bash.md

title: Bash

Clever one liners

# Search and replace in all files
perl -p -i -e "s/from/to/g" **/*.css

String substitutions by patterns

STR=/path/to/foo.c

echo ${STR%.c}      #=> "/path/to/foo"
echo ${STR%.c}.o    #=> "/path/to/foo.o"
echo ${STR##*.}     #=> "c" (extension)

BASE=${SRC##*/}     #=> "foo.c" (basepath)
DIR=${SRC%$BASE}    #=> "/path/to"

Substitutions by regex

echo ${STR/hi/hello}          # Replace first match
echo ${STR//hi/hello}         # Replace all matches
echo ${STR/#hi/hello}         # ^hi
echo ${STR/%hi/hello}         # hi$

echo "${STR:0:3}"             # .substr(0, 3) -- position, length
echo "${STR:-3:3}"            # Negative position = from the right

echo ${#line}                 # Length of $line

[ -z "$CC" ] && CC=gcc        # CC ||= "gcc"  assignment
${CC:=gcc}                    # $CC || "gcc"

Loops

for i in /etc/rc.*; do
  echo $i
done

Reading input

echo -n "Proceed? [y/n]: "
read ans
echo $ans

read -n 1 ans    # Just one character

Functions

Defining functions

myfunc() { ... }
fuction myfunc { ... }
fuction myfunc() { ... }

Returning strings

myfunc() {
    local myresult='some value'
    echo $myresult
}

result=$(myfunc)

Errors

myfunc() { return 1; }

Arguments

$#                          # Number of arguments
$*                          # All args
$1                          # First argument

Ifs - files

# File conditions
if [ -a FILE ]; then        # -e exists         -d directory        -f file
fi                          # -r readable       -w writeable        -x executable
                            # -h symlink        -s size > 0

# File comparisons
if [ FILE1 -nt FILE2 ]      # -nt   1 more recent than 2
                            # -ot   2 more recent than 1
                            # -ef   same files

Ifs

# String
if [ -z STRING ]            # empty?
if [ -n STRING ]            # not empty?

# Numeric
if [ $? -eq 0 ]             # -eq -ne -lt -le -gt -ge
                            # $? is exit status by the way

# Etc
if [ -o noclobber ]         # if OPTIONNAME is enabled
if [ ! EXPR ]               # not
if [ ONE -a TWO ]           # and
if [ ONE -o TWO ]           # or

# Regex
if [[ "A" =~ "." ]]

Numeric comparisons

if $(( $a < $b ))

Unset variables

Assume $FOO is not set. Doing this will result in that:

${FOO:-word}                # Returns word
${FOO:+word}                # Returns empty, or word if set
${FOO:=word}                # Sets parameter to word, returns word
${FOO:?message}             # Echoes message and exits

${FOO=word}                 # : is optional in all of the above

Numeric calculations

$((RANDOM%=200))            # Random number 0..200
$((a + 200))                # $ is optional

Arrays

Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"

# Declaring using declare -a
declare -a Fruits=('Apple' 'Banana' 'Orange')

echo ${Fruits[0]}           # Element #0
echo ${Fruits[@]}           # All elements, space-separated
echo ${#Fruits[@]}          # Number of elements
echo ${#Fruits}             # String length of the 1st element
echo ${#Fruits[3]}          # String length of the Nth element
echo ${Fruits[@]:3:2}       # Range (from position 3, length 2)

Fruits=("${Fruits[@]}" "Watermelon")    # Push
Fruits=( ${Fruits[@]/Ap*/} )            # Remove by regex match
unset Fruits[2]                         # Remove one item
Fruits=("${Fruits[@]}")                 # Duplicate
Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
lines=(`cat "logfile"`)                 # Read from file

Misc crap

command -V cd               #=> "cd is a function/alias/whatever"

Options

set -o noclobber            # Avoid overlay files (echo "hi" > foo)
set -o errexit              # Used to exit upon error, avoiding cascading errors
set -o pipefail             # Unveils hidden failures
set -o nounset              # Exposes unset variables

Glob options

set -o nullglob             # Non-matching globs are removed  ('*.foo' => '')
set -o failglob             # Non-matching globs throw errors
set -o nocaseglob           # Case insensitive globs
set -o dotglob              # Wildcards match dotfiles ("*.sh" => ".foo.sh")
set -o globstar             # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')

set GLOBIGNORE as a colon-separated list of patterns to be removed from glob matches.

Trap errors

trap 'echo Error at about $LINENO' ERR

or

traperr() {
  echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
}

set -o errtrace
trap traperr ERR

References

• http://wiki.bash-hackers.org/