Системно и Мрежово Администриране на Linux 1

1. Bash substitusions
man bash
search for 'Parameter Expansion'

$ a='ABCfilename123123'

  ${#parameter} - parameter length

$ echo ${#a}
24

Prefix substitusion
$ echo ${a#ABC}
filename123123
## does not work always

Suffix substitusion
$ echo ${a%123}
ABCfilename123
%% does not work always

Pattern Substitusion
$ echo ${a/123/DFG}
ABCfilename123DFG

$ echo ${a//123/DFG}
ABCfilenameDFGDFG

If pattern starts with # it must match at the beginning.
If pattern starts with % it must match at the end.
 
Upper ^ / Lower , case conversions
  ${var^}
  ${var^^}
  ${var,}
  ${var,,}

2. Magic variables:

$# - Number of arguments supplied to the script
$$ - PID of the currently running programm
$? - Error report code of the last executed script/command

3. Working with the shell
3.1. forground jobs
3.2. background jobs

  bg
  fg
  jobs


4. Codding style

4.1. empty var comparisons

VAR=
if [ -z $VAR ]; then  vs. if [ -z "$VAR" ]; then

4.2. bracketing
  if ( cmd ); then

vs.

  if ( cmd );
  then

  for i in *; do

vs.

  for i in *;
  do


4.3. Script width
  if [ ! -z "$var" ]; then
      for i in $var; do
            do_stuff ...
      done
  else
      for i in *; do
            do_stuff ...
      done
  fi
vs.
  tst="$var"
  if [ -z "$var" ]; then
      tst='*'
  fi
  for i in $var; do
      do_stuff ...
  done

3. commands

cat
cut
head
tail
sort
uniq
wc

hackman@gamelon:~/lsa$ head -n-2 file1
1 adasdasda
2 klwrtwfgs
3 gwfehrtgwerf
4 .gwegrewg
5 warwerfdsa
hackman@gamelon:~/lsa$ head -n+2 file1
1 adasdasda
2 klwrtwfgs
hackman@gamelon:~/lsa$ tail -n+2 file1
2 klwrtwfgs
3 gwfehrtgwerf
4 .gwegrewg
5 warwerfdsa
6 okmqtfsd
7 gwqwezeee
hackman@gamelon:~/lsa$ tail -n-2 file1
6 okmqtfsd
7 gwqwezeee