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edit loop section

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add more examples for 'if'
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@ -880,71 +880,75 @@ find . -name "*.mso" -size -74c -delete
``` ```
## Loops ## Loops
[[back to top](#handy-bash-oneliner-commands)] [[back to top](#handy-bash-oneliner-commands)]
##### While loop, column subtraction of a file (e.g. a 3 columns file)
```bash
while read a b c; do echo $(($c-$b));done < <(head filename)
#there is a space between the two '<'s
```
##### While loop, sum up column subtraction
```bash
i=0; while read a b c; do ((i+=$c-$b)); echo $i; done < <(head filename)
```
##### While loop, keep checking a running process (e.g. perl) and start another new process (e.g. python) immetiately after it. (BETTER use the wait command! Ctrl+F 'wait')
```bash
while [[ $(pidof perl) ]];do echo f;sleep 10;done && python timetorunpython.py
```
##### If loop ##### If loop
```bash ```bash
# if and else loop for string matching # if and else loop for string matching
if [[ "$c" == "read" ]]; then outputdir="seq"; else outputdir="write" ; fi if [[ "$c" == "read" ]]; then outputdir="seq"; else outputdir="write" ; fi
```
if (($j==$u+2)) # Test if myfile contains the string 'test':
#(( )) use for arithmetic operation if grep -q hello myfile; then …
```
```bash # Test if mydir is a directory, change to it and do other stuff:
if [[$age >21]] if cd mydir; then
#[[ ]] use for comparison echo 'some content' >myfile
``` else
echo >&2 "Fatal error. This script requires mydir."
fi
##### Test if file exist # Test if file exist
```bash
if [ -e 'filename' ] if [ -e 'filename' ]
then then
echo -e "file exists!" echo -e "file exists!"
fi fi
```
##### if else Test if file exist # Test if file exist but also including symbolic links:
```bash if [ -e myfile ] || [ -L myfile ]
if [ -e $filename ]; then echo -e "file exists!"; else mkdir $filename; fi then
echo -e "file exists!"
fi
# Test if the value of x is greater or equal than 5
if [ "$x" -ge 5 ]; then …
# Test if the value of x is greater or equal than 5, in bash/ksh/zsh:
if ((x >= 5)); then …
# Use (( )) for arithmetic operation
if (($j==$u+2))
# Use [[ ]] for comparison
if [[$age >21]]
``` ```
##### For loop ##### For loop
```bash ```bash
for i in $(ls); do echo file $i;done for i in $(ls); do echo file $i;done
```
##### for loop, press any key to continue each loop # Press any key to continue each loop
```bash
for i in $(cat tpc_stats_0925.log |grep failed|grep -o '\query\w\{1,2\}');do cat ${i}.log; read -rsp $'Press any key to continue...\n' -n1 key;done for i in $(cat tpc_stats_0925.log |grep failed|grep -o '\query\w\{1,2\}');do cat ${i}.log; read -rsp $'Press any key to continue...\n' -n1 key;done
```
##### for loop, print a file line by line when a key is pressed # Print a file line by line when a key is pressed
```bash
for line in $(cat myfile); do echo $line; read -n1; done for line in $(cat myfile); do echo $line; read -n1; done
``` ```
##### While loop,
```bash
# Column subtraction of a file (e.g. a 3 columns file)
while read a b c; do echo $(($c-$b));done < <(head filename)
#there is a space between the two '<'s
# Sum up column subtraction
i=0; while read a b c; do ((i+=$c-$b)); echo $i; done < <(head filename)
# Keep checking a running process (e.g. perl) and start another new process (e.g. python) immetiately after it. (BETTER use the wait command! Ctrl+F 'wait')
while [[ $(pidof perl) ]];do echo f;sleep 10;done && python timetorunpython.py
```
##### switch (case in bash) ##### switch (case in bash)
```bash ```bash
read type; read type;