WorkingOnServers/BetterBashing

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A collection of useful shell commands for the bash shell

A compilation on how to use the bash shell

  • changing the login shell

chsh /bin/bash

  • returning to the previous directory

cd -

  • brace expansion

echo b{a,e}d -> bad bed echo 1{2..4} -> 12 13 14

  • unix data filtering tools

cat copy input to output grep search for strings in the input sort sort lines in the input cut extract columns from the input sed perform editing operations on input tr translate characters in the input

  • cut with options

echo worm:track | cut -d: -f1 (use delimiter :, field #1) -> worm

  • backslash escape

echo \"2 \* 3 \> 5\" -> "2 * 3 > 5"

  • find with options

find . -name '*.txt' (wildcard needs quote)

  • line breaks

echo this is a very \ long line for echo

  • control keys

CTRL-C stop current command CTRL-D end of input CTRL-\ force stop command (try CTRL-C first) CTRL-S halt screen output CTRL-Q restart screen output CTRL-? erase last character CTRL-U erase command line CTRL-Z suspend current comand

  • command line editing (EMACS-mode)

set -o emacs

TAB attempt completion ESC-? list completion options ESC-TAB attempt completion from history CTRL-O execute command and go to next line in history CTRL-U kill line from beginning to current position ESC-U uppercase following words ESC-L lowercase following words ESC-. bring up last word from previous line in history

  • command line editing (vi-mode)

set -o vi

moving, deleting as usual i insert at current position a append after current position I insert at beginning of line A append after end of line R overwrite k or - up in history j or + down in history nG position n in history /string search backward for string ?string search forward for string n/N search again in same/opposite direction f/F find character t/T find character, back one

/, repeat character find in same/opposite direction

n| move to column n \ command completion

  • command completion and list on command line

= command completion and list below

  • setting the prompt

PS1 shell prompt variable \u user name \s shell name \! history number \# command number \w pwd \W basename of pwd \t time \h hostname

  • setting the search path

CDPATH default search paths for cd PWD present working directory OLDPWD previous PWD

  • ls with options

ls -F lists directories with /, links with @, and executables with *

  • defining shell functions

function functname{ 

 shell commands}

-or-

functname () { shell commands; }

  • function arguments

$0 command name $1 argument 1 $@ all arguments, separated by double quotes $* all arguments separated by field separator IFS (default blank) $# number of arguments


  • return value

function () { statement; return $var }

  • string operators

${variable:-word} return word if variable is undefined ${variable:=word} use word as default for variable if variable is undefined ${variable:?message} print message and abort if variable does not exist ${variable:+word} return word if variable is defined, otherwise return 0 ${variable:offset:length} return substring between offset and length if offset omitted, use until the end of variable if offset < 0, start from end (mind space!) ${#variable} return length of variable value


  • echo with options

echo -e -n "This is\n a \t poem" -e interprets \n as linefeed, \t as tabulator, -E to turn backslash escape interpretation off -n surpresses the newline at the end

  • echo escape sequences

\a Alert (beep) \b Backspace \c no newline \e,\E escape character \f Formfeed \n newline \r return \t tab \v vertical tab \n ASCII character with code n \0nnn octal format \xHH hexadecimal format \\ single backslash

  • variable pattern matching

${variable#pattern} delete shortest matching pattern from beginning ${variable##pattern} delete longest matching pattern from the end ${variable%pattern} delete shortest matching pattern from beginning ${variable%%pattern} delete longest matching pattern from the end ${variable/pattern/string} replace first longest matching pattern ${variable//pattern/string} replace all matching patterns

  • replacing awk

awk -FC '{print $N}' filename

- is equivalent to -

cut -fN -dC filename

where C is a character and N is a number.

  • flow control

if condition; then 

 statements

elif condition; then 

 statements

else 

 statements

fi

  • using the unaliased command

command ls ~

  • using the builtin function

builtin cd ~

  • condition testing: strings

str1=str2 str1!=str2 str1<str2 str1>str2 -n str1 string is not null (zero length) -z str1 string is null (zero length)

  • condition testing: file attributes

-a file file exists -e file file exists -d file file exists and is a directory -f file file exists and is a regular file -r file have read permission to file -s file file exists and is not empty -w file have write permission to file -x file have execute permission to file -N file file was modified since last read -O file own file -G file files group ID matches mine file1 -nt file2 file1 is newer than file2 file1 -ot file2 file1 is older than file2

Use && and || outside conditional expressions Use -a and -o inside conditional expressions

if [ -a live ] && [ \( -x live \) -a \( -w live \) ]; then 

 echo "OK"

else 

echo "not OK"

fi

  • condition testing: integers

-lt less than -le less or equal -eq equal -ge greater than -ge greater or equal -gt greater than -ne not equal

  • for loops

for name [in list]; do 

 statements

done

  • case statements

case expression in 

 pattern1 )
   statements ;;
 pattern2 )
   statements ;;

esac

  • while/until loops

while/until condition; do 

 statements

done

  • shift shell arguments

shift n shifts the contents of $m to $(m-n)

  • handling shell script options

while [ -n "$(echo $1 | grep '-')" ]; do 

 case $1 in
   -a )  process option -a ;;
   -b )  process option -b
          $2 is always the option's argument
          shift  ;;
   -c )  process option -c ;;
   *  )  echo 'usage: script [-a] [-b barg] [-c] args...'
          exit 1
 esac
 shift

done

  • handling shell script options with getopts

while getopts ":ab:c" opt; do 

 case $opt in
   a  )  process -a ;;
   b  )  process option -b
          $OPTARG is the option's argument
   c  )  process option -c ;;
   \? )  echo 'usage: script [-a] [-b barg] [-c] args...'
          exit 1
 esac

done shift $(($OPTIND - 1))

  • declare: typed variables

declare -a array declare -i integer declare -r read-only declare -x export

  • command evaluation

output=$( command )

  • integer arithmetic

output=$(( a + b )) (deprecated: $[ a + b ])

  • arithmetic operators

++ increment -- decrement + plus - minus

  • product

/ division % remainder

    • exponent

== equality (note: different for strings)

  • number base

16#ff hexadecimal 256 10#10 decimal 10

  • arithmetic for loop

for (( initialisation ; ending condition ; update )) do 

 statements

done

  • arrays

names[0]=hans names[1]=bob names[2]=jim

names=([2]=jim [0]=hans [1]=bob)

names=(jim hans bob)

echo ${names[2]} print array element echo ${names[@]} print array with double quotes echo ${names[*]} print array as single quoted string echo ${#names[2]} return length of array element echo ${#names[@]} return length of array

  • I/O redirection

cmd1 | cmd2 pipe > file stdout to file (same as 1> file) < file stdin from file (same as 0< file) >> stdout to file - append >| file stdout to file - overwrite <> file stdout and stdin to file &> file stdout and stderr to file 2> file stderr to file 2>&1 stderr to same as stdout

tee file copy stdin to screen and file

  • Here-documents

cat >> afile << EOF copy stdin to afile until EOF is encountered if EOF is surrounded by , no variable substitution will be performed on the input if EOF is prefixed with -, leading tabs (not spaces) are removed

  • printf

printf "hi %s\n" harald formated string printf "%7.2G\n" -89.2 formated number printf "%*.*G\n" 7 2 ${lat} dynamical field width specifiers, variable as argument

additional formatters:

- left-justify value in the field ' ' prefix positive values with space, negative with minus + always prefix numbers 0 pad output with zeros %b interpret escape sequences in the input %q transform spaces etc in strings in a shell-argument compatible way

printf "%q\n" "hello world" hello\ world

  • read

read var1 var2 ... breaks stdin into words and assigns it to the variables, using space (or IFS)

  • reading lines from files (not always useful)

IFS=':' while read parameter value; do 

 echo $parameter $value | grep "model name"

done < /proc/cpuinfo

  • float statement input redirection

aFunction() { 

 echo $i

} < logfile

while statement; do 

 something

done < inputfile

  • ignore comment lines in input

if [ -z "$(echo $dev | grep ^#)" ]; then 

 statements

fi

  • eval

eval "ls | more" perform command-line processing on "ls | more"

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