Bash

Documentation

• Advanced Bash-Scripting Guide
• http://www.ss64.com/bash/index.html (find Linux shell command if you know Windows shell command, and vice-versa)
• http://tille.xalasys.com/training/bash

Shell

Listing directory content:

ls --full-time               # To list files full date & time
ls -d directory-name         # List directory entries, not content
                             #  eg. ls -d /etc/rc*
ls -lS | head                # List biggest files first - print only first 10 entries
ls -lt | head -n 20          # List most recent files first, print only 20 first entries

Script Quick Tutorial

Warning / Common pitfalls

A set of warnings / common pitfalls when writing script in Bash:

Description	Example
Space! - Don't forget to add spaces whenever necessary, in particular around brace in function definition, or in test conditions for ifs.	if -space- [ -space- -f /etc/foo -space- ]; then ... function myfunc() { -space- echo Hello, World!; }
Quote - Always quote parameters, variables passed to test in if ... then ... else:	if [ "$name" -eq 5 ]; then ...
For loops with file - Use simply * to list files in for loops, not `ls *`:	for file in *; cat "$file"; done # SUCCEEDS, even if white space for file in `ls *`; cat "$file"; done # FAILS miserably

Environment

export MYVAR=myvalue && command-to-execute         #MYVAR defined for the current shell and invoked shell
MYVAR=myvalue && command-to-execute                #MYVAR defined for the current shell
MYVAR=myvalue command-to-execute                   #MYVAR only defined for the subsequent command

Quotation

echo "var is $var"	String enclosed in double quote is treated as a single argument. Variable and special characters are expanded.
echo 'it costs $10'	Same as double quote, but variables are not expanded.
x=`expr $x + 1`	Back-quote is replaced by the result of the invoked command.
x=$(expr $x + 1)	Same as back-quote, but somewhat clearer
x=$(($x + 1))	Built-in Bash expression evaluator. Accepts operation are +, -, *, /, %.

If ... then ... [elif ...] ... else ... fi

The basic syntax for if-then-else sequence control is

if list; then list; [ elif list; then list; ] ... [ else list; ] fi

Test conditions can be written using command test, or equivalently using the square brackets.

if test -f /etc/foo; then echo file found; else echo file NOT found; fi
if [ -f /etc/foo ]; then echo file found; else echo file NOT foudn; fi

Here's test's options:

-d FILE        # Check if the file is a directory
-e FILE        # Check if the file exists
-f FILE        # Check if the file is a regular file
-g FILE        # Check if the file hash SGID permissions
-r FILE        # Check if the file is readable
-s FILE        # Check if the file's size is not 0
-u FILE        # Check if the file has SUID permissions
-w FILE        # Check if the file is writeable
-x FILE        # Check if the file is executable
NBR1 -eq NBR2  # Check is NBR1 is equals to NBR2
NBR1 -ne NBR2  # Check if NBR1 is not equals to NBR2
NBR1 -ge NBR2  # Check if NBR1 is greater than or equal to NBR2
NBR1 -gt NBR2  # Check if NBR1 is greater than NBR2
NBR1 -le NBR2  # Check if NBR1 is less than or equal to NBR2
NBR1 -lt NBR2  # Check if NBR1 is less than NBR2
STR1 = STR2    # Check if STR1 is the same as STR2
STR1 != STR2   # Check if STR1 is not the same as STR2
-n STR         # Evaluates to true if STR is not null
-z STR         # Evaluates to true if STR is null.

Conditions can be combined with && and ||:

if [ $x -ge 5] && [ $x -le 10]; then ...
if [ $x -gt 5] || [ $x -lt 10]; then ...

Some examples:

if [ "$name" eq 5 ]; then echo one; else echo two; fi
if [ -d ~/var ]; then touch ~/var/done; else { mkdir ~/var; touch ~/var/done; } fi   # braces { ... } are optional here

Alternative: Using && and ||

Using && and ||, one can also build a if ... then ... else statement:

[ -f /etc/hosts ] && { echo one; echo two; } || { echo three; echo four; }

WHILE / UNTIL Loops

Syntax is

while list; do list; done
until list; do list; done

Some examples of While loops:

while true; do
   echo "Press CTRL-C to quit."
done

# Faster alternative using Bash built-in colon feature
while :; do
   echo "Press CTRL-C to quit."
done

# A more complete example
x=0;                                 # initialize x to 0
while [ "$x" -le 10 ]; do
    echo "Current value of x: $x"
    # increment the value of x:
    x=$(expr $x + 1)
    sleep 1
done

Some examples of until loops:

x=0
until [ "$x" -ge 10 ]; do
    echo "Current value of x: $x"
    x=$(expr $x + 1)
    sleep 1
done

FOR Loops

There are 2 syntax for for loops:

for name [ in word ] ; do list ; done
for (( expr1 ; expr2 ; expr3 )) ; do list ; done

The first syntax enumerates a list, as in:

# Counting from 1 to 10...
for dots in 1 2 3 4 5 6 7 8 9 10; do echo -n "$dots... "; done;
echo 'Done !'

# Enumerating a list
for fruit in Apple Pear Cherry; do
  echo "The value of variable fruit is: $fruit"
  sleep 1
done

# Acting on a directory list:
for file in *; do
  echo "Adding .html extension to $file..."
  mv $file $file.html
  sleep 1
done

# On several lines
for f in $(seq 33296 33330); do
  wget "http://i.techrepublic.com.com/gallery/${f}.jpg"
done

# On a single line
for i in `seq 278 328`; do wget http://i.techrepublic.com.com/gallery/33$i.jpg; done

case ... in ... esac

The syntax is

case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac

Some example

x=5     # initialize x to 5
# now check the value of x:
case $x in
   0) echo "Value of x is 0."
      ;;
   5) echo "Value of x is 5."
      ;;
   9) echo "Value of x is 9."
      ;;
   *) echo "Unrecognized value."
esac

Functions

Functions are like aliases, but accept parameters:

function myfunc1() { echo "$2"; echo "$1"; }
myfunc2() { MYVAR="$1"; echo Another $MYVAR function; }       #The keyword ''function'' is optional

Interactivity

Read User's password

Reading input from user is done with the commands read. To prevent password echo on display, one can use the command stty:

###### Preventing echo with stty ######
read -p "Username: " uname
stty -echo
read -p "Password: " passw; echo
stty echo

Preventing password echo using option -s:

###### Using -s ######
PASS="abc123"
read -s -p "Password: " mypassword
echo ""
[ "$mypassword" == "$PASS" ] && echo "Password accepted" || echo "Access denied"

Complete solution:

###### Complete solution ######
USER=wbi\\titeuf
echo "Mounting windows share... Please type password for user $USER..."
# time-out after 60sec, raw input no escaping, no echo, prompt
read -t 60 -r -s -p "Password: " PASSWORD
# delete prompt line
echo -e -n "\r"
mount -t smbfs -o username="$USER",password="$PASSWORD",iocharset=iso8859-1,codepage=cp437 //windows-host/C$ /mnt/c
mount -t smbfs -o username="$USER",password="$PASSWORD",iocharset=iso8859-1,codepage=cp437 //windows-host/D$ /mnt/d
mount -t smbfs -o username="$USER",password="$PASSWORD",iocharset=iso8859-1,codepage=cp437 //windows-host/F$ /mnt/f
# delete password variable
PASSWORD=------------------------------------
PASSWORD=abcdefghijklmnopqrstuvwxyz0123456789
PASSWORD=------------------------------------

Signal trapping

Use trap to trap signals send to a Bash script, and redirect execution to a given function/command.

#Trap Ctrl-C (signal SIGINT) by executing function "sorry"
trap sorry INT
sorry() { echo "I'm sorry Dave. I can't do that."; sleep 3; }

Signals can also be ignored or reset.

#reset the trap:
trap - INT
#do nothing when SIGINT is caught:
trap " INT

Miscellaneous

Use eval to execute the output (stdout) of a process	eval 'dircolors'
Use $$ as file name suffix or prefix to create unique name for temporary file	touch /tmp/mytmp.$$ echo some text>/tmp/mytmp.$$ cat /tmp/mytmp.$$

References

• [1], or with better formatting [2].