for $@)
printf '%s\n' "$x"
# hello world

# the $IFS char will be replaced by space
# you can also change the IFS temporarly

IFS='w'
printf '%s\n' $x
# hello
# orld

# note the space after hello and the missing 'w'

printf '%s\n' "$x"
# hello world

IFS=' ' # reset the IFS

Redirections

# Redirect stdout to FILE - override
cmd > FILE
# Redirect stdout to FILE - append
cmd >> FILE

# Redirect stdout and stderr to FILE
cmd &> FILE

# Redirect stderr to FILE
cmd 2> FILE

# Redirect stdout to FILE and stderr to stdout
cmd > FILE 2>&1
cmd >& FILE

# The standard output of cmd is connected via a pipe to the standard input of cmd2
cmd | cmd2

# cmd's standard error, in addition to its standard output, is connected to cmd2's standard input through the pipe
cmd |& cmd2

# stdin from FILE
cmd < FILE

# string as stdin
cmd <<< "string"
echo -n string | cmd

# stdin from FILE stdout to OUTPUT
cmd < FILE > OUTPUT

Test command

test -f myfile # return 1
touch myfile
test -f myfile # return 0

# You can also use the test binary with the '[' binary - it's the same form
# but you need a closing ']'

[ -f myfile ]