Bash constructs

{ code; } Command block

(code) Subshell

$'string' Quoting

${!var} Indirect Variable Reference

"'$var" | '"string' Char to Octal/Decimal/Hex

{ code; } - command block

Braces must be separated from the block by white space.
Must have semicolon or newline after last line in the block.
Acts like a function with no name.
A form of compound statement.
Will take standard I/O descriptors
Used to redirect I/O to or from an arbitrary group of commands
Doesn't create a separate process

(code) - subshell

Creates a separate process

${!var} (indirect variable reference)

var1=foo foo=bar echo ${!var1} # --> bar var2=${!var1} echo $var2 # --> bar

This is a replacement for eval var2=\$$var1

var1=foo foo=bar eval var2=\$$var1 echo $var2 # --> bar

Positional parameters

Syntax: set [options] [arg1 arg2 ...]

Example: set -- -1 -x

Explanation: -- turns off option processing so $1 is set to -1 and $2 is set to -x

Example: set -- *

Explanation: set the postitional parameters to the names of all the files in the current directory

Example: set --

Explanation: with no arguments, unset the positional parameters,

Quoting

Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\nnn the eight-bit character whose value is the octal value nnn (one to three digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\cx a control-x character

CR=$'\r' sets the variable CR to be a hex 0D (decimal 13)

LF=$'\n' sets the variable LF to be a hex 0A (decimal 10)

echo 'cat\ndog'
catndog

echo $'cat\ndog'
cat
dog

( execute in a subshell )

[ $a = 2 ] || (echo noa && echo xxx)
[ $a = 2 ] || { echo noa && echo xxx; }

Feed a block at the top with command output
(Beware the 2048 byte limit of temp files created by pipes)

command | while read LINE; do
  echo $LINE
done

also

command | while read WORD1 WORD2 REST; do
  do something with $WORD1
  do something with $WORD2
  do something with $REST
done

Feed a block at the bottom with command output

while read LINE; do
  echo $LINE
done < <(command)

The space between < and < is required
and this will not work with #!/bin/sh. use #!/bin/bash

Feed a block at the top with file output

cat file | while read LINE; do
  echo $LINE
done

Feed a block at the bottom with file output

while read LINE; do
  echo "$LINE"
done < /etc/joe/joerc|grep -v "^#"|grep -v '^$'

This example code also ignores comment and blank lines

Using the line command instead of read

while LINE=$(/usr/bin/line); do
  echo "$LINE"
done < /etc/joe/joerc|grep -v "^#"|grep -v '^$'

Using input file descriptors

exec 3<&0		# Associate stdin with fd3
exec 0> $INFILE		# Redirect $INFILE to stdin
while read LINE; do
  echo $LINE
done
exec 0<&3	# Restore stdin
exec 3>&-	# Close fd3

Using output file descriptors

exec 4<&1	# Associate stdout with fd4
exec 1> $OUTFILE	# Redirect stdout to $OUTFILE
while read LINE; do
  echo $LINE
done < $INFILE
exec 1<&4	# Restore stdout
exec 4>&-	# Close fd4

Indirect Expansion

bar=3 foo=bar echo ${!foo} 3 eval echo \$$foo 3 eval a=\$$foo echo $a 3

Array Key and Value Expansion

a=(cat dog hat rat) for key in ${!a[*]};do echo $key;done 0 1 2 3 for value in ${a[*]};do echo $value;done cat dog hat rat a=("black cat" "hot dog") for value in "${a[@]}";do echo $value;done black cat hot dog for value in "${a[*]}";do echo $value;done black cat hot dog

Command substitution

Command substitution allows the output of a command to replace the command name. There are two forms:

$(command) `command`

Bash performs the expansion by executing command and replacing the command substitution with the standard output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word splitting. The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).

If the substitution appears within double quotes, word splitting and filename expansion are not performed on the results.

echo -e $(cat now) Now is the time For all good men To go to the aid Of their country. echo -e "$(cat now)" Now is the time For all good men To go to the aid Of their country. for LINE in "$(cat now)"; do echo -e x${LINE}x;done xNow is the time For all good men To go to the aid Of their country.x for LINE in $(cat now); do echo -e x${LINE}x;done xNowx xisx xthex xtimex xForx xallx xgoodx xmenx xTox xgox xtox xthex xaidx xOfx xtheirx xcountry.x

Brace Expansion

echo {a,b}{c,d} --> ac ad bc bd
echo {a..d}{e,f} --> ae af be bf ce cf de df
cat {file1,file2,file3} > combined_file # Concatenates the files file1, file2, and file3 into combined_file.
cp /usr/local/bin/file22.{txt,backup} # Copies "/usr/local/bin/file22.txt" to "/usr/local/bin/file22.backup"

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	Many thanks to Debra and Ian Murdock for making Debian possible
	First created Apr 22, 2008 ~ Last revised August 27, 2010